A 3 dimensional array represented by a 2 dimensional array-

Question

So in WebGL, I can store a texture in up to 2 dimensions- and read it in the shader using texture2D(whatever);

If i wanted to store a 3 dimensional texture so that I

Answer 1

You can simulate a 3d texture by storing each plane of the 3d texture in a 2d texture

Then a function like this will let you use it as a 3d texture

vec4 sampleAs3DTexture(sampler2D tex, vec3 texCoord, float size) {
    float sliceSize = 1.0 / size;                         // space of 1 slice
    float slicePixelSize = sliceSize / size;              // space of 1 pixel
    float sliceInnerSize = slicePixelSize * (size - 1.0); // space of size pixels
    float zSlice0 = min(floor(texCoord.z * size), size - 1.0);
    float zSlice1 = min(zSlice0 + 1.0, size - 1.0);
    float xOffset = slicePixelSize * 0.5 + texCoord.x * sliceInnerSize;
    float s0 = xOffset + (zSlice0 * sliceSize);
    float s1 = xOffset + (zSlice1 * sliceSize);
    vec4 slice0Color = texture2D(tex, vec2(s0, texCoord.y));
    vec4 slice1Color = texture2D(tex, vec2(s1, texCoord.y));
    float zOffset = mod(texCoord.z * size, 1.0);
    return mix(slice0Color, slice1Color, zOffset);
 }

If your 3d texture was 8x8x8 then you'd make a 2d texture that is 64x8 and put each plane of the 3d texture in your 2d texture. Then, knowing that was originally 8x8x8 you'd pass in 8.0 for the size to sampleAs3DTexture

precision mediump float;
uniform sampler2D u_my3DTexture;
varying vec3 v_texCoord;

...

#define CUBE_SIZE 8.0

void main() {
  gl_FragColor = sampleAs3DTexture(u_my3DTexture, v_texCoord, CUBE_SIZE);
}

Note: the function above assumes you want bilinear filtering between the planes. If you don't you can simplify the function.

There's a video explanation of this code here which is from this sample.