Make discarding invisible RGB values (cleanup alpha) the default.

Rename the flag to exact instead of the opposite cleanup_alpha. Add the flag to
WebPConfig. Do the cleanup in the webp encoder library rather than the cwebp
binary, this will be needed for the next stage: smarter alpha cleanup for
better compression which cannot be done as a preprocessing due to depending on
predictor choices in the encoder.

Change-Id: I2fbf57f918a35f2da6186ef0b5d85e5fd0020eef

README

@@ -181,7 +181,7 @@ Options:
   -alpha_method <int> .... transparency-compression method (0..1)
   -alpha_filter <string> . predictive filtering for alpha plane,
                            one of: none, fast (default) or best
-  -alpha_cleanup ......... clean RGB values in transparent area
+  -exact ................. preserve RGB values in transparent area
   -blend_alpha <hex> ..... blend colors against background color
                            expressed as RGB values written in
                            hexadecimal, e.g. 0xc0e0d0 for red=0xc0

examples/cwebp.c

@@ -611,7 +611,8 @@ static void HelpLong(void) {
   printf("  -alpha_method <int> .... transparency-compression method (0..1)\n");
   printf("  -alpha_filter <string> . predictive filtering for alpha plane,\n");
   printf("                           one of: none, fast (default) or best\n");
-  printf("  -alpha_cleanup ......... clean RGB values in transparent area\n");
+  printf("  -exact ................. preserve RGB values in transparent area"
+         "\n");
   printf("  -blend_alpha <hex> ..... blend colors against background color\n"
          "                           expressed as RGB values written in\n"
          "                           hexadecimal, e.g. 0xc0e0d0 for red=0xc0\n"
@@ -763,7 +764,10 @@ int main(int argc, const char *argv[]) {
     } else if (!strcmp(argv[c], "-alpha_method") && c < argc - 1) {
       config.alpha_compression = ExUtilGetInt(argv[++c], 0, &parse_error);
     } else if (!strcmp(argv[c], "-alpha_cleanup")) {
-      keep_alpha = keep_alpha ? 2 : 0;
+      // This flag is obsolete, does opposite of -exact.
+      config.exact = 0;
+    } else if (!strcmp(argv[c], "-exact")) {
+      config.exact = 1;
     } else if (!strcmp(argv[c], "-blend_alpha") && c < argc - 1) {
       blend_alpha = 1;
       // background color is given in hex with an optional '0x' prefix
@@ -996,10 +1000,6 @@ int main(int argc, const char *argv[]) {
     WebPBlendAlpha(&picture, background_color);
   }
 
-  if (keep_alpha == 2) {
-    WebPCleanupTransparentArea(&picture);
-  }
-
   if (verbose) {
     const double read_time = StopwatchReadAndReset(&stop_watch);
     fprintf(stderr, "Time to read input: %.3fs\n", read_time);

man/cwebp.1

@@ -247,9 +247,9 @@ mode will just try to form an a priori guess without testing all modes.
 Specify the algorithm used for alpha compression: 0 or 1. Algorithm 0 denotes
 no compression, 1 uses WebP lossless format for compression. The default is 1.
 .TP
-.B \-alpha_cleanup
+.B \-exact
-Modify unseen RGB values under fully transparent area, to help compressibility.
+Preserve RGB values in transparent area. The default is off, to help
-The default is off.
+compressibility.
 .TP
 .BI \-blend_alpha " int
 This option blends the alpha channel (if present) with the source using the

src/enc/config.c

@@ -43,6 +43,7 @@ int WebPConfigInitInternal(WebPConfig* config,
   config->alpha_filtering = 1;
   config->alpha_quality = 100;
   config->lossless = 0;
+  config->exact = 0;
   config->image_hint = WEBP_HINT_DEFAULT;
   config->emulate_jpeg_size = 0;
   config->thread_level = 0;
@@ -139,6 +140,8 @@ int WebPValidateConfig(const WebPConfig* config) {
     return 0;
   if (config->low_memory < 0 || config->low_memory > 1)
     return 0;
+  if (config->exact < 0 || config->exact > 1)
+    return 0;
 #ifdef WEBP_EXPERIMENTAL_FEATURES
   if (config->delta_palettization < 0 || config->delta_palettization > 1)
     return 0;

src/enc/webpenc.c

@@ -321,6 +321,10 @@ int WebPEncode(const WebPConfig* config, WebPPicture* pic) {
   if (pic->width > WEBP_MAX_DIMENSION || pic->height > WEBP_MAX_DIMENSION)
     return WebPEncodingSetError(pic, VP8_ENC_ERROR_BAD_DIMENSION);
 
+  if (!config->exact) {
+    WebPCleanupTransparentArea(pic);
+  }
+
   if (pic->stats != NULL) memset(pic->stats, 0, sizeof(*pic->stats));
 
   if (!config->lossless) {

src/webp/encode.h

@@ -20,7 +20,7 @@
 extern "C" {
 #endif
 
-#define WEBP_ENCODER_ABI_VERSION 0x0208    // MAJOR(8b) + MINOR(8b)
+#define WEBP_ENCODER_ABI_VERSION 0x0209    // MAJOR(8b) + MINOR(8b)
 
 // Note: forward declaring enumerations is not allowed in (strict) C and C++,
 // the types are left here for reference.
@@ -136,12 +136,16 @@ struct WebPConfig {
 
   int near_lossless;      // Near lossless encoding [0 = off(default) .. 100].
                           // This feature is experimental.
+  int exact;              // if non-zero, preserve the exact RGB values under
+                          // transparent area. Otherwise, discard this invisible
+                          // RGB information for better compression. The default
+                          // value is 0.
 
 #ifdef WEBP_EXPERIMENTAL_FEATURES
   int delta_palettization;
-  uint32_t pad[3];        // padding for later use
+  uint32_t pad[2];        // padding for later use
 #else
-  uint32_t pad[4];        // padding for later use
+  uint32_t pad[3];        // padding for later use
 #endif  // WEBP_EXPERIMENTAL_FEATURES
 };