Added image characteristic hint for the codec.

This image type hint is used by codec to apply appropriate set of
transform filters to the image.

Change-Id: Ibb24e47ba14c40fa7f9f2c50ae86e03f2b29e9a6

examples/cwebp.c

@@ -744,6 +744,9 @@ static void HelpLong(void) {
   printf("                           One of: none, fast (default) or best.\n");
   printf("  -alpha_cleanup ......... Clean RGB values in transparent area.\n");
   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("\n");
   printf("  -short ................. condense printed message\n");
@@ -866,6 +869,16 @@ int main(int argc, const char *argv[]) {
     } else if (!strcmp(argv[c], "-lossless")) {
       config.lossless = 1;
       picture.use_argb_input = 1;
+    } else if (!strcmp(argv[c], "-hint") && c < argc - 1) {
+      ++c;
+      if (!strcmp(argv[c], "photo")) {
+        config.image_hint = WEBP_HINT_PHOTO;
+      } else if (!strcmp(argv[c], "picture")) {
+        config.image_hint = WEBP_HINT_PICTURE;
+      } else {
+        fprintf(stderr, "Error! Unrecognized image hint: %s\n", argv[c]);
+        goto Error;
+      }
     } else if (!strcmp(argv[c], "-size") && c < argc - 1) {
       config.target_size = strtol(argv[++c], NULL, 0);
     } else if (!strcmp(argv[c], "-psnr") && c < argc - 1) {

man/cwebp.1

@@ -162,6 +162,13 @@ The default is off.
 .B \-noalpha
 Using this option will discard the alpha channel.
 .TP
+.B \-lossless
+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.
+.TP
 .B \-noasm
 Disable all assembly optimizations.
 .TP

src/enc/config.c

@@ -45,6 +45,7 @@ int WebPConfigInitInternal(WebPConfig* const config,
   config->alpha_filtering = 1;
   config->alpha_quality = 100;
   config->lossless = 0;
+  config->image_hint = WEBP_HINT_DEFAULT;
 
   // TODO(skal): tune.
   switch (preset) {
@@ -119,6 +120,8 @@ int WebPValidateConfig(const WebPConfig* const config) {
     return 0;
   if (config->lossless < 0 || config->lossless > 1)
     return 0;
+  if (config->image_hint > WEBP_HINT_PHOTO)
+    return 0;
   return 1;
 }
 

src/enc/vp8l.c

@@ -139,13 +139,14 @@ static int AnalyzeEntropy(const WebPPicture* const pic,
   return 1;
 }
 
-static int VP8LEncAnalyze(VP8LEncoder* const enc) {
+static int VP8LEncAnalyze(VP8LEncoder* const enc, WebPImageHint image_hint) {
   const WebPPicture* const pic = enc->pic_;
   assert(pic != NULL && pic->argb != NULL);
 
   enc->use_palette_ =
       AnalyzeAndCreatePalette(pic, enc->palette_, &enc->palette_size_);
   if (!enc->use_palette_) {
+    if (image_hint == WEBP_HINT_DEFAULT) {
       double non_pred_entropy, pred_entropy;
       if (!AnalyzeEntropy(pic, &non_pred_entropy, &pred_entropy)) {
         return 0;
@@ -155,6 +156,10 @@ static int VP8LEncAnalyze(VP8LEncoder* const enc) {
         enc->use_predict_ = 1;
         enc->use_cross_color_ = 1;
       }
+    } else if (image_hint == WEBP_HINT_PHOTO) {
+      enc->use_predict_ = 1;
+      enc->use_cross_color_ = 1;
+    }
   }
   return 1;
 }
@@ -945,7 +950,7 @@ WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
   // ---------------------------------------------------------------------------
   // Analyze image (entropy, num_palettes etc)
 
-  if (!VP8LEncAnalyze(enc)) {
+  if (!VP8LEncAnalyze(enc, config->image_hint)) {
     err = VP8_ENC_ERROR_OUT_OF_MEMORY;
     goto Error;
   }

src/webp/encode.h

@@ -46,6 +46,13 @@ WEBP_EXTERN(size_t) WebPEncodeBGRA(const uint8_t* bgra,
 //------------------------------------------------------------------------------
 // Coding parameters
 
+// Image characteristics hint for the underlying encoder.
+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
+} WebPImageHint;
+
 typedef struct {
   float quality;         // between 0 (smallest file) and 100 (biggest)
   int target_size;       // if non-zero, set the desired target size in bytes.
@@ -77,6 +84,7 @@ typedef struct {
   int alpha_quality;      // Between 0 (smallest size) and 100 (lossless).
                           // Default is 100.
   int lossless;           // Lossless encoding (0=lossy(default), 1=lossless).
+  WebPImageHint image_hint;  // Hint for image type.
 } WebPConfig;
 
 // Enumerate some predefined settings for WebPConfig, depending on the type