From ed2160a935e247acd25408af98fc63e1b63fbe1e Mon Sep 17 00:00:00 2001
From: Zack Buhman <zack@buhman.org>
Date: Sat, 25 Jul 2026 22:08:36 -0500
Subject: [PATCH] png: always expand 3-byte-per-pixel textures as
 4-byte-per-pixel

---
 gltf.wgsl   |  2 +-
 index2.js   | 18 ++----------------
 src/png.cpp | 39 +++++++++++++++++++++++----------------
 3 files changed, 26 insertions(+), 33 deletions(-)

diff --git a/gltf.wgsl b/gltf.wgsl
index 48ca072..1246b3b 100644
--- a/gltf.wgsl
+++ b/gltf.wgsl
@@ -95,5 +95,5 @@ fn fragmentMain(input: FragmentInput) -> @location(0) vec4f
   let intensity = lighting(input.positionWorld, normalize(input.normal));
   let color = textureSample(colorTexture, linearSampler, input.texture);
 
-  return vec4(color.xyz * intensity, 1.0);
+  return vec4(color.xyz, 1.0);
 }
diff --git a/index2.js b/index2.js
index c6e6363..d98818d 100644
--- a/index2.js
+++ b/index2.js
@@ -278,15 +278,6 @@ function memoryString(memory, offset)
   return new TextDecoder().decode(new Uint8Array(s));
 }
 
-function pngComponents(colorType)
-{
-  if (colorType == 2)
-    return 3;
-  if (colorType == 6)
-    return 4;
-  throw new Error("non-implemented png color type");
-}
-
 async function loadTextures(device, memory, module, gltf)
 {
   const tempTable = module.instance.exports.mem_alloc(256 * 4);
@@ -326,11 +317,6 @@ async function loadTextures(device, memory, module, gltf)
 
     const width = tempHeaderU32[0];
     const height = tempHeaderU32[1];
-    const bitDepth = tempHeaderU32[2];
-    const colorType = tempHeaderU32[3];
-    const components = pngComponents(colorType);
-    console.log(components);
-
     const texture = device.createTexture({
       size: [width, height],
       format: 'rgba8unorm',
@@ -339,7 +325,7 @@ async function loadTextures(device, memory, module, gltf)
 
     device.queue.writeTexture({ texture: texture },
                               tempOutputU8,
-                              { bytesPerRow: width * components },
+                              { bytesPerRow: width * 4 },
                               { width: width, height: height });
 
     textures.push(texture);
@@ -368,7 +354,7 @@ async function loadGltf(device, canvasFormat, memory, module)
     code: await code,
   });
 
-  const renderer = new GltfRenderer(device, canvasFormat, shaderModule, gltf, textures[2]);
+  const renderer = new GltfRenderer(device, canvasFormat, shaderModule, gltf, textures[0]);
   console.log(renderer.nodeBuffer);
   device.queue.writeBuffer(renderer.nodeBuffer, 0,
                            memory.buffer, nodesHandle, (4 * 4 * 4) * gltf.json.nodes.length);
diff --git a/src/png.cpp b/src/png.cpp
index 4966873..7d863df 100644
--- a/src/png.cpp
+++ b/src/png.cpp
@@ -104,9 +104,11 @@ static int buffer_get(uint8_t const * buf, int stride, int x, int y)
 static int reconstruct(int function,
                        uint8_t const * ibuf, int istride,
                        uint8_t const * obuf, int ostride,
-                       int x, int y, int cpp)
+                       int ix, int y,
+                       int ox,
+                       int ocpp)
 {
-  int f = ibuf[y * istride + x + 1];
+  int f = ibuf[y * istride + ix + 1];
   switch (function) {
   case 0: // none
     {
@@ -114,25 +116,25 @@ static int reconstruct(int function,
     }
   case 1: // sub
     {
-      int a = buffer_get(obuf, ostride, x - cpp, y);
+      int a = buffer_get(obuf, ostride, ox - ocpp, y);
       return (f + a) & 0xff;
     }
   case 2: // up
     {
-      int b = buffer_get(obuf, ostride, x, y - 1);
+      int b = buffer_get(obuf, ostride, ox, y - 1);
       return (f + b) & 0xff;
     }
   case 3: // average
     {
-      int a = buffer_get(obuf, ostride, x - cpp, y);
-      int b = buffer_get(obuf, ostride, x, y - 1);
+      int a = buffer_get(obuf, ostride, ox - ocpp, y);
+      int b = buffer_get(obuf, ostride, ox, y - 1);
       return (f + ((a + b) / 2)) & 0xff;
     }
   case 4: // paeth
     {
-      int a = buffer_get(obuf, ostride, x - cpp, y);
-      int b = buffer_get(obuf, ostride, x, y - 1);
-      int c = buffer_get(obuf, ostride, x - cpp, y - 1);
+      int a = buffer_get(obuf, ostride, ox - ocpp, y);
+      int b = buffer_get(obuf, ostride, ox, y - 1);
+      int c = buffer_get(obuf, ostride, ox - ocpp, y - 1);
       return (f + paeth(a, b, c)) & 0xff;
     }
   default:
@@ -230,22 +232,27 @@ static int decode_iend(uint32_t length,
     return -(22 | (header->bit_depth << 8));
   }
 
-  int cpp = components_per_pixel(header);
-  if (cpp < 0) {
+  int icpp = components_per_pixel(header);
+  if (icpp < 0) {
     state->error = "unsupported color type";
     return -(3 | (header->color_type << 8));
   }
 
-  int obpp = cpp * 1;
+  int ocpp = 4;
+  int obpp = ocpp * 1;
   int ostride = header->width * obpp;
-  int ibpp = cpp * 1;
+  int ibpp = icpp * 1;
   int istride = header->width * ibpp + 1;
 
   for (int y = 0; y < header->height; y++) {
     int function = state->ibuf[y * istride];
-    for (int x = 0; x < header->width * cpp; x++) {
-      int value = reconstruct(function, state->ibuf, istride, obuf, ostride, x, y, cpp);
-      obuf[y * ostride + x] = value;
+    for (int x = 0; x < header->width; x++) {
+      for (int c = 0; c < icpp; c++) {
+        int ix = x * icpp + c;
+        int ox = x * ocpp + c;
+        int value = reconstruct(function, state->ibuf, istride, obuf, ostride, ix, y, ox, ocpp);
+        obuf[y * ostride + ox] = value;
+      }
     }
   }