psxrecomp/docs/GPU_GL_READBACK_REGIONS.md

OpenGL native readback regions

OpenGL owns the rendered video memory. A CPU read must see all prior GPU writes. The backend now keeps a separate conservative rectangle for GPU writes that have not reached the CPU array. It reads that rectangle into the full-width CPU array with an explicit row stride. It then clears the CPU readback debt.

The rectangle includes clipped primitives, fill segments and copy destinations. It also retains framebuffer clearing debt across depth24 transitions. GPU uploads and queued draws still finish before packing and reading. Texture sampling can consume packing debt without consuming CPU readback debt. A conservative union is safe for GPU-to-CPU reads because the GPU image is current; CPU-to-GPU uploads still use their exact rectangle list.

This does not change raster rules, read values, guest clocks, draw order or precision. It does not switch OpenGL to software raster ownership. The original interlaced row clipping remains in place. State recreation clears the new debt; CPU-authority mode retains its existing no-readback behavior.

Verification

runtime/tests/test_gl_readback_region.c uses an actual hidden SDL/OpenGL context. It checks the complete native CPU image against full hardware readback after ordered synthetic workloads. It covers single pixels, odd-width row stride, disjoint uploads, texture dependencies, wrapping fills, overlapping copies, masks, blending, clipping, precision margins and state restore. The hardware rasterizer is shared by the comparison; this proves coherence, not independent raster accuracy. No retail assets are required.

Run python runtime/tests/run_gl_readback_region.py --compiler-bin <mingw-bin> --sdl-root <deps> --output <new-directory>. The SDL dependency root must contain sdl3-src/include and sdl3-build/libSDL3.a. The Windows runner uses hidden windows. It tests native and4x scales. --gl-source <old-gpu_gl_renderer.c> --expect-unbounded checks the previous implementation: pixel comparisons pass, and only the small transfer assertion fails. Each command/result is saved in receipt.json under a fresh run directory inside the output root. Configure runtime/ with BUILD_TESTING=ON and PSX_GL_READBACK_SDL_ROOT=<deps> on MinGW to register gl_readback_region_test; run it with ctest --test-dir <build> -R gl_readback_region_test --output-on-failure. Missing dependencies print a configure-time message and do not count as a hardware pass.

Prior art and scope

Existing renderer coherence code supplied the ordering and dirty-area rules. DuckStation's software readback path was consulted as an alternative. This correction keeps the frozen branch's GPU authority and needs no title hook or controller policy change. Future work can reduce row submissions or use a more precise dirty set if measured demand justifies it.

For Clang/MinGW builds, set PSX_GL_READBACK_COMPILER_BIN to a GCC/MinGW bin directory containing both gcc.exe and g++.exe. CMake checks these files before registering the hardware test. gl_readback_runner_test separately checks strict result parsing without a GPU or compiler. A negative run passes only for exactly one failure named single-pixel bounded transfer.