Vertex Shader (transforms)

Let’s get our hands dirty right away and jump into it by showing the simplest forms of shaders in OpenGL. These are called transforms or transform feedback. Instead of drawing to the screen we simply capture the output of a shader into a Buffer.

The example below shows shader program with only a vertex shader. It has no input data, but we can still force it to run N times. The gl_VertexID (int) variable is a built-in value in vertex shaders containing an integer representing the vertex number being processed.

Input variables in vertex shaders are called attributes (we have no inputs in this example) while output values are called varyings.

import struct
import moderngl

ctx = moderngl.create_context(standalone=True)

program = ctx.program(
    vertex_shader="""
    #version 330

    // Output values for the shader. They end up in the buffer.
    out float value;
    out float product;

    void main() {
        // Implicit type conversion from int to float will happen here
        value = gl_VertexID;
        product = gl_VertexID * gl_VertexID;
    }
    """,
    # What out varyings to capture in our buffer!
    varyings=["value", "product"],
)

NUM_VERTICES = 10

# We always need a vertex array in order to execute a shader program.
# Our shader doesn't have any buffer inputs, so we give it an empty array.
vao = ctx.vertex_array(program, [])

# Create a buffer allocating room for 20 32 bit floats
# num of vertices (10) * num of varyings per vertex (2) * size of float in bytes (4)
buffer = ctx.buffer(reserve=NUM_VERTICES * 2 * 4)

# Start a transform with buffer as the destination.
# We force the vertex shader to run 10 times
vao.transform(buffer, vertices=NUM_VERTICES)

# Unpack the 20 float values from the buffer (copy from graphics memory to system memory).
# Reading from the buffer will cause a sync (the python program stalls until the shader is done)
data = struct.unpack("20f", buffer.read())
for i in range(0, 20, 2):
    print("value = {}, product = {}".format(*data[i:i+2]))

Output of the program is:

value = 0.0, product = 0.0
value = 1.0, product = 1.0
value = 2.0, product = 4.0
value = 3.0, product = 9.0
value = 4.0, product = 16.0
value = 5.0, product = 25.0
value = 6.0, product = 36.0
value = 7.0, product = 49.0
value = 8.0, product = 64.0
value = 9.0, product = 81.0

The GPU is at the very least slightly offended by the meager amount work we assigned it, but this at least shows the basic concept of transforms. We would in most situations also not read the results back into system memory because it’s slow, but sometimes it is needed.

This shader program could for example be modified to generate some geometry or data for any other purpose you might imagine useful. Using modulus (mod(number, divisor)) on gl_VertexID can get you pretty far.

Warning

One known bug in many OpenGL drivers is related to the mod(number, divisor) function. It lies in the fact that if the first argument is exactly 2 times the second, then instead of 0 you will receive the value of the second argument [divisor]. To avoid this error, it is recommended to use the following additional function (insert it into the shader before void main() and then always call mod2() instead of mod()):

float mod2(float number, float divisor) {
    float result = mod(number, divisor);
    if (result >= divisor) {
        result = 0.0;
    }
    return result;
}