Primitives addon

These functions are declared in the following header file. Link with allegro_primitives.

 #include <allegro5/allegro_primitives.h>

General

al_get_allegro_primitives_version

uint32_t al_get_allegro_primitives_version(void)

Source Code

Returns the (compiled) version of the addon, in the same format as al_get_allegro_version.

al_init_primitives_addon

bool al_init_primitives_addon(void)

Source Code

Initializes the primitives addon.

Returns: True on success, false on failure.

al_is_primitives_addon_initialized

bool al_is_primitives_addon_initialized(void)

Source Code

Returns true if the primitives addon is initialized, otherwise returns false.

Since: 5.2.6

al_shutdown_primitives_addon

void al_shutdown_primitives_addon(void)

Source Code

Shut down the primitives addon. This is done automatically at program exit, but can be called any time the user wishes as well.

High level drawing routines

High level drawing routines encompass the most common usage of this addon: to draw geometric primitives, both smooth (variations on the circle theme) and piecewise linear. Outlined primitives support the concept of thickness with two distinct modes of output: hairline lines and thick lines. Hairline lines are specifically designed to be exactly a pixel wide, and are commonly used for drawing outlined figures that need to be a pixel wide. Hairline thickness is designated as thickness less than or equal to 0. Unfortunately, the exact rasterization rules for drawing these hairline lines vary from one video card to another, and sometimes leave gaps where the lines meet. If that matters to you, then you should use thick lines. In many cases, having a thickness of 1 will produce 1 pixel wide lines that look better than hairline lines. Obviously, hairline lines cannot replicate thicknesses greater than 1. Thick lines grow symmetrically around the generating shape as thickness is increased.

Pixel-precise output

While normally you should not be too concerned with which pixels are displayed when the high level primitives are drawn, it is nevertheless possible to control that precisely by carefully picking the coordinates at which you draw those primitives.

To be able to do that, however, it is critical to understand how GPU cards convert shapes to pixels. Pixels are not the smallest unit that can be addressed by the GPU. Because the GPU deals with floating point coordinates, it can in fact assign different coordinates to different parts of a single pixel. To a GPU, thus, a screen is composed of a grid of squares that have width and length of 1. The top left corner of the top left pixel is located at (0, 0). Therefore, the center of that pixel is at (0.5, 0.5). The basic rule that determines which pixels are associated with which shape is then as follows: a pixel is treated to belong to a shape if the pixel’s center is located in that shape. The figure below illustrates the above concepts:

*Diagram showing a how pixel output is calculated by the GPU given the mathematical description of several shapes.*

This figure depicts three shapes drawn at the top left of the screen: an orange and green rectangles and a purple circle. On the left are the mathematical descriptions of pixels on the screen and the shapes to be drawn. On the right is the screen output. Only a single pixel has its center inside the circle, and therefore only a single pixel is drawn on the screen. Similarly, two pixels are drawn for the orange rectangle. Since there are no pixels that have their centers inside the green rectangle, the output image has no green pixels.

Here is a more practical example. The image below shows the output of this code:

/* blue vertical line */
al_draw_line(0.5, 0, 0.5, 6, color_blue, 1);
/* red horizontal line */
al_draw_line(2, 1, 6, 1, color_red, 2);
/* green filled rectangle */
al_draw_filled_rectangle(3, 4, 5, 5, color_green);
/* purple outlined rectangle */
al_draw_rectangle(2.5, 3.5, 5.5, 5.5, color_purple, 1);

*Diagram showing a practical example of pixel output resulting from the invocation of several primitives addon functions.*

It can be seen that lines are generated by making a rectangle based on the dashed line between the two endpoints. The thickness causes the rectangle to grow symmetrically about that generating line, as can be seen by comparing the red and blue lines. Note that to get proper pixel coverage, the coordinates passed to the al_draw_line had to be offset by 0.5 in the appropriate dimensions.

Filled rectangles are generated by making a rectangle between the endpoints passed to the al_draw_filled_rectangle.

Outlined rectangles are generated by symmetrically expanding an outline of a rectangle. With a thickness of 1, as depicted in the diagram, this means that an offset of 0.5 is needed for both sets of endpoint coordinates to exactly line up with the pixels of the display raster.

The above rules only apply when multisampling is turned off. When multisampling is turned on, the area of a pixel that is covered by a shape is taken into account when choosing what color to draw there. This also means that shapes no longer have to contain the pixel’s center to affect its color. For example, the green rectangle in the first diagram may in fact be drawn as two (or one) semi-transparent pixels. The advantages of multisampling is that slanted shapes will look smoother because they will not have jagged edges. A disadvantage of multisampling is that it may make vertical and horizontal edges blurry. While the exact rules for multisampling are unspecified, and may vary from GPU to GPU, it is usually safe to assume that as long as a pixel is either completely covered by a shape or completely not covered, then the shape edges will be sharp. The offsets used in the second diagram were chosen so that this is the case: if you use those offsets, your shapes (if they are oriented the same way as they are on the diagram) should look the same whether multisampling is turned on or off.

al_draw_line

void al_draw_line(float x1, float y1, float x2, float y2,
   ALLEGRO_COLOR color, float thickness)

Source Code

Draws a line segment between two points.

Parameters:

x1, y1, x2, y2 - Start and end points of the line
color - Color of the line
thickness - Thickness of the line, pass <= 0 to draw hairline lines

al_draw_triangle

void al_draw_triangle(float x1, float y1, float x2, float y2,
   float x3, float y3, ALLEGRO_COLOR color, float thickness)

Source Code

Draws an outlined triangle.

Parameters:

x1, y1, x2, y2, x3, y3 - Three points of the triangle
color - Color of the triangle
thickness - Thickness of the lines, pass <= 0 to draw hairline lines

Examples:

ex_prim.c

al_draw_filled_triangle

void al_draw_filled_triangle(float x1, float y1, float x2, float y2,
   float x3, float y3, ALLEGRO_COLOR color)

Source Code

Draws a filled triangle.

Parameters:

x1, y1, x2, y2, x3, y3 - Three points of the triangle
color - Color of the triangle

al_draw_rectangle

void al_draw_rectangle(float x1, float y1, float x2, float y2,
   ALLEGRO_COLOR color, float thickness)

Source Code

Draws an outlined rectangle.

Parameters:

x1, y1, x2, y2 - Upper left and lower right points of the rectangle
color - Color of the rectangle
thickness - Thickness of the lines, pass <= 0 to draw hairline lines

Examples:

al_draw_filled_rectangle

void al_draw_filled_rectangle(float x1, float y1, float x2, float y2,
   ALLEGRO_COLOR color)

Source Code

Draws a filled rectangle.

Parameters:

x1, y1, x2, y2 - Upper left and lower right points of the rectangle
color - Color of the rectangle

Examples:

al_draw_rounded_rectangle

void al_draw_rounded_rectangle(float x1, float y1, float x2, float y2,
   float rx, float ry, ALLEGRO_COLOR color, float thickness)

Source Code

Draws an outlined rounded rectangle.

Parameters:

x1, y1, x2, y2 - Upper left and lower right points of the rectangle
color - Color of the rectangle
rx, ry - The radii of the round
thickness - Thickness of the lines, pass <= 0 to draw hairline lines

Examples:

al_draw_filled_rounded_rectangle

void al_draw_filled_rounded_rectangle(float x1, float y1, float x2, float y2,
   float rx, float ry, ALLEGRO_COLOR color)

Source Code

Draws an filled rounded rectangle.

Parameters:

x1, y1, x2, y2 - Upper left and lower right points of the rectangle
color - Color of the rectangle
rx, ry - The radii of the round

Examples:

al_calculate_arc

void al_calculate_arc(float* dest, int stride, float cx, float cy,
   float rx, float ry, float start_theta, float delta_theta, float thickness,
   int num_points)

Source Code

When thickness <= 0 this function computes positions of num_points regularly spaced points on an elliptical arc. When thickness > 0 this function computes two sets of points, obtained as follows: the first set is obtained by taking the points computed in the thickness <= 0 case and shifting them by thickness / 2 outward, in a direction perpendicular to the arc curve. The second set is the same, but shifted thickness / 2 inward relative to the arc. The two sets of points are interleaved in the destination buffer (i.e. the first pair of points will be collinear with the arc center, the first point of the pair will be farther from the center than the second point; the next pair will also be collinear, but at a different angle and so on).

The destination buffer dest is interpreted as a set of regularly spaced pairs of floats, each pair holding the coordinates of the corresponding point on the arc. The two floats in the pair are adjacent, and the distance (in bytes) between the addresses of the first float in two successive pairs is stride. For example, if you have a tightly packed array of floats with no spaces between pairs, then stride will be exactly 2 * sizeof(float).

Example with thickness <= 0:

const int num_points = 4;
float points[num_points][2];
al_calculate_arc(&points[0][0], 2 * sizeof(float), 0, 0, 10, 10, 0, ALLEGRO_PI / 2, 0, num_points);

assert((int)points[0][0] == 10);
assert((int)points[0][1] == 0);

assert((int)points[num_points - 1][0] == 0);
assert((int)points[num_points - 1][1] == 10);

Example with thickness > 0:

const int num_points = 4;
float points[num_points * 2][2];
al_calculate_arc(&points[0][0], 2 * sizeof(float), 0, 0, 10, 10, 0, ALLEGRO_PI / 2, 2, num_points);

assert((int)points[0][0] == 11);
assert((int)points[0][1] == 0);
assert((int)points[1][0] == 9);
assert((int)points[1][1] == 0);

assert((int)points[(num_points - 1) * 2][0] == 0);
assert((int)points[(num_points - 1) * 2][1] == 11);
assert((int)points[(num_points - 1) * 2 + 1][0] == 0);
assert((int)points[(num_points - 1) * 2 + 1][1] == 9);

Parameters:

dest - The destination buffer
stride - Distance (in bytes) between starts of successive pairs of points
cx, cy - Center of the arc
rx, ry - Radii of the arc
start_theta - The initial angle from which the arc is calculated in radians
delta_theta - Angular span of the arc in radians (pass a negative number to switch direction)
thickness - Thickness of the arc
num_points - The number of points to calculate

Examples:

ex_vertex_buffer.c

al_draw_pieslice

void al_draw_pieslice(float cx, float cy, float r, float start_theta,
   float delta_theta, ALLEGRO_COLOR color, float thickness)

Source Code

Draws a pieslice (outlined circular sector).

Parameters:

cx, cy - Center of the pieslice
r - Radius of the pieslice
color - Color of the pieslice
start_theta - The initial angle from which the pieslice is drawn in radians
delta_theta - Angular span of the pieslice in radians (pass a negative number to switch direction)
thickness - Thickness of the circle, pass <= 0 to draw hairline pieslice

Since: 5.0.6, 5.1.0

al_draw_filled_pieslice

void al_draw_filled_pieslice(float cx, float cy, float r, float start_theta,
   float delta_theta, ALLEGRO_COLOR color)

Source Code

Draws a filled pieslice (filled circular sector).

Parameters:

cx, cy - Center of the pieslice
r - Radius of the pieslice
color - Color of the pieslice
start_theta - The initial angle from which the pieslice is drawn in radians
delta_theta - Angular span of the pieslice in radians (pass a negative number to switch direction)

Since: 5.0.6, 5.1.0

al_draw_ellipse

void al_draw_ellipse(float cx, float cy, float rx, float ry,
   ALLEGRO_COLOR color, float thickness)

Source Code

Draws an outlined ellipse.

Parameters:

cx, cy - Center of the ellipse
rx, ry - Radii of the ellipse
color - Color of the ellipse
thickness - Thickness of the ellipse, pass <= 0 to draw a hairline ellipse

Examples:

al_draw_filled_ellipse

void al_draw_filled_ellipse(float cx, float cy, float rx, float ry,
   ALLEGRO_COLOR color)

Source Code

Draws a filled ellipse.

Parameters:

cx, cy - Center of the ellipse
rx, ry - Radii of the ellipse
color - Color of the ellipse

Examples:

al_draw_circle

void al_draw_circle(float cx, float cy, float r, ALLEGRO_COLOR color,
   float thickness)

Source Code

Draws an outlined circle.

Parameters:

cx, cy - Center of the circle
r - Radius of the circle
color - Color of the circle
thickness - Thickness of the circle, pass <= 0 to draw a hairline circle

Examples:

al_draw_filled_circle

void al_draw_filled_circle(float cx, float cy, float r, ALLEGRO_COLOR color)

Source Code

Draws a filled circle.

Parameters:

cx, cy - Center of the circle
r - Radius of the circle
color - Color of the circle

Examples:

al_draw_arc

void al_draw_arc(float cx, float cy, float r, float start_theta,
   float delta_theta, ALLEGRO_COLOR color, float thickness)

Source Code

Draws an arc.

Parameters:

cx, cy - Center of the arc
r - Radius of the arc
color - Color of the arc
start_theta - The initial angle from which the arc is calculated in radians
delta_theta - Angular span of the arc in radians (pass a negative number to switch direction)
thickness - Thickness of the arc, pass <= 0 to draw hairline arc

Examples:

ex_prim.c

al_draw_elliptical_arc

void al_draw_elliptical_arc(float cx, float cy, float rx, float ry, float start_theta,
   float delta_theta, ALLEGRO_COLOR color, float thickness)

Source Code

Draws an elliptical arc.

Parameters:

cx, cy - Center of the arc
rx, ry - Radii of the arc
color - Color of the arc
start_theta - The initial angle from which the arc is calculated in radians
delta_theta - Angular span of the arc in radians (pass a negative number to switch direction)
thickness - Thickness of the arc, pass <= 0 to draw hairline arc

Since: 5.0.6, 5.1.0

al_calculate_spline

void al_calculate_spline(float* dest, int stride, const float points[8],
   float thickness, int num_segments)

Source Code

Calculates a Bézier spline given 4 control points. If thickness <= 0, then num_segments of points are required in the destination, otherwise twice as many are needed. The destination buffer should consist of regularly spaced (by distance of stride bytes) doublets of floats, corresponding to x and y coordinates of the vertices.

Parameters:

dest - The destination buffer
stride - Distance (in bytes) between starts of successive pairs of coordinates
points - An array of 4 pairs of coordinates of the 4 control points
thickness - Thickness of the spline ribbon
num_segments - The number of points to calculate

al_draw_spline

void al_draw_spline(const float points[8], ALLEGRO_COLOR color, float thickness)

Source Code

Draws a Bézier spline given 4 control points.

Parameters:

points - An array of 4 pairs of coordinates of the 4 control points
color - Color of the spline
thickness - Thickness of the spline, pass <= 0 to draw a hairline spline

al_calculate_ribbon

void al_calculate_ribbon(float* dest, int dest_stride, const float *points,
   int points_stride, float thickness, int num_segments)

Source Code

Calculates a ribbon given an array of points. The ribbon will go through all of the passed points. If thickness <= 0, then num_segments of points are required in the destination buffer, otherwise twice as many are needed. The destination and the points buffer should consist of regularly spaced doublets of floats, corresponding to x and y coordinates of the vertices.

Parameters:

dest - Pointer to the destination buffer
dest_stride - Distance (in bytes) between starts of successive pairs of coordinates in the destination buffer
points - An array of pairs of coordinates for each point
points_stride - Distance (in bytes) between starts of successive pairs of coordinates in the points buffer
thickness - Thickness of the spline ribbon
num_segments - The number of points to calculate

al_draw_ribbon

void al_draw_ribbon(const float *points, int points_stride, ALLEGRO_COLOR color,
   float thickness, int num_segments)

Source Code

Draws a ribbon given an array of points. The ribbon will go through all of the passed points. The points buffer should consist of regularly spaced doublets of floats, corresponding to x and y coordinates of the vertices.

Parameters:

points - An array of coordinate pairs (x and y) for each point
points_stride - Distance (in bytes) between starts of successive pairs of coordinates in the points buffer
color - Color of the spline
thickness - Thickness of the spline, pass <= 0 to draw hairline spline
num_segments - The number of segments

Low level drawing routines

Low level drawing routines allow for more advanced usage of the addon, allowing you to pass arbitrary sequences of vertices to draw to the screen. These routines also support using textures on the primitives with the following restrictions:

For maximum portability, you should only use textures that have dimensions that are a power of two, as not every videocard supports textures of different sizes completely. This warning is relaxed, however, if the texture coordinates never exit the boundaries of a single bitmap (i.e. you are not having the texture repeat/tile). As long as that is the case, any texture can be used safely. Sub-bitmaps work as textures, but cannot be tiled.

Some platforms also dictate a minimum texture size, which means that textures smaller than that size will not tile properly. The minimum size that will work on all platforms is 32 by 32.

A note about pixel coordinates. In OpenGL the texture coordinate (0, 0) refers to the top left corner of the pixel. This confuses some drivers, because due to rounding errors the actual pixel sampled might be the pixel to the top and/or left of the (0, 0) pixel. To make this error less likely it is advisable to offset the texture coordinates you pass to the al_draw_prim by (0.5, 0.5) if you need precise pixel control. E.g. to refer to pixel (5, 10) you’d set the u and v to 5.5 and 10.5 respectively.

al_draw_prim

int al_draw_prim(const void* vtxs, const ALLEGRO_VERTEX_DECL* decl,
   ALLEGRO_BITMAP* texture, int start, int end, int type)

Source Code

Draws a subset of the passed vertex array.

Parameters:

texture - Texture to use, pass NULL to use only color shaded primitves
vtxs - Pointer to an array of vertices
decl - Pointer to a vertex declaration. If set to NULL, the vertices are assumed to be of the ALLEGRO_VERTEX type
start - Start index of the subset of the vertex array to draw
end - One past the last index of the subset of the vertex array to draw
type - A member of the ALLEGRO_PRIM_TYPE enumeration, specifying what kind of primitive to draw

Returns: Number of primitives drawn

For example to draw a textured triangle you could use:

ALLEGRO_COLOR white = al_map_rgb_f(1, 1, 1);
ALLEGRO_VERTEX v[] = {
   {.x = 128, .y = 0, .z = 0, .color = white, .u = 128, .v = 0},
   {.x = 0, .y = 256, .z = 0, .color = white, .u = 0, .v = 256},
   {.x = 256, .y = 256, .z = 0, .color = white, .u = 256, .v = 256}};
al_draw_prim(v, NULL, texture, 0, 3, ALLEGRO_PRIM_TRIANGLE_LIST);

Examples:

al_draw_indexed_prim

int al_draw_indexed_prim(const void* vtxs, const ALLEGRO_VERTEX_DECL* decl,
   ALLEGRO_BITMAP* texture, const int* indices, int num_vtx, int type)

Source Code

Draws a subset of the passed vertex array. This function uses an index array to specify which vertices to use.

Parameters:

texture - Texture to use, pass NULL to use only color shaded primitves
vtxs - Pointer to an array of vertices
decl - Pointer to a vertex declaration. If set to NULL, the vtxs are assumed to be of the ALLEGRO_VERTEX type
indices - An array of indices into the vertex array
num_vtx - Number of indices from the indices array you want to draw
type - A member of the ALLEGRO_PRIM_TYPE enumeration, specifying what kind of primitive to draw

Returns: Number of primitives drawn

Examples:

al_draw_vertex_buffer

int al_draw_vertex_buffer(ALLEGRO_VERTEX_BUFFER* vertex_buffer,
   ALLEGRO_BITMAP* texture, int start, int end, int type)

Source Code

Draws a subset of the passed vertex buffer. The vertex buffer must not be locked. Additionally, to draw onto memory bitmaps or with memory bitmap textures the vertex buffer must support reading (i.e. it must be created with the ALLEGRO_PRIM_BUFFER_READWRITE).

Parameters:

vertex_buffer - Vertex buffer to draw
texture - Texture to use, pass NULL to use only color shaded primitves
start - Start index of the subset of the vertex buffer to draw
end - One past the last index of the subset of the vertex buffer to draw
type - A member of the ALLEGRO_PRIM_TYPE enumeration, specifying what kind of primitive to draw

Returns: Number of primitives drawn

Since: 5.1.3

Examples:

al_draw_indexed_buffer

int al_draw_indexed_buffer(ALLEGRO_VERTEX_BUFFER* vertex_buffer,
   ALLEGRO_BITMAP* texture, ALLEGRO_INDEX_BUFFER* index_buffer,
   int start, int end, int type)

Source Code

Draws a subset of the passed vertex buffer. This function uses an index buffer to specify which vertices to use. Both buffers must not be locked. Additionally, to draw onto memory bitmaps or with memory bitmap textures both buffers must support reading (i.e. they must be created with the ALLEGRO_PRIM_BUFFER_READWRITE).

Parameters:

vertex_buffer - Vertex buffer to draw
texture - Texture to use, pass NULL to use only color shaded primitves
index_buffer - Index buffer to use
start - Start index of the subset of the index buffer to draw
end - One past the last index of the subset of the index buffer to draw
type - A member of the ALLEGRO_PRIM_TYPE enumeration, specifying what kind of primitive to draw. Note that ALLEGRO_PRIM_LINE_LOOP and ALLEGRO_PRIM_POINT_LIST are not supported.

Returns: Number of primitives drawn

Since: 5.1.8

Examples:

ex_prim.c

al_draw_soft_triangle

void al_draw_soft_triangle(
   ALLEGRO_VERTEX* v1, ALLEGRO_VERTEX* v2, ALLEGRO_VERTEX* v3, uintptr_t state,
   void (*init)(uintptr_t, ALLEGRO_VERTEX*, ALLEGRO_VERTEX*, ALLEGRO_VERTEX*),
   void (*first)(uintptr_t, int, int, int, int),
   void (*step)(uintptr_t, int),
   void (*draw)(uintptr_t, int, int, int))

Source Code

Draws a triangle using the software rasterizer and user supplied pixel functions. For help in understanding what these functions do, see the implementation of the various shading routines in addons/primitives/tri_soft.c. The triangle is drawn in two segments, from top to bottom. The segments are deliniated by the vertically middle vertex of the triangle. One of the two segments may be absent if two vertices are horizontally collinear.

Parameters:

v1, v2, v3 - The three vertices of the triangle
state - A pointer to a user supplied struct, this struct will be passed to all the pixel functions
init - Called once per call before any drawing is done. The three points passed to it may be altered by clipping.
first - Called twice per call, once per triangle segment. It is passed 4 parameters, the first two are the coordinates of the initial pixel drawn in the segment. The second two are the left minor and the left major steps, respectively. They represent the sizes of two steps taken by the rasterizer as it walks on the left side of the triangle. From then on, each step will either be classified as a minor or a major step, corresponding to the above values.
step - Called once per scanline. The last parameter is set to 1 if the step is a minor step, and 0 if it is a major step.
draw - Called once per scanline. The function is expected to draw the scanline starting with a point specified by the first two parameters (corresponding to x and y values) going to the right until it reaches the value of the third parameter (the x value of the end point). All coordinates are inclusive.

al_draw_soft_line

void al_draw_soft_line(ALLEGRO_VERTEX* v1, ALLEGRO_VERTEX* v2, uintptr_t state,
   void (*first)(uintptr_t, int, int, ALLEGRO_VERTEX*, ALLEGRO_VERTEX*),
   void (*step)(uintptr_t, int),
   void (*draw)(uintptr_t, int, int))

Source Code

Draws a line using the software rasterizer and user supplied pixel functions. For help in understanding what these functions do, see the implementation of the various shading routines in addons/primitives/line_soft.c. The line is drawn top to bottom.

Parameters:

v1, v2 - The two vertices of the line
state - A pointer to a user supplied struct, this struct will be passed to all the pixel functions
first - Called before drawing the first pixel of the line. It is passed the coordinates of this pixel, as well as the two vertices above. The passed vertices may have been altered by clipping.
step - Called once per pixel. The second parameter is set to 1 if the step is a minor step, and 0 if this step is a major step. Minor steps are taken only either in x or y directions. Major steps are taken in both directions diagonally. In all cases, the absolute value of the change in coordinate is at most 1 in either direction.
draw - Called once per pixel. The function is expected to draw the pixel at the coordinates passed to it.

Custom vertex declaration routines

al_create_vertex_decl

ALLEGRO_VERTEX_DECL* al_create_vertex_decl(const ALLEGRO_VERTEX_ELEMENT* elements, int stride)

Source Code

Creates a vertex declaration, which describes a custom vertex format.

Parameters:

elements - An array of ALLEGRO_VERTEX_ELEMENT structures.
stride - Size of the custom vertex structure

Returns: Newly created vertex declaration.

Examples:

al_destroy_vertex_decl

void al_destroy_vertex_decl(ALLEGRO_VERTEX_DECL* decl)

Source Code

Destroys a vertex declaration.

Parameters:

decl - Vertex declaration to destroy

Examples:

ex_prim_shader.c

Vertex buffer routines

al_create_vertex_buffer

ALLEGRO_VERTEX_BUFFER* al_create_vertex_buffer(ALLEGRO_VERTEX_DECL* decl,
   const void* initial_data, int num_vertices, int flags)

Source Code

Creates a vertex buffer. Can return NULL if the buffer could not be created (e.g. the system only supports write-only buffers).

Note:

This is an advanced feature, often unsupported on lower-end video cards. Be extra mindful of this function failing and make arrangements for fallback drawing functionality or a nice error message for users with such lower-end cards.

Parameters:

decl - Vertex type that this buffer will hold. NULL implies that this buffer will hold ALLEGRO_VERTEX vertices
initial_data - Memory buffer to copy from to initialize the vertex buffer. Can be NULL, in which case the buffer is uninitialized.
num_vertices - Number of vertices the buffer will hold
flags - A combination of the ALLEGRO_PRIM_BUFFER_FLAGS flags specifying how this buffer will be created. Passing 0 is the same as passing ALLEGRO_PRIM_BUFFER_STATIC.

Since: 5.1.3

Examples:

al_destroy_vertex_buffer

void al_destroy_vertex_buffer(ALLEGRO_VERTEX_BUFFER* buffer)

Source Code

Destroys a vertex buffer. Does nothing if passed NULL.

Since: 5.1.3

Examples:

al_lock_vertex_buffer

void* al_lock_vertex_buffer(ALLEGRO_VERTEX_BUFFER* buffer, int offset,
   int length, int flags)

Source Code

Locks a vertex buffer so you can access its data. Will return NULL if the parameters are invalid, if reading is requested from a write only buffer, or if the buffer is already locked.

Parameters:

buffer - Vertex buffer to lock
offset - Vertex index of the start of the locked range
length - How many vertices to lock
flags - ALLEGRO_LOCK_READONLY, ALLEGRO_LOCK_WRITEONLY or ALLEGRO_LOCK_READWRITE

Since: 5.1.3

Examples:

al_unlock_vertex_buffer

void al_unlock_vertex_buffer(ALLEGRO_VERTEX_BUFFER* buffer)

Source Code

Unlocks a previously locked vertex buffer.

Since: 5.1.3

Examples:

al_get_vertex_buffer_size

int al_get_vertex_buffer_size(ALLEGRO_VERTEX_BUFFER* buffer)

Source Code

Returns the size of the vertex buffer

Since: 5.1.8

Index buffer routines

al_create_index_buffer

ALLEGRO_INDEX_BUFFER* al_create_index_buffer(int index_size,
    const void* initial_data, int num_indices, int flags)

Source Code

Creates a index buffer. Can return NULL if the buffer could not be created (e.g. the system only supports write-only buffers).

Note:

This is an advanced feature, often unsupported on lower-end video cards. Be extra mindful of this function failing and make arrangements for fallback drawing functionality or a nice error message for users with such lower-end cards.

Parameters:

index_size - Size of the index in bytes. Supported sizes are 2 for short integers and 4 for integers
initial_data - Memory buffer to copy from to initialize the index buffer. Can be NULL, in which case the buffer is uninitialized.
num_indices - Number of indices the buffer will hold
flags - A combination of the ALLEGRO_PRIM_BUFFER_FLAGS flags specifying how this buffer will be created. Passing 0 is the same as passing ALLEGRO_PRIM_BUFFER_STATIC.

Since: 5.1.8

Examples:

ex_prim.c

al_destroy_index_buffer

void al_destroy_index_buffer(ALLEGRO_INDEX_BUFFER* buffer)

Source Code

Destroys a index buffer. Does nothing if passed NULL.

Since: 5.1.8

Examples:

ex_prim.c

al_lock_index_buffer

void* al_lock_index_buffer(ALLEGRO_INDEX_BUFFER* buffer, int offset,
    int length, int flags)

Source Code

Locks a index buffer so you can access its data. Will return NULL if the parameters are invalid, if reading is requested from a write only buffer and if the buffer is already locked.

Parameters:

buffer - Index buffer to lock
offset - Element index of the start of the locked range
length - How many indices to lock
flags - ALLEGRO_LOCK_READONLY, ALLEGRO_LOCK_WRITEONLY or ALLEGRO_LOCK_READWRITE

Since: 5.1.8

Examples:

ex_prim.c

al_unlock_index_buffer

void al_unlock_index_buffer(ALLEGRO_INDEX_BUFFER* buffer)

Source Code

Unlocks a previously locked index buffer.

Since: 5.1.8

Examples:

ex_prim.c

al_get_index_buffer_size

int al_get_index_buffer_size(ALLEGRO_INDEX_BUFFER* buffer)

Source Code

Returns the size of the index buffer

Since: 5.1.8

Polygon routines

al_draw_polyline

void al_draw_polyline(const float* vertices, int vertex_stride,
   int vertex_count, int join_style, int cap_style,
   ALLEGRO_COLOR color, float thickness, float miter_limit)

Source Code

Draw a series of line segments.

vertices - Interleaved array of (x, y) vertex coordinates
vertex_stride - the number of bytes between pairs of vertices (the stride)
vertex_count - Number of vertices in the array
join_style - Member of ALLEGRO_LINE_JOIN specifying how to render the joins between line segments
cap_style - Member of ALLEGRO_LINE_CAP specifying how to render the end caps
color - Color of the line
thickness - Thickness of the line, pass <= 0 to draw hairline lines
miter_limit - Parameter for miter join style

The stride is normally 2 * sizeof(float) but may be more if the vertex coordinates are in an array of some structure type, e.g.

struct VertexInfo {
   float x;
   float y;
   int id;
};

void my_draw(struct VertexInfo verts[], int vertex_count, ALLEGRO_COLOR c)
{
   al_draw_polyline((float *)verts, sizeof(VertexInfo), vertex_count,
      ALLEGRO_LINE_JOIN_NONE, ALLEGRO_LINE_CAP_NONE, c, 1.0, 1.0);
}

The stride may also be negative if the vertices are stored in reverse order.

Since: 5.1.0

Examples:

ex_polygon.c

al_draw_polygon

void al_draw_polygon(const float *vertices, int vertex_count,
   int join_style, ALLEGRO_COLOR color, float thickness, float miter_limit)

Source Code

Draw an unfilled polygon. This is the same as passing ALLEGRO_LINE_CAP_CLOSED to al_draw_polyline.

vertex - Interleaved array of (x, y) vertex coordinates
vertex_count - Number of vertices in the array
join_style - Member of ALLEGRO_LINE_JOIN specifying how to render the joins between line segments
color - Color of the line
thickness - Thickness of the line, pass <= 0 to draw hairline lines
miter_limit - Parameter for miter join style

Since: 5.1.0

Examples:

ex_polygon.c

al_draw_filled_polygon

void al_draw_filled_polygon(const float *vertices, int vertex_count,
   ALLEGRO_COLOR color)

Source Code

Draw a filled, simple polygon. Simple means it does not have to be convex but must not be self-overlapping.

vertices - Interleaved array of (x, y) vertex coordinates
vertex_count - Number of vertices in the array
color - Color of the filled polygon

When the y-axis is facing downwards (the usual), the coordinates must be ordered anti-clockwise.

Since: 5.1.0

Examples:

ex_polygon.c

al_draw_filled_polygon_with_holes

void al_draw_filled_polygon_with_holes(const float *vertices,
   const int *vertex_counts, ALLEGRO_COLOR color)

Source Code

Draws a filled simple polygon with zero or more other simple polygons subtracted from it - the holes. The holes cannot touch or intersect with the outline of the filled polygon.

vertices - Interleaved array of (x, y) vertex coordinates for each of the polygons, including holes.
vertex_counts - Number of vertices for each polygon. The number of vertices in the filled polygon is given by vertex_counts[0] and must be at least three. Subsequent elements indicate the number of vertices in each hole. The array must be terminated with an element with value zero.
color - Color of the filled polygon

When the y-axis is facing downwards (the usual) the filled polygon coordinates must be ordered anti-clockwise. All hole vertices must use the opposite order (clockwise with y down). All hole vertices must be inside the main polygon and no hole may overlap the main polygon.

For example:

float vertices[] = {
      0,   0, // filled polygon, upper left corner
      0, 100, // filled polygon, lower left corner
    100, 100, // filled polygon, lower right corner
    100,   0, // filled polygon, upper right corner
     10,  10, // hole, upper left
     90,  10, // hole, upper right
     90,  90  // hole, lower right
};
int vertex_counts[] = {
   4, // number of vertices for filled polygon
   3, // number of vertices for hole
   0  // terminator
};

There are 7 vertices: four for an outer square from (0, 0) to (100, 100) in anti-clockwise order, and three more for an inner triangle in clockwise order. The outer main polygon uses vertices 0 to 3 (inclusive) and the hole uses vertices 4 to 6 (inclusive).

Since: 5.1.0

Examples:

ex_polygon.c

al_triangulate_polygon

bool al_triangulate_polygon(
   const float* vertices, size_t vertex_stride, const int* vertex_counts,
   void (*emit_triangle)(int, int, int, void*), void* userdata)

Source Code

Divides a simple polygon into triangles, with zero or more other simple polygons subtracted from it - the holes. The holes cannot touch or intersect with the outline of the main polygon. Simple means the polygon does not have to be convex but must not be self-overlapping.

Parameters:

vertices - Interleaved array of (x, y) vertex coordinates for each of the polygons, including holes.
vertex_stride - distance (in bytes) between successive pairs of vertices in the array.
vertex_counts - Number of vertices for each polygon. The number of vertices in the main polygon is given by vertex_counts[0] and must be at least three. Subsequent elements indicate the number of vertices in each hole. The array must be terminated with an element with value zero.
emit_triangle - a function to be called for every set of three points that form a triangle. The function is passed the indices of the points in vertices and userdata.
userdata - arbitrary data to be passed to emit_triangle.

Since: 5.1.0

Structures and types

ALLEGRO_VERTEX

typedef struct ALLEGRO_VERTEX ALLEGRO_VERTEX;

Source Code

Defines the generic vertex type, with a 3D position, color and texture coordinates for a single texture. Note that at this time, the software driver for this addon cannot render 3D primitives. If you want a 2D only primitive, set z to 0. Note that you must initialize all members of this struct when you’re using it. One exception to this rule are the u and v variables which can be left uninitialized when you are not using textures.

Fields:

x, y, z - Position of the vertex (float)
u, v - Texture coordinates measured in pixels (float)
color - ALLEGRO_COLOR structure, storing the color of the vertex

ALLEGRO_VERTEX_DECL

typedef struct ALLEGRO_VERTEX_DECL ALLEGRO_VERTEX_DECL;

Source Code

A vertex declaration. This opaque structure is responsible for describing the format and layout of a user defined custom vertex. It is created and destroyed by specialized functions.

Examples:

ALLEGRO_VERTEX_ELEMENT

typedef struct ALLEGRO_VERTEX_ELEMENT ALLEGRO_VERTEX_ELEMENT;

Source Code

A small structure describing a certain element of a vertex. E.g. the position of the vertex, or its color. These structures are used by the al_create_vertex_decl function to create the vertex declaration. For that they generally occur in an array. The last element of such an array should have the attribute field equal to 0, to signify that it is the end of the array. Here is an example code that would create a declaration describing the ALLEGRO_VERTEX structure (passing this as vertex declaration to al_draw_prim would be identical to passing NULL):

/* On compilers without the offsetof keyword you need to obtain the
 * offset with sizeof and make sure to account for packing.
 */
ALLEGRO_VERTEX_ELEMENT elems[] = {
   {ALLEGRO_PRIM_POSITION, ALLEGRO_PRIM_FLOAT_3, offsetof(ALLEGRO_VERTEX, x)},
   {ALLEGRO_PRIM_TEX_COORD_PIXEL, ALLEGRO_PRIM_FLOAT_2, offsetof(ALLEGRO_VERTEX, u)},
   {ALLEGRO_PRIM_COLOR_ATTR, 0, offsetof(ALLEGRO_VERTEX, color)},
   {0, 0, 0}
};
ALLEGRO_VERTEX_DECL* decl = al_create_vertex_decl(elems, sizeof(ALLEGRO_VERTEX));

Fields:

attribute - A member of the ALLEGRO_PRIM_ATTR enumeration, specifying what this attribute signifies
storage - A member of the ALLEGRO_PRIM_STORAGE enumeration, specifying how this attribute is stored
offset - Offset in bytes from the beginning of the custom vertex structure. The C function offsetof is very useful here.

Examples:

ALLEGRO_PRIM_TYPE

typedef enum ALLEGRO_PRIM_TYPE

Source Code

Enumerates the types of primitives this addon can draw.

ALLEGRO_PRIM_POINT_LIST - A list of points, each vertex defines a point
ALLEGRO_PRIM_LINE_LIST - A list of lines, sequential pairs of vertices define disjointed lines
ALLEGRO_PRIM_LINE_STRIP - A strip of lines, sequential vertices define a strip of lines
ALLEGRO_PRIM_LINE_LOOP - Like a line strip, except at the end the first and the last vertices are also connected by a line
ALLEGRO_PRIM_TRIANGLE_LIST - A list of triangles, sequential triplets of vertices define disjointed triangles
ALLEGRO_PRIM_TRIANGLE_STRIP - A strip of triangles, sequential vertices define a strip of triangles
ALLEGRO_PRIM_TRIANGLE_FAN - A fan of triangles, all triangles share the first vertex

ALLEGRO_PRIM_ATTR

typedef enum ALLEGRO_PRIM_ATTR

Source Code

Enumerates the types of vertex attributes that a custom vertex may have.

ALLEGRO_PRIM_POSITION - Position information, can be stored only in ALLEGRO_PRIM_SHORT_2, ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_FLOAT_3.
ALLEGRO_PRIM_COLOR_ATTR - Color information, stored in an ALLEGRO_COLOR. The storage field of ALLEGRO_VERTEX_ELEMENT is ignored
ALLEGRO_PRIM_TEX_COORD - Texture coordinate information, can be stored only in ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_SHORT_2. These coordinates are normalized by the width and height of the texture, meaning that the bottom-right corner has texture coordinates of (1, 1).
ALLEGRO_PRIM_TEX_COORD_PIXEL - Texture coordinate information, can be stored only in ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_SHORT_2. These coordinates are measured in pixels.
ALLEGRO_PRIM_USER_ATTR - A user specified attribute. You can use any storage for this attribute. You may have at most ALLEGRO_PRIM_MAX_USER_ATTR (currently 10) of these that you can specify by adding an index to the value of ALLEGRO_PRIM_USER_ATTR, e.g. the first user attribute is ALLEGRO_PRIM_USER_ATTR + 0, the second is ALLEGRO_PRIM_USER_ATTR + 1 and so on.

To access these custom attributes from GLSL shaders you need to declare attributes that follow this nomenclature: al_user_attr_# where # is the index of the attribute.

For example to have a position and a normal vector for each vertex you could declare it like this:
```
ALLEGRO_VERTEX_ELEMENT elements[3] = {
    {ALLEGRO_PRIM_POSITION, ALLEGRO_PRIM_FLOAT_3, 0},
    {ALLEGRO_PRIM_USER_ATTR + 0, ALLEGRO_PRIM_FLOAT_3, 12},
    {0, 0, 0}};
```
And then in your vertex shader access it like this:
```
attribute vec3 al_pos; // ALLEGRO_PRIM_POSITION
attribute vec3 al_user_attr_0; // ALLEGRO_PRIM_USER_ATTR + 0
varying float light;
const vec3 light_direction = vec3(0, 0, 1);
void main() {
    light = dot(al_user_attr_0, light_direction);
    gl_Position = al_pos;
}
```
To access these custom attributes from HLSL you need to declare a parameter with the following semantics: TEXCOORD{# + 2} where # is the index of the attribute. E.g. the first attribute can be accessed via TEXCOORD2, second via TEXCOORD3 and so on.

Since: 5.1.6

ALLEGRO_PRIM_STORAGE

typedef enum ALLEGRO_PRIM_STORAGE

Source Code

Enumerates the types of storage an attribute of a custom vertex may be stored in. Many of these can only be used for ALLEGRO_PRIM_USER_ATTR attributes and can only be accessed via shaders. Usually no matter what the storage is specified the attribute gets converted to single precision floating point when the shader is run. Despite that, it may be advantageous to use more dense storage formats (e.g. ALLEGRO_PRIM_NORMALIZED_UBYTE_4 instead of ALLEGRO_PRIM_FLOAT_4) when bandwidth (amount of memory sent to the GPU) is an issue but precision is not.

ALLEGRO_PRIM_FLOAT_1 - A single float

Since: 5.1.6
ALLEGRO_PRIM_FLOAT_2 - A doublet of floats
ALLEGRO_PRIM_FLOAT_3 - A triplet of floats
ALLEGRO_PRIM_FLOAT_4 - A quad of floats

Since: 5.1.6
ALLEGRO_PRIM_SHORT_2 - A doublet of shorts
ALLEGRO_PRIM_SHORT_4 - A quad of shorts

Since: 5.1.6
ALLEGRO_PRIM_UBYTE_4 - A quad of unsigned bytes

Since: 5.1.6
ALLEGRO_PRIM_NORMALIZED_SHORT_2 - A doublet of shorts. Before being sent to the shader, each component is divided by 32767. Each component of the resultant float doublet ranges between -1.0 and 1.0

Since: 5.1.6
ALLEGRO_PRIM_NORMALIZED_SHORT_4 - A quad of shorts. Before being sent to the shader, each component is divided by 32767. Each component of the resultant float quad ranges between -1.0 and 1.0

Since: 5.1.6
ALLEGRO_PRIM_NORMALIZED_UBYTE_4 - A quad of unsigned bytes. Before being sent to the shader, each component is divided by 255. Each component of the resultant float quad ranges between 0.0 and 1.0

Since: 5.1.6
ALLEGRO_PRIM_NORMALIZED_USHORT_2 - A doublet of unsigned shorts. Before being sent to the shader, each component is divided by 65535. Each component of the resultant float doublet ranges between 0.0 and 1.0

Since: 5.1.6
ALLEGRO_PRIM_NORMALIZED_USHORT_4 - A quad of unsigned shorts. Before being sent to the shader, each component is divided by 65535. Each component of the resultant float quad ranges between 0.0 and 1.0

Since: 5.1.6
ALLEGRO_PRIM_HALF_FLOAT_2 - A doublet of half-precision floats. Note that this storage format is not supported on all platforms. al_create_vertex_decl will return NULL if you use it on those platforms

Since: 5.1.6
ALLEGRO_PRIM_HALF_FLOAT_4 - A quad of half-precision floats. Note that this storage format is not supported on all platforms. al_create_vertex_decl will return NULL if you use it on those platforms.

Since: 5.1.6

ALLEGRO_VERTEX_CACHE_SIZE

#define ALLEGRO_VERTEX_CACHE_SIZE 256

Source Code

Defines the size of the transformation vertex cache for the software renderer. If you pass less than this many vertices to the primitive rendering functions you will get a speed boost. This also defines the size of the cache vertex buffer, used for the high-level primitives. This corresponds to the maximum number of line segments that will be used to form them.

ALLEGRO_PRIM_QUALITY

#define ALLEGRO_PRIM_QUALITY 10

Source Code

Controls the quality of the approximation of curved primitives (e.g. circles). Curved primitives are drawn by approximating them with a sequence of line segments. By default, this roughly corresponds to error of less than half of a pixel.

ALLEGRO_LINE_JOIN

typedef enum ALLEGRO_LINE_JOIN

Source Code

ALLEGRO_LINE_JOIN_NONE
ALLEGRO_LINE_JOIN_BEVEL
ALLEGRO_LINE_JOIN_ROUND
ALLEGRO_LINE_JOIN_MITER

See the picture for the difference.

The maximum miter length (relative to the line width) can be specified as parameter to the polygon functions.

Since: 5.1.0

ALLEGRO_LINE_CAP

typedef enum ALLEGRO_LINE_CAP

Source Code

ALLEGRO_LINE_CAP_NONE
ALLEGRO_LINE_CAP_SQUARE
ALLEGRO_LINE_CAP_ROUND
ALLEGRO_LINE_CAP_TRIANGLE
ALLEGRO_LINE_CAP_CLOSED

See the picture for the difference.

ALLEGRO_LINE_CAP_CLOSED is different from the others - it causes the polygon to have no caps. (And the ALLEGRO_LINE_JOIN style will determine how the vertex looks.)

Since: 5.1.0

ALLEGRO_VERTEX_BUFFER

typedef struct ALLEGRO_VERTEX_BUFFER ALLEGRO_VERTEX_BUFFER;

Source Code

A GPU vertex buffer that you can use to store vertices on the GPU instead of uploading them afresh during every drawing operation.

Since: 5.1.3

Examples:

ALLEGRO_INDEX_BUFFER

typedef struct ALLEGRO_INDEX_BUFFER ALLEGRO_INDEX_BUFFER;

Source Code

A GPU index buffer that you can use to store indices of vertices in a vertex buffer on the GPU instead of uploading them afresh during every drawing operation.

Since: 5.1.8

Examples:

ex_prim.c

ALLEGRO_PRIM_BUFFER_FLAGS

typedef enum ALLEGRO_PRIM_BUFFER_FLAGS

Source Code

Flags to specify how to create a vertex or an index buffer.

ALLEGRO_PRIM_BUFFER_STREAM - Hints to the driver that the buffer is written to often, but used only a few times per frame
ALLEGRO_PRIM_BUFFER_STATIC - Hints to the driver that the buffer is written to once and is used often
ALLEGRO_PRIM_BUFFER_DYNAMIC - Hints to the driver that the buffer is written to often and is used often
ALLEGRO_PRIM_BUFFER_READWRITE - Specifies that you want to be able read from this buffer. By default this is disabled for performance. Some platforms (like OpenGL ES) do not support reading from vertex buffers, so if you pass this flag to al_create_vertex_buffer or al_create_index_buffer the call will fail.

Since: 5.1.3

Allegro version 5.2.10 (20231119) - Last updated: 2024-11-21 07:18:40 UTC