Executing a simple GLSL compute shader with modernGL does not work

Question

I recently discovered the advantages of compute shaders. However, somehow I couldn't get them to write data correctly.

In the minimal example below, I want to use a compute shader to rotate the first three channels of an image. However, when using the shader, the resulting output image is always zero.

When filling the destination image blue before executing the shader, it also becomes black after execution, which shows that the shader is doing something to the texture. What am I missing?

import moderngl
import numpy as np
import cv2

compute_shader_source = """
#version 450

layout (local_size_x = 16, local_size_y = 16) in;

layout (binding = 0, rgba8ui) uniform   uimage2D src_texture;
layout (binding = 1, rgba8ui) uniform   uimage2D dest_texture;

// rotate channels
void main() {
    ivec2 texel_pos = ivec2(gl_GlobalInvocationID.xy);
    uvec4 color = imageLoad(src_texture, texel_pos);
    uvec4 flipped_color = color.gbra;
    imageStore(dest_texture, texel_pos, flipped_color);
}
"""

dims            = (256, 128)

ctx             = moderngl.create_standalone_context()
compute_shader  = ctx.compute_shader(compute_shader_source)
compute_shader['src_texture']   = 0
compute_shader['dest_texture']  = 1

# Input Image (RED)
in_color    = np.array([0, 0, 100, 255] * dims[0] * dims[1], dtype=np.uint8)
in_texture  = ctx.texture(dims, 4, data = in_color)

# Output Image (initialized BLUE)
out_color   = np.array([100, 0, 0, 255] * dims[0] * dims[1], dtype=np.uint8)
out_texture = ctx.texture(dims, 4, data = out_color)

# Bind textures to units
in_texture.bind_to_image(0, read=True, write=True)
out_texture.bind_to_image(1, read=True, write=True)

# Input
a = np.frombuffer(in_texture.read(), dtype=np.uint8).reshape(in_texture.height, in_texture.width, -1)
cv2.imshow('input', a)

# Before shader execution
a = np.frombuffer(out_texture.read(), dtype=np.uint8).reshape(in_texture.height, in_texture.width, -1)
cv2.imshow('destination before', a)

# Execute the compute shader
compute_shader.run(dims[0] // 16, dims[1] // 16, 1)

# After shader execution (should be GREEN, is BLACK)
a = np.frombuffer(out_texture.read(), dtype=np.uint8).reshape(in_texture.height, in_texture.width, -1)
cv2.imshow('destination after', a)
cv2.waitKey()

Answer 1

I finally figured out what went wrong. According to the docs the data type needs to be set explicitly when creating a texture, otherwise it is assumed float, or 'f1'. As my image consist of unsigned ints dtype needs to be 'u1':

in_texture   = ctx.texture(dims, 4, data = in_color, dtype='u1')
out_texture  = ctx.texture(dims, 4, data = out_color, dtype='u1')

This finally produces a green image!