add a -z option to cwebp, and WebPConfigLosslessPreset() function

These are presets for lossless coding, similar to zlib.
The shortcut for lossless coding is now, e.g.:
   cwebp -z 5 in.png -o out_lossless.webp

There are 10 possible values for -z parameter:
   0 (fastest, lowest compression)
to 9 (slowest, best compression)

A reasonable tradeoff is -z 6, e.g.
-z 9 can be quite slow, so use with care.

This -z option is just a shortcut for some pre-defined
'-lossless -m xx -q yy' combinations.

Change-Id: I6ae716456456aea065469c916c2d5ca4d6c6cf04

examples/cwebp.c

@@ -568,6 +568,8 @@ static void HelpLong(void) {
   printf("                            default, photo, picture,\n");
   printf("                            drawing, icon, text\n");
   printf("     -preset must come first, as it overwrites other parameters.");
+  printf("  -z <int> ............... Activates lossless preset with given "
+         "                           level in [0:fast, ..., 9:slowest]\n");
   printf("\n");
   printf("  -m <int> ............... compression method (0=fast, 6=slowest)\n");
   printf("  -segments <int> ........ number of segments to use (1..4)\n");
@@ -672,6 +674,8 @@ int main(int argc, const char *argv[]) {
   uint32_t background_color = 0xffffffu;
   int crop = 0, crop_x = 0, crop_y = 0, crop_w = 0, crop_h = 0;
   int resize_w = 0, resize_h = 0;
+  int lossless_preset = 6;
+  int use_lossless_preset = -1;  // -1=unset, 0=don't use, 1=use it
   int show_progress = 0;
   int keep_metadata = 0;
   int metadata_written = 0;
@@ -726,8 +730,13 @@ int main(int argc, const char *argv[]) {
       picture.height = strtol(argv[++c], NULL, 0);
     } else if (!strcmp(argv[c], "-m") && c < argc - 1) {
       config.method = strtol(argv[++c], NULL, 0);
+      use_lossless_preset = 0;   // disable -z option
     } else if (!strcmp(argv[c], "-q") && c < argc - 1) {
       config.quality = (float)strtod(argv[++c], NULL);
+      use_lossless_preset = 0;   // disable -z option
+    } else if (!strcmp(argv[c], "-z") && c < argc - 1) {
+      lossless_preset = strtol(argv[++c], NULL, 0);
+      if (use_lossless_preset != 0) use_lossless_preset = 1;
     } else if (!strcmp(argv[c], "-alpha_q") && c < argc - 1) {
       config.alpha_quality = strtol(argv[++c], NULL, 0);
     } else if (!strcmp(argv[c], "-alpha_method") && c < argc - 1) {
@@ -916,6 +925,13 @@ int main(int argc, const char *argv[]) {
     goto Error;
   }
 
+  if (use_lossless_preset == 1) {
+    if (!WebPConfigLosslessPreset(&config, lossless_preset)) {
+      fprintf(stderr, "Invalid lossless preset (-z %d)\n", lossless_preset);
+      goto Error;
+    }
+  }
+
   // Check for unsupported command line options for lossless mode and log
   // warning for such options.
   if (!quiet && config.lossless == 1) {

man/cwebp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH CWEBP 1 "December 12, 2013"
+.TH CWEBP 1 "March 11, 2014"
 .SH NAME
 cwebp \- compress an image file to a WebP file
 .SH SYNOPSIS
@@ -32,6 +32,14 @@ Print the version number (as major.minor.revision) and exit.
 .BI \-q " float
 Specify the compression factor for RGB channels between 0 and 100. The default
 is 75.
+.TP
+.BI \-z " int
+Switch on \fBlossless\fP compression mode with the specified level between 0
+and 9, with level 0 being the fastest, 9 being the slowest. Fast mode
+produce larger file size than fast ones. A good default is \-z 6.
+This option is actually a shortcut for some predefined setting for quality
+and method. If options \-q  or \-m are subsequently used, they will invalidate
+the effect of this \-z option.
 .br
 In case of lossy compression (default), a small factor produces a smaller file
 with lower quality. Best quality is achieved by using a value of 100.

src/enc/config.c

@@ -138,3 +138,23 @@ int WebPValidateConfig(const WebPConfig* config) {
 
 //------------------------------------------------------------------------------
 
+#define MAX_LEVEL 9
+
+// Mapping between -z level and -m / -q parameter settings.
+static const struct {
+  uint8_t method_;
+  uint8_t quality_;
+} kLosslessPresets[MAX_LEVEL + 1] = {
+  { 0,  0 }, { 1, 20 }, { 2, 25 }, { 3, 30 }, { 3, 50 },
+  { 4, 50 }, { 4, 75 }, { 4, 90 }, { 5, 90 }, { 6, 100 }
+};
+
+int WebPConfigLosslessPreset(WebPConfig* config, int level) {
+  if (config == NULL || level < 0 || level > MAX_LEVEL) return 0;
+  config->lossless = 1;
+  config->method = kLosslessPresets[level].method_;
+  config->quality = kLosslessPresets[level].quality_;
+  return 1;
+}
+
+//------------------------------------------------------------------------------

src/webp/encode.h

@@ -20,7 +20,7 @@
 extern "C" {
 #endif
 
-#define WEBP_ENCODER_ABI_VERSION 0x0202    // MAJOR(8b) + MINOR(8b)
+#define WEBP_ENCODER_ABI_VERSION 0x0203    // MAJOR(8b) + MINOR(8b)
 
 // Note: forward declaring enumerations is not allowed in (strict) C and C++,
 // the types are left here for reference.
@@ -167,6 +167,14 @@ static WEBP_INLINE int WebPConfigPreset(WebPConfig* config,
                                 WEBP_ENCODER_ABI_VERSION);
 }
 
+// Activate the lossless compression mode with the desired efficiency level
+// between 0 (fastest, lowest compression) and 9 (slower, best compression).
+// A good default level is '6', providing a fair tradeoff between compression
+// speed and final compressed size.
+// This function will overwrite several fields from config: 'method', 'quality'
+// and 'lossless'. Returns false in case of parameter error.
+WEBP_EXTERN(int) WebPConfigLosslessPreset(WebPConfig* config, int level);
+
 // Returns true if 'config' is non-NULL and all configuration parameters are
 // within their valid ranges.
 WEBP_EXTERN(int) WebPValidateConfig(const WebPConfig* config);