mirror of
https://github.com/RetroPortingToolKit/psxrecomp
synced 2026-10-27 10:38:46 -04:00
Alexandros Mandravillis
56 lines
3.6 KiB
Markdown
56 lines
3.6 KiB
Markdown
# OpenGL native readback regions
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OpenGL owns the rendered video memory. A CPU read must see all prior GPU writes.
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The backend now keeps a separate conservative rectangle for GPU writes that
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have not reached the CPU array. It reads that rectangle into the full-width CPU
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array with an explicit row stride. It then clears the CPU readback debt.
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The rectangle includes clipped primitives, fill segments and copy destinations.
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It also retains framebuffer clearing debt across depth24 transitions. GPU
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uploads and queued draws still finish before packing and reading. Texture
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sampling can consume packing debt without consuming CPU readback debt. A
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conservative union is safe for GPU-to-CPU reads because the GPU image is current;
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CPU-to-GPU uploads still use their exact rectangle list.
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This does not change raster rules, read values, guest clocks, draw order or
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precision. It does not switch OpenGL to software raster ownership. The original
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interlaced row clipping remains in place. State recreation clears the new debt;
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CPU-authority mode retains its existing no-readback behavior.
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## Verification
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`runtime/tests/test_gl_readback_region.c` uses an actual hidden SDL/OpenGL
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context. It checks the complete native CPU image against full hardware readback
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after ordered synthetic workloads. It covers single pixels, odd-width row
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stride, disjoint uploads, texture dependencies, wrapping fills, overlapping
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copies, masks, blending, clipping, precision margins and state restore. A depth24 exit followed by an immediate
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CPU read verifies that cleared movie pixels are visible and newer overlapping
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texture uploads survive. It tests this existing clear policy, not full movie
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playback or depth24 raster accuracy.
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The hardware rasterizer is shared by the comparison; this proves coherence,
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not independent raster accuracy. No retail assets are required.
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Run `python runtime/tests/run_gl_readback_region.py --compiler-bin <mingw-bin>
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--sdl-root <deps> --output <new-directory>`. The SDL dependency root must contain
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`sdl3-src/include` and `sdl3-build/libSDL3.a`. The Windows runner uses hidden
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windows. It tests
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native and 4x scales. `--gl-source <old-gpu_gl_renderer.c> --expect-unbounded`
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accepts an unbounded implementation only when pixel comparisons pass and
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exactly the small-transfer assertion fails. An older implementation with a
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coherence defect (including the depth24 clear) is correctly rejected too. 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.
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## Prior art and scope
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Existing renderer coherence code supplied the ordering and dirty-area rules.
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DuckStation's software readback path was consulted as an alternative. This correction keeps the frozen branch's GPU
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authority and needs no title hook or controller policy change. Future work can
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reduce row submissions or use a more precise dirty set if measured demand
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justifies it.
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For Clang/MinGW builds, set `PSX_GL_READBACK_COMPILER_BIN` to a GCC/MinGW
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bin directory containing both gcc.exe and g++.exe. CMake checks these files
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before registering the hardware test. Both the configure step and the runner
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also validate the SDL3 include directory and static library. `gl_readback_runner_test` separately
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checks strict result parsing without a GPU or compiler. A negative run passes
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only for exactly one failure named single-pixel bounded transfer.
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