Passing In Uniforms

Uniforms are used to control a shader while it is running. They can pass in data such as object transforms, the time, the screen resolution or anything else really.

There are two parts to passing in a uniform:

1. Finding where the uniform is using gl.get_uniform_location(&program, name)
2. When the program is active setting the value of the uniform using gl.uniform* to set the value.

I've also changed the triangle to being a single full-screen quad. This means we can now do fancy pixel-shader-rendering:

Yes that's a single quad. The Shader is taken from shadertoy, written by "iq" and used under CC-BY-NC-SA 3.0.

For this I passed in a floating point number for time, and a float vec2 for resolution:

let uniform_resolution = gl.get_uniform_location(&program, "iResolution");
let uniform_time = gl.get_uniform_location(&program, "iTime");

<< snip >>

gl.use_program(Some(&self.program));

gl.uniform1f(self.uniform_time.as_ref(), self.time);
gl.uniform2f(
    self.uniform_resolution.as_ref(),
    self.resolution.0 as f32,
    self.resolution.1 as f32,
);

There are some gotcha's. The uniform name has to exist in the shader and be used. So if you have the a shader that declares uniform float iTime but then never uses it, the uniform will be compiled out, and get_uniform_location will return None. Because the gl.uniform* functions can handle None, the result is simply that it has no effect.

Another gotcha is that the program must be active ( gl.use_program(Some(&self.program)); ) when you set the uniform value. Otherwise you'll get a warning in console and nothing will happen.