Using textures in a shader

Texture coordinates are exposed to the shader code in the same way that we have any other vertex attribute; no surprises here. We'll want to include a two-element vector attribute in our vertex shader that will map to our texture coordinates:

attribute vec2 aVertexTextureCoords;

Additionally, we will also want to add a new uniform to the fragment shader that uses a type we haven't seen before: sampler2D. The sampler2D uniform is what allows us to access the texture data in the shader.

uniform sampler2D uSampler;

In the past, when we've used uniforms, we have always set them to the value that we want them to be in the shader, such as a light color. Samplers work a little differently, however. The following shows how to associate a texture with a specific sampler uniform:

gl.activeTexture(gl.TEXTURE0);
gl.bindTexture(gl.TEXTURE_2D, texture);
gl.uniform1i(Program.uSampler, 0);

So what's going on here? First off, we are changing the active texture index with gl.activeTexture...