quad-decomposition.c

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/*
 
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Copyright (c) 2026 PCSX-Redux authors
 
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*/
 
// Quad decomposition suite. The PS1 GPU decomposes 4-vertex polygons
// into two triangles internally; the audit at soft.cc:2493-2496 says
// the soft renderer splits as (1,3,2) + (0,1,2). Hardware truth here
// determines:
//   - Whether the diagonal seam pixel is drawn once, twice, or not at all.
//   - Whether the second triangle's edge rule sees the seam edge as
//     top-or-bottom (depends on winding).
//   - Whether 4-vertex coordinate ordering changes the fill set for
//     otherwise-identical destination geometry.
 
CESTER_BODY(
 
// Quad Q: 4x4 axis-aligned square, vertices in scan order
// (0,0),(4,0),(0,4),(4,4), BLUE.
static void rasterDrawQuadQ(void) {
    rasterReset();
    rasterClearTestRegion(0, 0, 16, 16);
    rasterFlatQuad(RASTER_CMD_BLUE, 0, 0, 4, 0, 0, 4, 4, 4);
    rasterFlushPrimitive();
}
 
// Quad R: same square but with vertices in reversed winding -
// (4,4),(0,4),(4,0),(0,0). The destination geometry is identical but the
// rasterizer's decomposition sees a different triangle pair. Captures
// whether winding affects the fill set on hardware.
static void rasterDrawQuadR(void) {
    rasterReset();
    rasterClearTestRegion(0, 0, 16, 16);
    rasterFlatQuad(RASTER_CMD_BLUE, 4, 4, 0, 4, 4, 0, 0, 0);
    rasterFlushPrimitive();
}
 
// Quad S: 4-vertex polygon where the 4th vertex creates a non-convex
// shape (the second-triangle decomposition draws OUTSIDE the apparent
// quad). Vertices (0,0),(4,0),(0,4),(2,2). The two-triangle split
// produces tri (4,0)-(2,2)-(0,4) (smaller, interior) and tri
// (0,0)-(4,0)-(0,4) (large, full upper-left). Documents what hardware
// does with degenerate-concave 4-vertex input.
static void rasterDrawQuadS(void) {
    rasterReset();
    rasterClearTestRegion(0, 0, 16, 16);
    rasterFlatQuad(RASTER_CMD_GREEN, 0, 0, 4, 0, 0, 4, 2, 2);
    rasterFlushPrimitive();
}
 
)  // CESTER_BODY
 
// --------------------------------------------------------------------------
// Quad Q: 4x4 axis-aligned, scan-order winding
// --------------------------------------------------------------------------
 
CESTER_TEST(quadQ_pixel_0_0, gpu_raster_phase1,
    rasterDrawQuadQ();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_0_0, 0, 0);
)
 
CESTER_TEST(quadQ_pixel_3_0_top_right, gpu_raster_phase1,
    rasterDrawQuadQ();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_3_0, 3, 0);
)
 
CESTER_TEST(quadQ_pixel_4_0_right_edge, gpu_raster_phase1,
    rasterDrawQuadQ();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_4_0, 4, 0);
)
 
CESTER_TEST(quadQ_pixel_0_3_bottom_left, gpu_raster_phase1,
    rasterDrawQuadQ();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_0_3, 0, 3);
)
 
CESTER_TEST(quadQ_pixel_3_3_interior_corner, gpu_raster_phase1,
    rasterDrawQuadQ();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_3_3, 3, 3);
)
 
CESTER_TEST(quadQ_pixel_4_4_outside_bottom_right, gpu_raster_phase1,
    rasterDrawQuadQ();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_4_4, 4, 4);
)
 
CESTER_TEST(quadQ_pixel_0_4_bottom_edge, gpu_raster_phase1,
    rasterDrawQuadQ();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_0_4, 0, 4);
)
 
CESTER_TEST(quadQ_pixel_2_2_diagonal_seam, gpu_raster_phase1,
    rasterDrawQuadQ();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_2_2, 2, 2);
)
 
// --------------------------------------------------------------------------
// Quad R: same destination geometry, reversed winding
// --------------------------------------------------------------------------
//
// Expected: identical fill set to Quad Q if hardware is winding-agnostic
// for flat untextured. If results differ, the rasterizer is reading
// winding for fill-rule purposes.
 
CESTER_TEST(quadR_reversed_winding_pixel_0_0, gpu_raster_phase1,
    rasterDrawQuadR();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_0_0, 0, 0);
)
 
CESTER_TEST(quadR_reversed_winding_pixel_3_3, gpu_raster_phase1,
    rasterDrawQuadR();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_3_3, 3, 3);
)
 
CESTER_TEST(quadR_reversed_winding_pixel_2_2_seam, gpu_raster_phase1,
    rasterDrawQuadR();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_2_2, 2, 2);
)
 
CESTER_TEST(quadR_reversed_winding_pixel_4_4_outside, gpu_raster_phase1,
    rasterDrawQuadR();
    ASSERT_PIXEL_EQ(EXPECT_QUAD_Q_PIXEL_4_4, 4, 4);
)
 
// --------------------------------------------------------------------------
// Quad S: non-convex 4-vertex (4th vertex inside triangle of first three)
// --------------------------------------------------------------------------
//
// Pure characterization: no EXPECT macros (added once hardware run lands).
// Reads back a 5x5 grid; emits OBS lines for every pixel. cester assertions
// are absent here because the behavior is fully undefined by psx-spx and
// the test exists only to capture ground truth.
 
CESTER_TEST(quadS_nonconvex_dump_5x5, gpu_raster_phase1,
    rasterDrawQuadS();
    for (int y = 0; y < 5; y++) {
        for (int x = 0; x < 5; x++) {
            uint16_t v = rasterReadPixel((int16_t)x, (int16_t)y);
            ramsyscall_printf("OBS quadS x=%d y=%d val=0x%04x\n",
                              x, y, (unsigned)v);
        }
    }
    // Assert at least one pixel is non-sentinel so the test fails if
    // the GPU drew nothing at all (which would itself be a finding).
    cester_assert_uint_ne((unsigned)RASTER_SENTINEL,
                          (unsigned)rasterReadPixel(0, 0));
)