compute shader won't write into buffer

Question

I am trying to use a compute shader to write into a buffer.

Setting up the buffer:

glCreateBuffers(1, &m_ssbo);
glNamedBufferStorage(m_ssbo, 1920 * 1080 * 4 * sizeof(GLfloat), nullptr, GL_MAP_WRITE_BIT | GL_MAP_READ_BIT | GL_DYNAMIC_STORAGE_BIT);

Compute Shader:

#version 450 core
layout (local_size_x = 1) in;
layout(std430, binding = 0) restrict writeonly buffer SSBO {
    vec4 color[];
};
void main() {
    color[0] = vec4(1., 0., 0., 1.);
}

Dispatch and Reading:

m_comp_prog.use();
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, m_ssbo);
glDispatchCompute(1, 1, 0);
glMemoryBarrier(GL_SHADER_STORAGE_BARRIER_BIT);

{
    auto * ptr = m_ssbo.map(0, static_cast<unsigned int>(sizeof(GLfloat)) * 4, GL_MAP_READ_BIT);
    GLfloat * fptr = static_cast<GLfloat *>(ptr);
    std::cout << fptr[0] << std::endl;
    m_ssbo.unmap();
}

The output however is 0 and not 1.

BDL · Accepted Answer · 2016-02-01 18:24:41Z

3

It looks as if you are never executing the shader:

glDispatchCompute(1, 1, 0);

Although not mentioned in the reference, this page indicatest that parsing 0 to one of the arguments is not allowed. This can be explained since num_group_x * num_group_y * num_group_z workgroups are created, which means that non are created with this parameters (1*1*0 = 0).

To fix the problem, try to call

glDispatchCompute(1, 1, 1);
                        ^

instead, which will create exactly one workgroup.

answered Feb 1, 2016 at 18:24

BDL

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