Applying part of a texture (sprite sheet / texture map) to a point sprite in iOS OpenGL ES 2.0

Question

It seems this should be easy but I'm having a lot of difficulty using part of a texture with a point sprite. I have googled around extensively and turned up various answers but none of these deal with the specific issue I'm having.

What I've learned so far:

1. Basics of point sprite drawing
2. How to deal with point sprites rendering as solid squares
3. How to alter orientation of a point sprite
4. How to use multiple textures with a point sprite, getting closer here..
5. That point sprites + sprite sheets has been done before, but is only possible in OpenGL ES 2.0 (not 1.0)

Here is a diagram of what I'm trying to achieve

Where I'm at:

• I have a set of working point sprites all using the same single square image. Eg: a 16x16 image of a circle works great.
• I have an Objective-C method which generates a 600x600 image containing a sprite-sheet with multiple images. I have verified this is working by applying the entire sprite sheet image to a quad drawn with GL_TRIANGLES.
• I have used the above method successfully to draw parts of a sprite sheet on to quads. I just cant get it to work with point sprites.
• Currently I'm generating texture coordinates pointing to the center of the sprite on the sprite sheet I'm targeting. Eg: Using the image at the bottom; star: 0.166,0.5; cloud: 0.5,0.5; heart: 0.833,0.5.

Code:

Vertex Shader

uniform mat4 Projection;
uniform mat4 Modelview;
uniform float PointSize;

attribute vec4 Position;
attribute vec2 TextureCoordIn;

varying vec2 TextureCoord;

void main(void)
{
    gl_Position = Projection * Modelview * Position;
    TextureCoord = TextureCoordIn;
    gl_PointSize = PointSize;
}

Fragment Shader

varying mediump vec2 TextureCoord;
uniform sampler2D Sampler;

void main(void)
{
    // Using my TextureCoord just draws a grey square, so
    // I'm likely generating texture coords that texture2D doesn't like.
    gl_FragColor = texture2D(Sampler, TextureCoord);

    // Using gl_PointCoord just draws my whole sprite map
    // gl_FragColor = texture2D(Sampler, gl_PointCoord);
}

What I'm stuck on:

1. I don't understand how to use the gl_PointCoord variable in the fragment shader. What does gl_PointCoord contain initially? Why? Where does it get its data?
2. I don't understand what texture coordinates to pass in. For example, how does the point sprite choose what part of my sprite sheet to use based on the texture coordinates? I'm used to drawing quads which have effectively 4 sets of texture coordinates (one for each vertex), how is this different (clearly it is)?

Answer 1

A colleague of mine helped with the answer. It turns out the trick is to utilize both the size of the point (in OpenGL units) and the size of the sprite (in texture units, (0..1)) in combination with a little vector math to render only part of the sprite-sheet onto each point.

Vertex Shader

uniform mat4 Projection;
uniform mat4 Modelview;
// The radius of the point in OpenGL units, eg: "20.0"
uniform float PointSize;
// The size of the sprite being rendered. My sprites are square
// so I'm just passing in a float.  For non-square sprites pass in
// the width and height as a vec2.
uniform float TextureCoordPointSize;

attribute vec4 Position;
attribute vec4 ObjectCenter;
// The top left corner of a given sprite in the sprite-sheet
attribute vec2 TextureCoordIn;

varying vec2 TextureCoord;
varying vec2 TextureSize;

void main(void)
{
    gl_Position = Projection * Modelview * Position;
    TextureCoord = TextureCoordIn;
    TextureSize = vec2(TextureCoordPointSize, TextureCoordPointSize);

    // This is optional, it is a quick and dirty way to make the points stay the same
    // size on the screen regardless of distance.
    gl_PointSize = PointSize / Position.w;
}

Fragment Shader

varying mediump vec2 TextureCoord;
varying mediump vec2 TextureSize;
uniform sampler2D Sampler;

void main(void)
{
    // This is where the magic happens.  Combine all three factors to render
    // just a portion of the sprite-sheet for this point
    mediump vec2 realTexCoord = TextureCoord + (gl_PointCoord * TextureSize);
    mediump vec4 fragColor = texture2D(Sampler, realTexCoord);

    // Optional, emulate GL_ALPHA_TEST to use transparent images with
    // point sprites without worrying about z-order.
    // see: http://stackoverflow.com/a/5985195/806988
    if(fragColor.a == 0.0){
        discard;
    }

    gl_FragColor = fragColor;
}