d2520682cc7cac8838925cf1c9f7d9159a82df92
braney
  Sun Aug 16 10:44:02 2026 -0700
pngwrite: replace leftover experiment marker with a real comment, refs #38107

The png_set_filter call in 443dc863b48 kept a scratch comment that said
"EXPERIMENT ONLY, refs #38094 - do not commit".  The code is correct and
intended, and the justification is #38107, not #38094.  Comment only, no
code change.

diff --git src/lib/pngwrite.c src/lib/pngwrite.c
index d49d508ce46..10d52e7ec28 100644
--- src/lib/pngwrite.c
+++ src/lib/pngwrite.c
@@ -50,31 +50,34 @@
 // But that should not happen because png_error should call pngAbort which calls errAbort.
 if (setjmp(png_jmpbuf(png)))
     {
     png_destroy_write_struct(&png, &info);
     fclose(png_file);
     errAbort("pngwrite: setjmp nonzero.  "
 	     "why didn't png_error..pngAbort..errAbort stop execution before this errAbort?");
     return FALSE;
     }
 
 // Configure PNG output params:
 png_init_io(png, png_file);
 png_set_IHDR(png, info, mg->width, mg->height, 8, // 8=bit_depth
              PNG_COLOR_TYPE_RGBA, PNG_INTERLACE_NONE,
              PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
-// EXPERIMENT ONLY, refs #38094 - do not commit
+// Pin the row filter to UP instead of letting libpng try all five filters on
+// every row.  UP was picked by measuring speed and output size on real browser
+// images (refs #38107).  A row filter is a lossless per-row transform, so the
+// decoded image is unchanged.
 png_set_filter(png, PNG_FILTER_TYPE_BASE, PNG_FILTER_UP);
 
 // Write header/params, write pixels, close and clean up.
 // PNG wants a 2D array of pointers to byte offsets into palette/colorMap.
 // mg has a 1D array of byte offsets.  Make row pointers for PNG:
 
 png_byte **row_pointers = needMem(mg->height * sizeof(png_byte *));
 int i;
 for (i = 0;  i < mg->height;  i++)
     row_pointers[i] = (unsigned char *)&(mg->pixels[i*mg->width]);
 png_set_rows(png, info, row_pointers);
 png_write_png(png, info, PNG_TRANSFORM_IDENTITY, // no transform
 	      NULL); // unused as of PNG 1.2
 png_destroy_write_struct(&png, &info);
 return TRUE;