From e15cca96239909f664ff17cbb66a0eea5541044a Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Mon, 17 Apr 2023 22:23:54 -0700
Subject: [PATCH] GBA Video: Fix interpolation issues with OpenGL renderer

---
 CHANGES                |  1 +
 src/gba/renderers/gl.c | 18 +++++++++---------
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/CHANGES b/CHANGES
index e76ff61c7..8358a7f33 100644
--- a/CHANGES
+++ b/CHANGES
@@ -8,6 +8,7 @@ Emulation fixes:
  - GBA SIO: Fix unconnected normal mode SIOCNT SI bit (fixes mgba.io/i/2810)
  - GBA SIO: Normal mode transfers with no clock should not finish (fixes mgba.io/i/2811)
  - GBA Timers: Cascading timers don't tick when disabled (fixes mgba.io/i/2812)
+ - GBA Video: Fix interpolation issues with OpenGL renderer
 Other fixes:
  - Core: Allow sending thread requests to a crashed core (fixes mgba.io/i/2784)
  - FFmpeg: Force lower sample rate for codecs not supporting high rates (fixes mgba.io/i/2869)
diff --git a/src/gba/renderers/gl.c b/src/gba/renderers/gl.c
index b6ffa2aac..4a751c706 100644
--- a/src/gba/renderers/gl.c
+++ b/src/gba/renderers/gl.c
@@ -289,12 +289,12 @@ static const char* const _renderMode2 =
 	"	}\n"
 	"	loadAffine(int(incoord.y), mat, offset);\n"
 	"	float y = fract(incoord.y);\n"
-	"	float start = 0.75;\n"
+	"	float start = 2. / 3.;\n"
 	"	if (int(incoord.y) - range.x < 4) {\n"
 	"		y = incoord.y - float(range.x);\n"
-	"		start = 0.;\n"
+	"		start -= 1.;\n"
 	"	}\n"
-	"	float lin = start + y * 0.25;\n"
+	"	float lin = start + y / 3.;\n"
 	"	vec2 mixedTransform = interpolate(mat, lin);\n"
 	"	vec2 mixedOffset = interpolate(offset, lin);\n"
 	"	int paletteEntry = fetchTile(ivec2(mixedTransform * incoord.x + mixedOffset));\n"
@@ -340,12 +340,12 @@ static const char* const _renderMode35 =
 	"	}\n"
 	"	loadAffine(int(incoord.y), mat, offset);\n"
 	"	float y = fract(incoord.y);\n"
-	"	float start = 0.75;\n"
+	"	float start = 2. / 3.;\n"
 	"	if (int(incoord.y) - range.x < 4) {\n"
 	"		y = incoord.y - float(range.x);\n"
-	"		start = 0.;\n"
+	"		start -= 1.;\n"
 	"	}\n"
-	"	float lin = start + y * 0.25;\n"
+	"	float lin = start + y / 3.;\n"
 	"	vec2 mixedTransform = interpolate(mat, lin);\n"
 	"	vec2 mixedOffset = interpolate(offset, lin);\n"
 	"	ivec2 coord = ivec2(mixedTransform * incoord.x + mixedOffset);\n"
@@ -401,12 +401,12 @@ static const char* const _renderMode4 =
 	"	}\n"
 	"	loadAffine(int(incoord.y), mat, offset);\n"
 	"	float y = fract(incoord.y);\n"
-	"	float start = 0.75;\n"
+	"	float start = 2. / 3.;\n"
 	"	if (int(incoord.y) - range.x < 4) {\n"
 	"		y = incoord.y - float(range.x);\n"
-	"		start = 0.;\n"
+	"		start -= 1.;\n"
 	"	}\n"
-	"	float lin = start + y * 0.25;\n"
+	"	float lin = start + y / 3.;\n"
 	"	vec2 mixedTransform = interpolate(mat, lin);\n"
 	"	vec2 mixedOffset = interpolate(offset, lin);\n"
 	"	ivec2 coord = ivec2(mixedTransform * incoord.x + mixedOffset);\n"