From 36a559c27c4c496d16868bedee52c851526addcf Mon Sep 17 00:00:00 2001
From: pogoflow <6523921+pogoflow@users.noreply.github.com>
Date: Sun, 25 Oct 2026 18:40:51 +0100
Subject: [PATCH] Draw BG modes 4-6 with the compatibility renderer under
 NewRenderer

PpuDrawWholeLine implements modes 0-3 and treats every other mode as
mode 7. A mode 4, 5 or 6 screen therefore sampled BG1's tilemap as a
mode-7 pixel map: a mode 5 menu with hi-res text over a 4bpp background
came out as vertical colour stripes, while VRAM, CGRAM and OAM matched a
reference emulator. Found on a HiROM title.

ppu_runLine now routes modes 4-6 to ppu_draw_whole_line_legacy, which
already handles 16-pixel-wide tiles, the hi-res column split and
offset-per-tile, until the optimized renderer grows them. Modes 0-3 and 7
are unchanged.

tests/ppu/ppu_hires_mode_test.c: for modes 4, 5 and 6 the optimized
renderer must produce the compatibility renderer's scanline, mode 5 must
show the 16-pixel tile split, and mode 1 stays on the optimized path.
Registered in tests/ppu/run.ps1 and tests/run_c_tests.sh.
---
 runner/src/snes/ppu.c           |  15 ++-
 tests/ppu/ppu_hires_mode_test.c | 163 ++++++++++++++++++++++++++++++++
 tests/ppu/run.ps1               |   2 +-
 tests/run_c_tests.sh            |   4 +
 4 files changed, 182 insertions(+), 2 deletions(-)
 create mode 100644 tests/ppu/ppu_hires_mode_test.c

diff --git a/runner/src/snes/ppu.c b/runner/src/snes/ppu.c
index c5728bd9..13a8220d 100644
--- a/runner/src/snes/ppu.c
+++ b/runner/src/snes/ppu.c
@@ -770,6 +770,18 @@ int ppu_wlog_collect(uint32_t frame, PpuWlogEntry *out, int cap, int *lost) {
   return count;
 }
 
+/* BG modes the optimized line renderer implements. PpuDrawWholeLine knows
+ * modes 0-3 and treats everything else as mode 7, so a mode 4/5/6 screen
+ * (8bpp with offset-per-tile, and the two 512-pixel hi-res modes) came out
+ * as mode-7 garbage (a mode 5 menu with hi-res text over a 4bpp background
+ * showed up as vertical colour stripes). Those modes take the
+ * compatibility renderer's scanline until the optimized one grows them;
+ * it samples the main screen from the odd hi-res column, as it always did
+ * with NewRenderer=0. Regression: tests/ppu/ppu_hires_mode_test.c. */
+static bool PpuOptimizedRendererHandlesMode(uint mode) {
+  return mode <= 3 || mode == 7;
+}
+
 void ppu_runLine(Ppu* ppu, int line) {
   PPU_T0_DECL
   /* Writes from here to the next ppu_runLine land on line + 1; no register
@@ -844,7 +856,8 @@ void ppu_runLine(Ppu* ppu, int line) {
              sizeof(ppu->overlayBuffers[kPpuOverlaySource_Obj]));
     ppu->lineHasSprites = !PPU_forcedBlank(ppu) && ppu_evaluateSprites(ppu, line - 1);
 
-    if (ppu->renderFlags & kPpuRenderFlags_NewRenderer) {
+    if ((ppu->renderFlags & kPpuRenderFlags_NewRenderer) &&
+        PpuOptimizedRendererHandlesMode(PPU_mode(ppu))) {
       debug_server_on_ppu_line_drawn(line, 1, ppu->bgmode);
       PPU_T0; PpuDrawWholeLine(ppu, line); PPU_ACC(g_ppu_sec_line_ms);
 #ifdef SNESRECOMP_INTERP_PROFILE
diff --git a/tests/ppu/ppu_hires_mode_test.c b/tests/ppu/ppu_hires_mode_test.c
new file mode 100644
index 00000000..4dca02ec
--- /dev/null
+++ b/tests/ppu/ppu_hires_mode_test.c
@@ -0,0 +1,163 @@
+/* Regression: BG modes the optimized renderer does not implement (4, 5, 6)
+ * must come out of the same scanline as the compatibility renderer instead
+ * of falling through to the mode-7 path.
+ *
+ * Seen on a mode 5 menu (hi-res text over a 4bpp background): with
+ * NewRenderer=1 the screen was vertical colour stripes, because the mode
+ * dispatch in PpuDrawWholeLine knew modes 0-3 and treated everything else
+ * as mode 7, so BG1's tilemap was sampled as a mode-7 pixel map.
+ *
+ * No game ROM, generated data, or platform frontend is required. */
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "snes/ppu.h"
+#include "snes/snes.h"
+
+Snes *g_snes;
+
+/* ppu.c references these runtime globals, but this harness links ppu.c alone. */
+int snes_frame_counter;
+unsigned char g_snesrecomp_last_hdmaen;
+
+uint16_t WsShadowTile(int layer, int screen_x, uint32_t wrapped_y,
+                      uint16_t real_tile) {
+    (void)layer;
+    (void)screen_x;
+    (void)wrapped_y;
+    return real_tile;
+}
+
+bool WsShadowLayerActive(int layer) {
+    (void)layer;
+    return false;
+}
+
+uint32_t WsShadowWorldX(int layer) {
+    (void)layer;
+    return 0;
+}
+
+uint32_t WsShadowPresentWorldY(int layer, int screen_x) {
+    (void)layer;
+    (void)screen_x;
+    return 0;
+}
+
+uint32_t WsShadowScrollY(int layer) {
+    (void)layer;
+    return 0;
+}
+
+void WsShadowOnVramWrite(uint16_t word_adr, uint16_t value) {
+    (void)word_adr;
+    (void)value;
+}
+
+static int check(bool condition, const char *message) {
+    if (!condition) fprintf(stderr, "FAIL: %s\n", message);
+    return condition ? 0 : 1;
+}
+
+/* One solid 4bpp tile (16 words) at word address tile_base. */
+static void fill_solid_4bpp_tile(Ppu *ppu, uint16_t tile_base, uint8_t color) {
+    uint16_t lo = 0, hi = 0;
+    for (int bit = 0; bit < 8; bit++) {
+        if (color & 1) lo |= (uint16_t)(1u << bit);
+        if (color & 2) lo |= (uint16_t)(1u << (8 + bit));
+        if (color & 4) hi |= (uint16_t)(1u << bit);
+        if (color & 8) hi |= (uint16_t)(1u << (8 + bit));
+    }
+    for (int row = 0; row < 8; row++) {
+        ppu->vram[tile_base + row] = lo;
+        ppu->vram[tile_base + 8 + row] = hi;
+    }
+}
+
+/* BG1 only, tilemap at word 0, characters at word 0x1000. Every tilemap
+ * entry names tile 1; in the 16-pixel-wide tile modes the right half is
+ * tile 2. Tile 1 is colour 1, tile 2 is colour 2, so a hi-res scanline is
+ * 8 pixels of colour 1 followed by 8 of colour 2, repeating. */
+static void setup_two_tone_bg1(Ppu *ppu, uint8_t mode) {
+    ppu_reset(ppu);
+    ppu->inidisp = 0x0f;
+    ppu->bgmode = mode;
+    ppu->screenEnabled[0] = 1;
+    ppu->bgXsc[0] = 0x00;
+    ppu->bgTileAdr = 0x0001;
+    memset(ppu->vram, 0, sizeof ppu->vram);
+    for (int i = 0; i < 32 * 32; i++) ppu->vram[i] = 1;
+    fill_solid_4bpp_tile(ppu, 0x1000 + 1 * 16, 1);
+    fill_solid_4bpp_tile(ppu, 0x1000 + 2 * 16, 2);
+    for (size_t i = 0; i < sizeof ppu->cgram / sizeof ppu->cgram[0]; i++)
+        ppu->cgram[i] = 0;
+    ppu->cgram[1] = 0x001f;  /* red */
+    ppu->cgram[2] = 0x7c00;  /* blue */
+}
+
+static void render_line(Ppu *ppu, uint8_t *pixels, size_t pitch, uint32_t flags) {
+    memset(pixels, 0, pitch);
+    PpuBeginDrawing(ppu, pixels, pitch, flags);
+    ppu_runLine(ppu, 0);
+    ppu_runLine(ppu, 1);
+}
+
+static bool pixel_is(const uint8_t *pixels, int x, uint8_t r, uint8_t g, uint8_t b) {
+    const uint8_t *p = pixels + (size_t)x * 4;  /* BGRX */
+    return p[0] == b && p[1] == g && p[2] == r;
+}
+
+int main(void) {
+    enum { kPitch = kPpuXPixels * 4 };
+    uint8_t legacy[kPitch], optimized[kPitch];
+    Ppu *ppu = ppu_init();
+    int failures = 0;
+    if (!ppu) return 2;
+
+    static const uint8_t kModes[] = { 4, 5, 6 };
+    for (size_t m = 0; m < sizeof kModes; m++) {
+        char msg[96];
+        uint8_t mode = kModes[m];
+
+        setup_two_tone_bg1(ppu, mode);
+        render_line(ppu, legacy, kPitch, 0);
+        setup_two_tone_bg1(ppu, mode);
+        render_line(ppu, optimized, kPitch, kPpuRenderFlags_NewRenderer);
+
+        snprintf(msg, sizeof msg, "mode %u: optimized renderer matches the compatibility renderer", mode);
+        failures += check(!memcmp(legacy, optimized, kPitch), msg);
+
+        snprintf(msg, sizeof msg, "mode %u: compatibility renderer draws the layer (not backdrop)", mode);
+        failures += check(!pixel_is(legacy, 0, 0, 0, 0), msg);
+    }
+
+    /* Mode 5 samples the main screen from the odd hi-res column, so the
+     * 256-wide output shows 4 pixels of tile 1 then 4 of tile 2. */
+    setup_two_tone_bg1(ppu, 5);
+    render_line(ppu, optimized, kPitch, kPpuRenderFlags_NewRenderer);
+    failures += check(pixel_is(optimized, 0, 0xff, 0, 0) && pixel_is(optimized, 3, 0xff, 0, 0),
+                      "mode 5: pixels 0-3 come from the left 8 hi-res columns (tile 1, red)");
+    failures += check(pixel_is(optimized, 4, 0, 0, 0xff) && pixel_is(optimized, 7, 0, 0, 0xff),
+                      "mode 5: pixels 4-7 come from the right 8 hi-res columns (tile 2, blue)");
+    failures += check(pixel_is(optimized, 8, 0xff, 0, 0),
+                      "mode 5: the 16-pixel tile repeats at pixel 8");
+
+    /* Modes 0-3 and 7 stay on the optimized path: a mode-1 scene must not
+     * change because of the fallback. */
+    setup_two_tone_bg1(ppu, 1);
+    render_line(ppu, legacy, kPitch, 0);
+    setup_two_tone_bg1(ppu, 1);
+    render_line(ppu, optimized, kPitch, kPpuRenderFlags_NewRenderer);
+    failures += check(pixel_is(optimized, 0, 0xff, 0, 0) && pixel_is(optimized, 7, 0xff, 0, 0) &&
+                      pixel_is(optimized, 8, 0xff, 0, 0),
+                      "mode 1: 8-pixel tiles, tile 1 fills pixels 0-15");
+
+    ppu_free(ppu);
+    if (failures) {
+        fprintf(stderr, "%d check(s) failed\n", failures);
+        return 1;
+    }
+    printf("ppu_hires_mode_test: ok\n");
+    return 0;
+}
diff --git a/tests/ppu/run.ps1 b/tests/ppu/run.ps1
index 09f8d9ec..b01565a9 100644
--- a/tests/ppu/run.ps1
+++ b/tests/ppu/run.ps1
@@ -1,7 +1,7 @@
 $ErrorActionPreference = "Stop"
 $root = (Resolve-Path (Join-Path $PSScriptRoot "..\..")).Path
 $gcc = (Get-Command gcc).Source
-$tests = @("ppu_sprite_limit_test", "ppu_world_mirror_test", "ppu_elastic_band_test", "ppu_mode7_hd_test", "ppu_raster_journal_test")
+$tests = @("ppu_sprite_limit_test", "ppu_world_mirror_test", "ppu_elastic_band_test", "ppu_mode7_hd_test", "ppu_raster_journal_test", "ppu_hires_mode_test")
 $failed = @()
 
 foreach ($test in $tests) {
diff --git a/tests/run_c_tests.sh b/tests/run_c_tests.sh
index 59d44c6d..0b3a3b10 100644
--- a/tests/run_c_tests.sh
+++ b/tests/run_c_tests.sh
@@ -76,6 +76,10 @@ echo "=== PPU sprite limits ==="
     -lm -o "$OUT/ppu_sprite_limit_test"
 "$OUT/ppu_sprite_limit_test"
 
+echo "=== PPU modes 4-6 fall back to the compatibility renderer ==="
+"$CC" -std=c11 -Wall -Wextra -O1     -DSNESRECOMP_REVERSE_DEBUG=0     -I "$ROOT/runner/src" -I "$ROOT/runner/src/snes"     "$ROOT/tests/ppu/ppu_hires_mode_test.c"     "$ROOT/runner/src/snes/ppu.c"     "$ROOT/runner/src/snes/ppu_legacy.c"     -lm -o "$OUT/ppu_hires_mode_test"
+"$OUT/ppu_hires_mode_test"
+
 echo "=== PPU widescreen world-mirror band ==="
 "$CC" -std=c11 -Wall -Wextra -O1     -DSNESRECOMP_REVERSE_DEBUG=0     -I "$ROOT/runner/src" -I "$ROOT/runner/src/snes"     "$ROOT/tests/ppu/ppu_world_mirror_test.c"     "$ROOT/runner/src/snes/ppu.c"     "$ROOT/runner/src/snes/ppu_legacy.c"     -lm -o "$OUT/ppu_world_mirror_test"
 "$OUT/ppu_world_mirror_test"