From 9e5db7c33df67d80f49f4efd7b8ace5f27e2e7cb Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Sun, 1 Feb 2026 18:35:45 -0800
Subject: Fix bilinear filter

---
 src/swgfx.c | 38 +++++++++++++++++++++-----------------
 1 file changed, 21 insertions(+), 17 deletions(-)

diff --git a/src/swgfx.c b/src/swgfx.c
index 2262dd8..85844e0 100644
--- a/src/swgfx.c
+++ b/src/swgfx.c
@@ -53,12 +53,13 @@ typedef struct swgfx {
 
 static inline int mod(int a, int m) { return (m + (a % m)) % m; }
 static inline R   frac(R a) { return a - (R)((int)a); }
-
 static inline int imin(int a, int b) { return (a <= b) ? a : b; }
 static inline int imax(int a, int b) { return (a >= b) ? a : b; }
-static inline R   rmin(R   a, R   b) { return (a <= b) ? a : b; }
-static inline R   rmax(R   a, R   b) { return (a >= b) ? a : b; }
-static inline R   lerp(R   a, R   b, R t) { return a + t*(b-a); }
+
+static inline R   rmin(R a, R b) { return (a <= b) ? a : b; }
+static inline R   rmax(R a, R b) { return (a >= b) ? a : b; }
+static inline R   lerp(R a, R b, R t) { return a + t*(b-a); }
+static inline R   mod1(R a, R m) { return fmodf(1.f + fmodf(a, m), 1.f); }
 
 static inline sgVec2i min2i(sgVec2i a, sgVec2i b) { return (sgVec2i){.x = imin(a.x, b.x), .y = imin(a.y, b.y) }; }
 static inline sgVec2i max2i(sgVec2i a, sgVec2i b) { return (sgVec2i){.x = imax(a.x, b.x), .y = imax(a.y, b.y) }; }
@@ -69,6 +70,7 @@ static inline sgVec2  sub2(sgVec2 a, sgVec2 b) { return (sgVec2){a.x - b.x, a.y
 static inline sgVec2  scale2(sgVec2 v, R s) { return (sgVec2){v.x * s, v.y * s}; }
 static inline sgVec2  frac2(sgVec2 v) { return (sgVec2){frac(v.x), frac(v.y)}; }
 static inline sgVec2  lerp2(sgVec2 a, sgVec2 b, R t) { return add2(a, scale2(sub2(b,a), t)); }
+static inline sgVec2  mod2(sgVec2 v, R m) { return (sgVec2){mod1(v.x, m), mod1(v.y, m)}; }
 
 static inline sgVec3 add3(sgVec3 a, sgVec3 b) { return (sgVec3){a.x + b.x, a.y + b.y, a.z + b.z}; }
 static inline sgVec3 neg3(sgVec3 v) { return (sgVec3){-v.x, -v.y, -v.z}; }
@@ -314,33 +316,37 @@ static inline sgVec4 ReadTextureFloat(const sgImage* texture, sgVec2i xy) {
   return PixelToVec4(ReadTexture(texture, xy));
 }
 
+static inline sgVec2i UvToIndex(const sgImage* texture, sgVec2 uv) {
+  assert(texture);
+  return (sgVec2i){
+    (int)(uv.x * (R)(texture->width - 1)),
+    (int)(uv.y * (R)(texture->height - 1))};
+}
+
 static inline sgVec2i TextureRepeat(const sgImage* texture, sgVec2i p) {
   return (sgVec2i){mod(p.x, texture->width), mod(p.y, texture->height)};
 }
 
-static inline sgPixel SampleNearest(const sgImage* texture, sgVec2 uv) {
+static inline sgPixel FilterNearest(const sgImage* texture, sgVec2 uv) {
   assert(texture);
   assert(texture->pixels);
-  const sgVec2i xy = {
-    (int)(uv.x * (R)texture->width),
-    (int)(uv.y * (R)texture->height)};
+  const sgVec2i xy  = UvToIndex(texture, uv);
   const sgVec2i xy2 = TextureRepeat(texture, xy);
   return ReadTexture(texture, xy2);
 }
 
-static inline sgPixel SampleBilinear(const sgImage* texture, sgVec2 uv) {
+static inline sgPixel FilterBilinear(const sgImage* texture, sgVec2 uv) {
   assert(texture);
   assert(texture->pixels);
 #define ADDR(x,y) TextureRepeat(texture, (sgVec2i){x,y})
+  const sgVec2 uv01 = mod2(uv, 1.f);
   // Find the closest grid vertex, then interpolate the 4 neighbouring pixel
   // centers.
-  const sgVec2i tl = ADDR(
-    (int)(uv.x * (R)(texture->width  - 1)),
-    (int)(uv.y * (R)(texture->height - 1)));
+  const sgVec2i tl = UvToIndex(texture, uv01);
   const sgVec2i tr = ADDR(tl.x+1, tl.y);
   const sgVec2i bl = ADDR(tl.x,   tl.y+1);
   const sgVec2i br = ADDR(tl.x+1, tl.y+1);
-  const sgVec2   t = frac2(uv);
+  const sgVec2   t = frac2(uv01);
   const sgVec4 tl_pix = ReadTextureFloat(texture, tl);
   const sgVec4 tr_pix = ReadTextureFloat(texture, tr);
   const sgVec4 bl_pix = ReadTextureFloat(texture, bl);
@@ -355,10 +361,8 @@ static inline sgPixel SampleBilinear(const sgImage* texture, sgVec2 uv) {
 // TODO: Clamping and other addressing strategies.
 static inline sgPixel Sample(const sgImage* texture, sgVec2 uv) {
   // TODO: Add a member to sgImage that determines how it should be filtered.
-  return SampleNearest(texture, uv);
-  // TODO: Debug bilinear. It's producing random colours on the ground, likely
-  //  out-of-bounds memory accesses.
-  //return SampleBilinear(texture, uv);
+  //return FilterNearest(texture, uv);
+  return FilterBilinear(texture, uv);
 }
 
 static inline sgAABB2 TriangleAabb2(sgVec2 p0, sgVec2 p1, sgVec2 p2) {
-- 
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