Compute shaders issues

Scripting Issues
1

Say I want to generate a 512x512 image inside a compute shader by passing an array of size 512x3, and painting each column in an image accordingly
Example: if array[8] = [0.8, 0.2, 0.3], then the column img[8] in the final image will be of this color.

I relied on this post and the code addressed in this post, but I couldn’t get it to work.

Expectation
image
reault
image
close up
image

main.py

import panda3d.core as p3d
from direct.showbase.ShowBase import ShowBase
from array import array

immutableTextureStore = p3d.ConfigVariableBool("gl-immutable-texture-storage", False)
immutableTextureStore.setValue(True)

base = ShowBase(windowType='offscreen')


_TOTAL_ = 512
floats = []
for i in range(_TOTAL_):
    floats += [i/_TOTAL_, 0, 0]
initial_data = array('f', floats)
buffer = p3d.ShaderBuffer('elements', initial_data.tobytes(), p3d.GeomEnums.UH_static)

img = p3d.Texture()
img.setup_2d_texture(512, 512, p3d.Texture.T_unsigned_byte, p3d.Texture.F_rgba8)

shader = p3d.Shader.load_compute(p3d.Shader.SL_GLSL, "shaders/c_dummy.glsl")
node_p = p3d.NodePath("dummy")
node_p.set_shader(shader)
node_p.set_shader_input("img", img)
node_p.set_shader_input("ElementBuffer", buffer)

sattr = node_p.get_attrib(p3d.ShaderAttrib)
base.graphicsEngine.dispatchCompute((_TOTAL_//16, 1, 1), sattr, base.win.get_gsg())
base.graphicsEngine.extractTextureData(img, base.win.get_gsg())

frame = p3d.PNMImage()
img.store(frame)
frame.write("test.png")

compute_shader.glsl

#version 430

layout (local_size_x = 16, local_size_y = 1) in;

struct Element
{
    vec3 color;
};

layout(std430) buffer ElementBuffer { Element elements[]; };
uniform writeonly image2D img;

float size = 512.;
void main()
{
  ivec2 texelCoords = ivec2(gl_GlobalInvocationID.xy);
  int id = texelCoords.x;

  vec3 c = elements[id].color;
  for (int i=0; i < size; i++)
    imageStore(img, ivec2(id, i), vec4(c, 1));
}

any ideas?

2

The problem is the oddball alignment rules for vec3. Really, you should avoid using the vec3 type in SSBOs. Either use three individual float variables or switch to vec4.

1 Like
3

@rdb
What if I want to have an output buffer from the shader?
I extended my input buffer to be 512x512 and I want it to write both to an image and to buffer back to the cpu

How can I access the values in elements_out?

modified shader

#version 430

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

struct Element
{
    vec4 color;
};

buffer ElementBufferIn { Element elements_in[]; };
buffer ElementBufferOut { Element elements_out[]; };
uniform writeonly image2D img;

float size = pow(512., 2);
void main()
{
  ivec2 texelCoords = ivec2(gl_GlobalInvocationID.xy);
  int id = texelCoords.x * 512 + texelCoords.y;
  vec4 c = elements_in[id].color;
  
  elements_out[id].color = c;
  imageStore(img, texelCoords, c);
}

added this to python

initial_data_out = array('f', list(range(512 * 512 * 4))) # init empty list
buffer_out = p3d.ShaderBuffer('elements_out', initial_data_out.tobytes(), p3d.GeomEnums.UH_static)
node_p.set_shader_input("ElementBufferOut", buffer_out)
4

It’s not possible to directly download the contents of a ShaderBuffer at the moment.

You should instead write to a buffer texture and you can download that with extract_texture_data.