I'm writing a simple test compute shader that writes a value of 5.0 to every element in a buffer. The buffer's values are initialized to -1, so that I know whether or not creating the buffer and reading the buffer are the problem.
class ComputeShaderWindow : public QOpenGLWindow {
public:
void initializeGL() {
// Create the opengl functions object
gl = context()->versionFunctions<QOpenGLFunctions_4_3_Core>();
m_compute_program = new QOpenGLShaderProgram(this);
auto compute_shader_s = fs::readFile(
"test_assets/example_compute_shader.comp");
// Adds the compute shader, then links and binds it
m_compute_program->addShaderFromSourceCode(QOpenGLShader::Compute,
compute_shader_s);
m_compute_program->link();
m_compute_program->bind();
// Fills the buffer with -1, so we know whether the problem
// is the compute shader not being invoked or not reading
// the buffer correctly afterwards.
GLfloat* default_values = new GLfloat[NUM_INVOCATIONS];
std::fill(default_values, default_values + NUM_INVOCATIONS, -1.0);
GLuint ssbo;
gl->glGenBuffers(1, &ssbo);
gl->glBindBuffer(GL_SHADER_STORAGE_BUFFER, ssbo);
gl->glBufferData(GL_SHADER_STORAGE_BUFFER,
NUM_INVOCATIONS,
default_values,
GL_DYNAMIC_DRAW);
gl->glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);
gl->glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, 0);
gl->glDispatchCompute(NUM_INVOCATIONS / WORKGROUP_SIZE, 1, 1);
gl->glMemoryBarrier(GL_SHADER_STORAGE_BARRIER_BIT);
gl->glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, 0);
gl->glBindBuffer(GL_SHADER_STORAGE_BUFFER, ssbo);
// Now map the buffer so that we can check its values
GLfloat* read_data = (GLfloat*) gl->glMapBuffer(GL_SHADER_STORAGE_BUFFER,
GL_READ_ONLY);
std::vector<GLfloat> buffer_data(NUM_INVOCATIONS);
for (int i = 0; i < NUM_INVOCATIONS; i++) {
buffer_data[i] = read_data[i];
}
gl->glUnmapBuffer(GL_SHADER_STORAGE_BUFFER);
for (int i = 0; i < NUM_INVOCATIONS; i++) {
DEBUG(buffer_data[i]);
}
assert(gl->glGetError() == GL_NO_ERROR);
}
void resizeGL(int width, int height) {
}
void paintGL() {
}
void teardownGL() {
}
private:
QOpenGLFunctions_4_3_Core* gl;
QOpenGLShaderProgram* m_compute_program;
static constexpr int NUM_INVOCATIONS = 9000;
static constexpr int WORKGROUP_SIZE = 128;
};
My compute shader is fairly simple:
#version 430 core
layout(std430, binding = 0) writeonly buffer SSBO {
float data[];
};
layout(local_size_x = 128) in;
void main() {
uint ident = gl_GlobalInvocationID.x;
data[ident] = 5.0f;
}
When I read the buffer, most it is -1, but some of the data is comprised of random float values (-nan, 0, etc). What's going on here?
