Add image-hint for low-color images.

For low-color images, it may be better to not use color-palettes.
Users should treat this as one another hint (as with Photo &
Picture) and another parameter for tuning the compression density.
The optimum compression can still be obtained by running (outer loop)
compression with all possible tunable parameters.

Change-Id: Icb1a4face2a84774e16e801aee4a8ae97e232e8a

README

@@ -168,7 +168,7 @@ options:
   -noalpha ............... discard any transparency information.
   -lossless .............. Encode image losslessly.
   -hint <string> ......... Specify image characteristics hint.
-                           One of: photo or picture
+                           One of: photo, picture or graph.
 
   -short ................. condense printed message
   -quiet ................. don't print anything.

examples/cwebp.c

@@ -844,7 +844,7 @@ static void HelpLong(void) {
   printf("  -noalpha ............... discard any transparency information.\n");
   printf("  -lossless .............. Encode image losslessly.\n");
   printf("  -hint <string> ......... Specify image characteristics hint.\n");
-  printf("                           One of: photo or picture\n");
+  printf("                           One of: photo, picture or graph\n");
 
   printf("\n");
   printf("  -short ................. condense printed message\n");
@@ -973,6 +973,8 @@ int main(int argc, const char *argv[]) {
         config.image_hint = WEBP_HINT_PHOTO;
       } else if (!strcmp(argv[c], "picture")) {
         config.image_hint = WEBP_HINT_PICTURE;
+      } else if (!strcmp(argv[c], "graph")) {
+        config.image_hint = WEBP_HINT_GRAPH;
       } else {
         fprintf(stderr, "Error! Unrecognized image hint: %s\n", argv[c]);
         goto Error;

man/cwebp.1

@@ -164,7 +164,7 @@ Encode the image without any loss.
 .TP
 .B \-hint string
 Specify the hint about input image type. Possible values are:
-\fBphoto\fP, and \fBpicture\fP.
+\fBphoto\fP, \fBpicture\fP or \fBgraph\fP.
 .TP
 .B \-noasm
 Disable all assembly optimizations.

src/enc/config.c

@@ -120,7 +120,7 @@ int WebPValidateConfig(const WebPConfig* config) {
     return 0;
   if (config->lossless < 0 || config->lossless > 1)
     return 0;
-  if (config->image_hint > WEBP_HINT_PHOTO)
+  if (config->image_hint >= WEBP_HINT_LAST)
     return 0;
   return 1;
 }

src/enc/vp8l.c

@@ -141,7 +141,7 @@ static int VP8LEncAnalyze(VP8LEncoder* const enc, WebPImageHint image_hint) {
   const WebPPicture* const pic = enc->pic_;
   assert(pic != NULL && pic->argb != NULL);
 
-  enc->use_palette_ =
+  enc->use_palette_ = (image_hint == WEBP_HINT_GRAPH) ? 0 :
       AnalyzeAndCreatePalette(pic, enc->palette_, &enc->palette_size_);
   if (!enc->use_palette_) {
     if (image_hint == WEBP_HINT_DEFAULT) {
@@ -162,7 +162,6 @@ static int VP8LEncAnalyze(VP8LEncoder* const enc, WebPImageHint image_hint) {
   return 1;
 }
 
-
 static int GetHuffBitLengthsAndCodes(
     const VP8LHistogramSet* const histogram_image,
     HuffmanTreeCode* const huffman_codes) {

src/webp/encode.h

@@ -69,7 +69,9 @@ WEBP_EXTERN(size_t) WebPEncodeLosslessBGRA(const uint8_t* bgra,
 typedef enum {
   WEBP_HINT_DEFAULT = 0,  // default preset.
   WEBP_HINT_PICTURE,      // digital picture, like portrait, inner shot
-  WEBP_HINT_PHOTO         // outdoor photograph, with natural lighting
+  WEBP_HINT_PHOTO,        // outdoor photograph, with natural lighting
+  WEBP_HINT_GRAPH,        // Discrete tone image (graph, map-tile etc).
+  WEBP_HINT_LAST
 } WebPImageHint;
 
 typedef struct {