OpenGL backend, restore state?

[ronen]

Generalizing #1165, it seems there are several items of OpenGL state that end up changed after halide_opengl_run() finishes, which leads to surprisingly wrong--and hard to debug--results when a halide filter is used as part of an existing OpenGL app.

Here are ones @shoaibkamil and I have come across (there may be others, but I haven't trolled through the source to find out):

• glUseProgram() as per OpenGL backend should call glUseProgram(0) #1165
• glViewport()
• GL_CULL_FACE state
• GL_DEPTH_TEST state

So my current code wraps the halide filter with:

// save state that halide will clobber
GLint program;
GLint viewport[4];
GLboolean cull_face;
GLboolean depth_test;
glGetIntegerv(GL_VIEWPORT, viewport);
glGetIntegerv(GL_CURRENT_PROGRAM, &program);
glGetBooleanv(GL_CULL_FACE, &cull_face);
glGetBooleanv(GL_DEPTH_TEST, &depth_test);

halide_filter(&input_buf, &output_buf);   // AOT compiled filter

// restore state
glUseProgram(program);
glViewport(viewport[0], viewport[1], viewport[2], viewport[3]);
(cull_face ? glEnable : glDisable)(GL_CULL_FACE);
(depth_test ? glEnable : glDisable)(GL_DEPTH_TEST);

Of course it might be that there's a good reason not to restore some part of the state; in which case though WBN to document that clearly.

[zvookin]

@ronen @shoaibkamil Can this be closed now or is there more to do?

[ronen]

Can be closed. Closing.