Question:

I'm creating a 2D rendering engine with WebGL in which I'm using Texture Atlases and batching hundreds of entities at once. I need to set a unique alpha value for each entity, and I'm at a loss of how to do this.

My current fragment shader is this:

    precision mediump float;

    uniform float u_alpha;
    uniform sampler2D u_image;

    varying vec2 v_texCoord;

    void main () {
        vec4 texture = texture2D(u_image, v_texCoord);
        gl_FragColor = vec4(texture.rgb, texture.a * u_alpha);
    }

I'd like to change that global uniform into a uniform buffer or an array of unique values that will be applied to the v_texCoord, but I don't know how to do this.

Answer:

You need to pass those values in as an attribute just like v_texCoord.

in vertex shader

attribute float a_alpha;
varying float v_alpha;
...
void main() {
   ...
   v_alpha = a_alpha;  // pass the alpha values to the fragment shader.
   ...
}

in fragment shader

varying float v_alpha;
...
gl_FragColor = vec4(texture.rgb, texture.a * v_alpha);

Instead of using a separate attribute you could also just make your texture coordinates have 3 values. u, v, and alpha. In other words, change v_texCoord to a vec3. Update the attribute in the vertex shader to take a vec3. Update your UV data so each UV also has an alpha. Change your fragment shader to.

varying vec3 v_texCoord;

void main () {
    vec4 texture = texture2D(u_image, v_texCoord.xy);
    gl_FragColor = vec4(texture.rgb, texture.a * v_texCoord.z);
}