Sunday, December 14, 2008

iP* programming tip #7

This time I will cover Point Sprites in the iPhone / iPod touch programming tip. The idea is that a set of points -as the simplest primitive in OpenGL ES rendering- describes the positions of Point Sprites, and their appearance comes from the current texture map. This way, Point Sprites are screen-aligned sprites that offer a reduced geometry footprint and transform cost because they are represented by one point == vertex. This is useful for particle systems, lens flare, light glow and other 2-D effects.
  • glEnable(GL_POINT_SPRITES_OES) - this is the global switch that turns point sprites on. Once enabled, all points will be drawn as point sprites.
  • glTexEnvi(GL_POINT_SPRITES_OES, GL_COORD_REPLACE_OES, GL_TRUE) - this enables [0..1] texture coordinate generation for the four corners of the point sprite. It can be set per-texture unit. If disabled, all corners of the quad have the same texture coordinate.
  • glPointParametervf(GLenum pname, const GLfloat * params) - this is used to set the point attenuation as described below.
The point size of a point sprite can be derived with the formula:
user_clamp represents GL_POINT_SIZE_MIN and GL_POINT_SIZE_MIN settings of the glPointParametervf(). impl_clamp represents an implementation-dependent point size range.
GL_POINT_DISTANCE_ATTENUATION is used to pass in params as an array containing the distance attenuation coefficients a, b, and c, in that order.
In case multisampling is used (not officially supported), the point size is clamped to have a minimum threshold, and the alpha value of the point is modulated by the following equation:
GL_POINT_FADE_THRESHOLD_SIZE specifies the point alpha fade threshold.
Check out the Oolong engine example Particle System for an implementation. It uses 600 point sprites with nearly 60 fps. Increasing the number of point sprites to 3000 lets the framerate drop to around 20 fps.

4 comments:

blackpawn said...

thanks for the iP* tips keep em coming! :)

Biosopher said...

You have a slight typo here. The point sprite extension should be GL_OES_point_sprite.

Cheers,
Biosopher

Claire Blackshaw said...

I found your article really helpful today.

Just to add the iPhone has glPointSizePointerOES for people who are working in Ortho mode.

Anonymous said...

Is there any way to have the point sprite use just a part of a larger texture? Like from a sprite sheet/atlas texture?