Added OptiPNG's opngreduc.c functionality to iff2html. The results are fantastic for typical SPR# images, reducing images to about half the size they were before.

On diner_steel2:
113073 bytes -> 57004 bytes; 57004/113073= 0.5041 or 50% original size

Tools/iff2html/CMakeLists.txt

@@ -5,6 +5,7 @@ set(IFF2HTML_SOURCES
     iff2html.c
     md5.c
     image.c
+    opngreduc.c
     ../../Libraries/FileHandler/bmp/read_bmp.c
 )
 

Tools/iff2html/image.c

@@ -21,6 +21,7 @@
 #include <bmp/read_bmp.h>
 #include <setjmp.h>
 #include <libpng/png.h>
+#include "opngreduc.h"
 
 int WritePNG(const char * OutName, const IFFChunk * ChunkData, const IFFSprite * Sprite, size_t * Width, size_t * Height){
     FILE * hFile;
@@ -121,6 +122,7 @@ int WritePNG(const char * OutName, const IFFChunk * ChunkData, const IFFSprite *
         PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
 
     png_set_rows(png_ptr, info_ptr, row_pointers);
+    opng_reduce_image(png_ptr, info_ptr, OPNG_REDUCE_ALL);
     png_write_png(png_ptr, info_ptr, PNG_TRANSFORM_BGR, NULL);
 
     png_destroy_write_struct(&png_ptr, &info_ptr);

Tools/iff2html/opngreduc.h

@@ -0,0 +1,97 @@
+/*
+ * opngreduc.h - libpng extension: lossless image reductions.
+ *
+ * Copyright (C) 2003-2011 Cosmin Truta.
+ * This software is distributed under the same licensing and warranty terms
+ * as libpng.
+ *
+ * This code is functional, although it is still work in progress.
+ * Upon completion, it will be submitted for incorporation into libpng.
+ */
+
+#ifndef OPNGREDUC_H
+#define OPNGREDUC_H
+
+#include <libpng/png.h>
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+#ifdef PNG_INFO_IMAGE_SUPPORTED
+
+/*
+ * Indicate whether the image information is valid, i.e.
+ * all the required critical information is present in the png structures.
+ */
+int PNGAPI opng_validate_image(png_structp png_ptr, png_infop info_ptr);
+
+#endif /* PNG_INFO_IMAGE_SUPPORTED */
+
+
+#ifndef OPNG_NO_IMAGE_REDUCTIONS
+#define OPNG_IMAGE_REDUCTIONS_SUPPORTED
+#endif
+
+#ifdef OPNG_IMAGE_REDUCTIONS_SUPPORTED
+
+#ifndef PNG_INFO_IMAGE_SUPPORTED
+#error OPNG_IMAGE_REDUCTIONS_SUPPORTED requires PNG_INFO_IMAGE_SUPPORTED
+#endif
+
+#ifndef PNG_tRNS_SUPPORTED
+#error OPNG_IMAGE_REDUCTIONS_SUPPORTED requires proper transparency support
+#endif
+
+/*
+ * Reduce the image (bit depth + color type + palette) without
+ * losing any information.  The image data must be present
+ * (e.g. after calling png_set_rows(), or after loading IDAT).
+ */
+png_uint_32 PNGAPI opng_reduce_image(png_structp png_ptr, png_infop info_ptr,
+   png_uint_32 reductions);
+
+/*
+ * PNG reduction flags.
+ */
+#define OPNG_REDUCE_NONE             0x0000
+#define OPNG_REDUCE_16_TO_8          0x0001  /* discard bits 8-15 */
+#define OPNG_REDUCE_8_TO_4_2_1       0x0002  /* discard bits 4-7, 2-7 or 1-7 */
+#define OPNG_REDUCE_RGB_TO_GRAY      0x0004  /* ...also RGBA to GA */
+#define OPNG_REDUCE_STRIP_ALPHA      0x0008  /* ...and create tRNS if needed */
+#define OPNG_REDUCE_RGB_TO_PALETTE   0x0010  /* ...also RGBA to palette/tRNS */
+#define OPNG_REDUCE_PALETTE_TO_RGB   0x0020  /* TODO */
+#define OPNG_REDUCE_GRAY_TO_PALETTE  0x0040  /* ...also GA to palette/tRNS */
+#define OPNG_REDUCE_PALETTE_TO_GRAY  0x0080  /* ...also palette/tRNS to GA */
+#define OPNG_REDUCE_PALETTE_SLOW     0x0100  /* TODO: remove all sterile entries
+                                                and reorder PLTE */
+#define OPNG_REDUCE_PALETTE_FAST     0x0200  /* remove trailing sterile entries
+                                                only; do not reorder PLTE */
+#define OPNG_REDUCE_ANCILLARY        0x1000  /* TODO */
+
+#define OPNG_REDUCE_BIT_DEPTH  \
+   (OPNG_REDUCE_16_TO_8 | OPNG_REDUCE_8_TO_4_2_1)
+
+#define OPNG_REDUCE_COLOR_TYPE  \
+   (OPNG_REDUCE_RGB_TO_GRAY | OPNG_REDUCE_STRIP_ALPHA | \
+    OPNG_REDUCE_RGB_TO_PALETTE | OPNG_REDUCE_PALETTE_TO_RGB | \
+    OPNG_REDUCE_GRAY_TO_PALETTE | OPNG_REDUCE_PALETTE_TO_GRAY)
+
+#define OPNG_REDUCE_PALETTE  \
+   (OPNG_REDUCE_PALETTE_SLOW | OPNG_REDUCE_PALETTE_FAST)
+
+#define OPNG_REDUCE_ALL  \
+   (OPNG_REDUCE_BIT_DEPTH | OPNG_REDUCE_COLOR_TYPE | \
+    OPNG_REDUCE_PALETTE | OPNG_REDUCE_ANCILLARY)
+
+#endif /* OPNG_IMAGE_REDUCTIONS_SUPPORTED */
+
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+
+#endif /* OPNGREDUC_H */