Shaders

Learn OpenGL - Shaders

So we can adjust the vertex shader and the fragment shader to make a red rectangle if we want, but that's so trivial we won't bother. Instead we'll skip right to setting a uniform.

Uniforms

First we get the time. GLFW's getTime action gives us a Maybe Double, so we will convert that around into the GLfloat that Open GL will accept.

timeValue <- maybe 0 realToFrac <$> GLFW.getTime
let greenValue = sin timeValue / 2 + 0.5

And then we find the uniform we're after with glGetUniformLocation and assign that value into the uniform with glUniform4f.

-- before the loop we do this once
ourColor <- newCString "ourColor"

-- inside the loop we do this every frame
glUseProgram shaderProgram
vertexColorLocation <- glGetUniformLocation shaderProgram ourColor
glUniform4f vertexColorLocation 0.0 greenValue 0.0 1.0

Full Code

More Attributes!

We can place more attributes in our vertex info,

-- assign the attribute pointer information
let floatSize = (fromIntegral $ sizeOf (0.0::GLfloat)) :: GLsizei
-- position attribute
glVertexAttribPointer 0 3 GL_FLOAT GL_FALSE (6*floatSize) nullPtr
glEnableVertexAttribArray 0
-- color attribute
let threeFloatOffset = castPtr $ plusPtr nullPtr (fromIntegral $ 3*floatSize)
glVertexAttribPointer 1 3 GL_FLOAT GL_FALSE (6*floatSize) threeFloatOffset
glEnableVertexAttribArray 1

And then adjust how we're drawing so that it matches our new vertex stuff,

-- event poll
GLFW.pollEvents
-- clear the screen
glClearColor 0.2 0.3 0.3 1.0
glClear GL_COLOR_BUFFER_BIT
-- draw our triangle
glUseProgram shaderProgram
glBindVertexArray vao
glDrawArrays GL_TRIANGLES 0 3
glBindVertexArray 0
-- swap buffers and go again
GLFW.swapBuffers window
loop

And now we've got a multi-colorized triangle.

Full Code

Our Own Shader Class?

I don't think that we need to make our own shader class, in the C++ class sense. We have a loadShader function, and it does the error handling stuff. We can load the string using readFile, and the fact that readFile is lazy won't affect us much, since we force the whole String right away when we push it out to a Ptr value and all that.