Implement early depth test

[skyfloogle]

Is there an existing issue for this?

• I have searched the existing issues

What feature are you suggesting?

The PICA200 offers an "early depth test" feature, which performs a depth test before fragment processing happens. This appears to at least be widespread enough to have a page on the OpenGL wiki. I've looked through the code of Azahar's predecessors, and this doesn't appear to be implemented.

Why would this feature be useful?

I'm interested in using this for Red Viper. Some quick tests on my 3DS suggest that the early depth test could help rendering performance significantly, but it doesn't seem to behave in quite the way I expect it to. I usually throw Citra into RenderDoc to investigate graphical oddities, but that's not currently possible for this.

[skyfloogle]

To my understanding, there's an automatic aspect in modern GPUs, but it can also be manually configured. In OpenGL, fragment shaders can be manually configured to force early depth testing. The 3DS is even more configurable in this regard: it can be toggled with 2 different registers, the compare function can be chosen, and a 24-bit clear value can be specified (problemkaputt, 3dbrew).

[rtiangha]

OK, dumb question since I'm not an OpenGL guy: Is it enough to set depth.test_enabled to GL_TRUE in src/video_core/renderer_opengl/gl_state.cpp or would a depth buffer also have to be created for it to work? Or am I totally missing the mark?

    depth.test_enabled = false;
    depth.test_func = GL_LESS;
    depth.write_mask = GL_TRUE;