Textured points in OpenGL ES 2.0?

Question

I'm trying to implement textured points (e.g. point sprites) in OpenGL ES 2.0 for a particle system. Problem I'm having is the points all render as solid black squares, rather than having the texture properly mapped.

I have verified that gl_PointCoord is in fact returning x/y values from 0.0 to 1.0, which would map across the entire texture. The texture2D call always seems to return black though.

My vertex shader :

attribute vec4 aPosition;
attribute float aAlpha;
attribute float aSize;
varying float vAlpha;
uniform mat4 uMVPMatrix;

void main() {
  gl_PointSize = aSize;
  vAlpha = aAlpha;
  gl_Position = uMVPMatrix * aPosition;
}

And my fragment shader :

precision mediump float;
uniform sampler2D tex;
varying float vAlpha;

void main () {
    vec4 texColor = texture2D(tex, gl_PointCoord);
    gl_FragColor = vec4(texColor.rgb, texColor.a * vAlpha);
}

The texture in question is 16x16. I am able to successfully map this texture to other geometry, but for some reason not to points.

My platform is a Motorola Droid, running Android 2.2.

Answer 1

You select your texture as active and bind it - like normally. But you must pass your texture unit as uniform to the shader program. So you can use the gl_PointCoord as texture coordinates in your fragment shader.

glUniform1i(MY_TEXTURE_UNIFORM_LOCATION, 0);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, yourTextureId);
glEnable(GL_TEXTURE_2D);
glDrawArrays(GL_POINTS, 0, pointCount);
glDisable(GL_TEXTURE_2D);

And your fragment shader should look like:

uniform sampler2D texture;
void main ( )
{
    gl_FragColor = texture2D(texture, gl_PointCoord);
}