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
@@ -1,97 +1,100 @@
 /* pngwrite.c - write out a memGfx to a PNG file, using the reference library libpng */
 /* (libpng is available from sourceforge and included in many open source distros,
  *  has a wide-open license intended to encourage usage of the PNG format, and the lib
  *  has been under development and testing for 14 years -- http://libpng.org/) */
 
 
 #include "png.h"   // MUST come before common.h, due to setjmp checking  in pngconf.h 
 #include "common.h"
 #include "memgfx.h"
 
 
 static void pngAbort(png_structp png, png_const_charp errorMessage)
 /* type png_error wrapper around errAbort */
 {
 errAbort("%s", (char *)errorMessage);
 }
 
 static void pngWarn(png_structp png, png_const_charp warningMessage)
 /* type png_error wrapper around warn */
 {
 warn("%s", (char *)warningMessage);
 }
 
 boolean mgSaveToPng(FILE *png_file, struct memGfx *mg, boolean useTransparency)
 /* Save PNG to an already open file.
  * If useTransparency, then the first color in memgfx's colormap/palette is
  * assumed to be the image background color, and pixels of that color
  * are made transparent. */
 /* Reference: http://libpng.org/pub/png/libpng-1.2.5-manual.html */
 {
 if (!png_file || !mg)
     errAbort("mgSaveToPng: called with a NULL");
 png_structp png = png_create_write_struct(PNG_LIBPNG_VER_STRING,
 					  NULL, // don't need pointer to data for err/warn handlers
 					  pngAbort, pngWarn);
 if (!png)
     {
     errAbort("png_write_struct failed");
     return FALSE;
     }
 png_infop info = png_create_info_struct(png);
 if (!info)
     {
     errAbort("png create_info_struct failed");
     png_destroy_write_struct(&png, NULL);
     return FALSE;
     }
 
 // If setjmp returns nonzero, it means png_error is returning control here.
 // 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;
 }
 
 void mgSavePng(struct memGfx *mg, char *filename, boolean useTransparency)
 /* Save memory bitmap to filename as a PNG.
  * If useTransparency, then the first color in memgfx's colormap/palette is
  * assumed to be the image background color, and pixels of that color
  * are made transparent. */
 {
 FILE *pngFile = mustOpen(filename, "wb");
 if (!mgSaveToPng(pngFile, mg, useTransparency))
     {
     remove(filename);
     errAbort("Couldn't save %s", filename);
     }
 if (fclose(pngFile) != 0)
     errnoAbort("fclose failed");
 }