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I'm not so sure that there is such an interoperability between the two graphics APIs; for one, OpenCL was meant to replace OpenGL. OpenGL didn't get compute shaders until v4.3. That said, you can always read from the texture GPU buffer into a CPU buffer.

const size_t num_output_channels = 1;
vector<float> output_pixels(res * res * num_output_channels, 0.0f);

glGenTextures(1, &tex_output);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, tex_output);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_R32F, tex_w, tex_h, 0, GL_RED, GL_FLOAT, NULL);
glBindImageTexture(0, tex_output, 0, GL_FALSE, 0, GL_WRITE_ONLY, GL_R32F);

glActiveTexture(GL_TEXTURE0);
glBindImageTexture(0, tex_output, 0, GL_FALSE, 0, GL_WRITE_ONLY, GL_R32F);
glGetTexImage(GL_TEXTURE_2D, 0, GL_RED, GL_FLOAT, &output_pixels[0]);

I'm not so sure that there is such an interoperability between the two graphics APIs; for one, OpenCL was meant to replace OpenGL. OpenGL didn't get compute shaders until v4.3. That said, you can always read from the texture GPU buffer into a CPU buffer.

const size_t num_output_channels = 1;
vector<float> output_pixels(res * res output_pixels(tex_w* tex_h * num_output_channels, 0.0f);

glGenTextures(1, &tex_output);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, tex_output);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_R32F, tex_w, tex_h, 0, GL_RED, GL_FLOAT, NULL);
glBindImageTexture(0, tex_output, 0, GL_FALSE, 0, GL_WRITE_ONLY, GL_R32F);

glActiveTexture(GL_TEXTURE0);
glBindImageTexture(0, tex_output, 0, GL_FALSE, 0, GL_WRITE_ONLY, GL_R32F);
glGetTexImage(GL_TEXTURE_2D, 0, GL_RED, GL_FLOAT, &output_pixels[0]);