stop wasting bytes decoding PLTE, support indexed transparency

(cherry picked from commit a38fca5)

Author: matmen

utils/png.js

@@ -85,19 +85,26 @@ export async function decode(array) {
     let c_offset = 33;
     const chunks = [];
 
-        let palette;
-    if (array[25] === 3)
-        palette = new Uint32Array(2 ** bpc);
+        let palette, alphaPalette;
     let type;
     while (type !== 1229278788) {
         type = view.getUint32(4 + c_offset);
 
         // IDAT
         if (type === 1229209940)
             chunks.push(array.subarray(8 + c_offset, 8 + c_offset + view.getUint32(c_offset)));
-        else if (type === 1347179589) {
+        else if (type === 1347179589) { // PLTE
+            if (palette)
+                throw new Error('PLTE can only occur once in an image');
+            palette = new Uint32Array(view.getUint32(c_offset));
             for (let pxlOffset = 0; pxlOffset < palette.length * 8; pxlOffset += 3)
                 palette[pxlOffset / 3] = array[8 + c_offset + pxlOffset] << 24 | array[8 + c_offset + pxlOffset + 1] << 16 | array[8 + c_offset + pxlOffset + 2] << 8 | 0xff;
+        } else if (type === 1951551059) { // tRNS
+            if (alphaPalette)
+                throw new Error('tRNS can only occur once in an image');
+            alphaPalette = new Uint8Array(view.getUint32(c_offset));
+            for (let i = 0; i < alphaPalette.length; i++)
+                alphaPalette[i] = array[8 + c_offset + i];
         }
 
         c_offset += 4 + 4 + 4 + view.getUint32(c_offset);
@@ -118,7 +125,14 @@ export async function decode(array) {
         p_offset += row_length;
     }
 
-        if (channels === 1 && palette) {
+    if (pixel_type === 3) {
+        if (!palette)
+            throw new Error('Indexed color PNG has no PLTE');
+
+        if (alphaPalette)
+            for (let i = 0; i < alphaPalette.length; i++)
+                palette[i] &= 0xffffff00 | alphaPalette[i];
+
         channels = 4;
         const newPixels = new Uint8Array(width * height * 4);
         const pixelView = new DataView(newPixels.buffer, newPixels.byteOffset, newPixels.byteLength);
@@ -206,4 +220,4 @@ function filter_4(slice, pixels, p_offset, bytespp, row_length) {
             pixels[i + p_offset] = slice[i++] + ((pa <= pb && pa <= pc) ? a : ((pb <= pc) ? b : c));
         }
     }
-}
+}