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widescreen: fix vertical-double-buffer top/bottom flicker (rt_wide Y-clip)

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The native-wide mirror target clipped Y to the full surface (0..511), but a
vertically double-buffered game (MMX6: front y0..239 / back y240..479, both
sharing base_x=0) keeps both buffers in one shared wide surface. The background
tile loop starts a partial top row at y = buffer_top - (scrollY & 0xf), so
back-buffer draws bled across the band boundary into the front buffer's region
of the wide surface -> top/bottom edge flicker as the buffers flipped. Canonical
VRAM never showed it because its draw-area clip already confines each buffer.

Clip the wide target's Y to the canonical draw area (g_clip_y1..g_clip_y2),
exactly like rt_native, so each buffer's draws stay in its own band. X still
spans the full wide width (that is where the reveal margins live). User-confirmed
the top/bottom flicker is fixed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
Matthew Stanley
2026-06-20 12:27:42 -07:00
parent a8f37c1f76
commit 2e7b751bfc
1 changed files with 12 additions and 6 deletions
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runtime/src/gpu_sw_renderer.c
+12 -6
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@@ -146,17 +146,23 @@ static inline RTarget rt_hires(void) {
return t;
}
/* Native-wide mirror target: the active wide surface (g_wide_cur). Clip is the
* full surface; callers pass coordinates already translated into surface space
* (vram_x - base_x + OFFSET) and y-clipped to the buffer's band via the source
* primitive. Scale matches the SSAA scale. Only used when g_wide_cur != NULL. */
/* Native-wide mirror target: the active wide surface (g_wide_cur). X spans the
* full wide width (that is where the revealed margins live), but Y is clipped to
* the canonical DRAW AREA — exactly like rt_native — so a primitive that bleeds
* past the current framebuffer's vertical band is confined to it. This matters
* for VERTICALLY double-buffered games (e.g. MMX6: front y0..239 / back
* y240..479, same base_x): the background tile loop starts a partial row at
* y = buffer_top - (scrollY & 0xf), which without the band clip would draw up
* into the OTHER buffer's band in the shared wide surface and corrupt it
* (top/bottom edge flicker as the buffers flip). Canonical VRAM never showed
* this because its draw-area clip already confines it. Scale matches SSAA. */
static inline RTarget rt_wide(void) {
int s = g_scale;
RTarget t;
t.buf = g_wide_cur;
t.w = g_wide_w * s; t.h = VRAM_HEIGHT * s; t.s = s;
t.cx1 = 0; t.cy1 = 0;
t.cx2 = g_wide_w * s - 1; t.cy2 = VRAM_HEIGHT * s - 1;
t.cx1 = 0; t.cy1 = g_clip_y1 * s;
t.cx2 = g_wide_w * s - 1; t.cy2 = g_clip_y2 * s + (s - 1);
return t;
}