diff --git a/src/gba/renderers/video-glsl.c b/src/gba/renderers/video-glsl.c
index b7c6cb0ff..1b5f3a0c6 100644
--- a/src/gba/renderers/video-glsl.c
+++ b/src/gba/renderers/video-glsl.c
@@ -35,10 +35,19 @@ static const GLchar* _fragmentShader =
 	"vec4 backgroundMode0(int bgcnt, int hofs, int vofs) {\n"
 	"	int charBase = ((bgcnt / 4) & 3) * 8192;\n"
 	"	int screenBase = ((bgcnt / 256) & 0x1F) * 1024;\n"
+	"	int size = bgcnt >> 14;\n"
 	"	int localX = hofs + int(x);\n"
 	"	int localY = vofs + int(y);\n"
 	"	int xBase = localX & 0xF8;\n"
 	"	int yBase = localY & 0xF8;\n"
+	"	if (size == 1) {\n"
+	"		xBase += (localX & 0x100) << 5;\n"
+	"	} else if (size == 2) {\n"
+	"		yBase += localY & 0x100;\n"
+	"	} else if (size == 3) {\n"
+	"		xBase += (localX & 0x100) << 5;\n"
+	"		yBase += (localY & 0x100) << 1;\n"
+	"	}\n"
 	"	screenBase = screenBase + (xBase / 8) + (yBase * 4);\n"
 	"	int mapData = DESERIALIZE(texture2D(vram, VRAM_INDEX(screenBase)));\n"
 	"	charBase = charBase + ((mapData & 0x3FF) * 16) + (localX & 0x4) / 4 + (localY & 0x7) * 2;\n"