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)

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)

Initializes the primitives addon.

Returns: True on success, false on failure.

See also: al_shutdown_primitives_addon

al_shutdown_primitives_addon

void al_shutdown_primitives_addon(void)

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

See also: al_init_primitives_addon

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.

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.

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)

Draws a line segment between two points.

Parameters:

See also: al_draw_soft_line

al_draw_triangle

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

Draws an outlined triangle.

Parameters:

See also: al_draw_filled_triangle, al_draw_soft_triangle

al_draw_filled_triangle

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

Draws a filled triangle.

Parameters:

See also: al_draw_triangle

al_draw_rectangle

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

Draws an outlined rectangle.

Parameters:

See also: al_draw_filled_rectangle, al_draw_rounded_rectangle

al_draw_filled_rectangle

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

Draws a filled rectangle.

Parameters:

See also: al_draw_rectangle, al_draw_filled_rounded_rectangle

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)

Draws an outlined rounded rectangle.

Parameters:

See also: al_draw_filled_rounded_rectangle, al_draw_rectangle

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)

Draws an filled rounded rectangle.

Parameters:

See also: al_draw_rounded_rectangle, al_draw_filled_rectangle

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_segments)

Calculates an elliptical arc, and sets the vertices in the destination buffer to the calculated positions. If thickness <= 0, then num_points 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:

See also: al_draw_arc, al_calculate_spline, al_calculate_ribbon

al_draw_pieslice

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

Draws a pieslice (outlined circular sector).

Parameters:

Since: 5.0.6, 5.1.0

See also: al_draw_filled_pieslice

al_draw_filled_pieslice

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

Draws a filled pieslice (filled circular sector).

Parameters:

Since: 5.0.6, 5.1.0

See also: al_draw_pieslice

al_draw_ellipse

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

Draws an outlined ellipse.

Parameters:

See also: al_draw_filled_ellipse, al_draw_circle

al_draw_filled_ellipse

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

Draws a filled ellipse.

Parameters:

See also: al_draw_ellipse, al_draw_filled_circle

al_draw_circle

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

Draws an outlined circle.

Parameters:

See also: al_draw_filled_circle, al_draw_ellipse

al_draw_filled_circle

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

Draws a filled circle.

Parameters:

See also: al_draw_circle, al_draw_filled_ellipse

al_draw_arc

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

Draws an arc.

Parameters:

See also: al_calculate_arc, al_draw_elliptical_arc

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)

Draws an elliptical arc.

Parameters:

Since: 5.0.6, 5.1.0

See also: al_calculate_arc, al_draw_arc

al_calculate_spline

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

Calculates a B├ęzier 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:

See also: al_draw_spline, al_calculate_arc, al_calculate_ribbon

al_draw_spline

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

Draws a B├ęzier spline given 4 control points.

Parameters:

See also: al_calculate_spline

al_calculate_ribbon

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

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:

See also: al_draw_ribbon, al_calculate_arc, al_calculate_spline

al_draw_ribbon

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

Draws a series of straight lines given an array of points. The ribbon will go through all of the passed points.

Parameters:

See also: al_calculate_ribbon

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 them 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.

See also: [Pixel-precise output]

al_draw_prim

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

Draws a subset of the passed vertex buffer.

Parameters:

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);

See also: ALLEGRO_VERTEX, ALLEGRO_PRIM_TYPE, ALLEGRO_VERTEX_DECL, al_draw_indexed_prim

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)

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

Parameters:

Returns: Number of primitives drawn

See also: ALLEGRO_VERTEX, ALLEGRO_PRIM_TYPE, ALLEGRO_VERTEX_DECL, al_draw_prim

al_create_vertex_decl

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

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

Parameters:

Returns: Newly created vertex declaration.

See also: ALLEGRO_VERTEX_ELEMENT, ALLEGRO_VERTEX_DECL, al_destroy_vertex_decl

al_destroy_vertex_decl

void al_destroy_vertex_decl(ALLEGRO_VERTEX_DECL* decl)

Destroys a vertex declaration.

Parameters:

See also: ALLEGRO_VERTEX_ELEMENT, ALLEGRO_VERTEX_DECL, al_create_vertex_decl

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))

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 each segment may be absent if two vertices are horizontally collinear.

Parameters:

See also: al_draw_triangle

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))

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:

Structures and types

ALLEGRO_VERTEX

typedef struct ALLEGRO_VERTEX ALLEGRO_VERTEX;

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 when 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:

See also: ALLEGRO_PRIM_ATTR

ALLEGRO_VERTEX_DECL

typedef struct ALLEGRO_VERTEX_DECL ALLEGRO_VERTEX_DECL;

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.

See also: al_create_vertex_decl, al_destroy_vertex_decl, ALLEGRO_VERTEX_ELEMENT

ALLEGRO_VERTEX_ELEMENT

typedef struct ALLEGRO_VERTEX_ELEMENT ALLEGRO_VERTEX_ELEMENT;

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:

See also: al_create_vertex_decl, ALLEGRO_VERTEX_DECL, ALLEGRO_PRIM_ATTR, ALLEGRO_PRIM_STORAGE

ALLEGRO_PRIM_TYPE

typedef enum ALLEGRO_PRIM_TYPE

Enumerates the types of primitives this addon can draw.

ALLEGRO_PRIM_ATTR

typedef enum ALLEGRO_PRIM_ATTR

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

See also: ALLEGRO_VERTEX_DECL, ALLEGRO_PRIM_STORAGE

ALLEGRO_PRIM_STORAGE

typedef enum ALLEGRO_PRIM_STORAGE

Enumerates the types of storage an attribute of a custom vertex may be stored in.

See also: ALLEGRO_PRIM_ATTR

ALLEGRO_VERTEX_CACHE_SIZE

#define ALLEGRO_VERTEX_CACHE_SIZE 256

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

Defines the quality of the quadratic primitives. At 10, this roughly corresponds to error of less than half of a pixel.

Allegro version 5.0.8 - Last updated: 2012-11-18 06:28:55 UTC