VP8EstimateQuality(): roughty estimate webp bitstream quality factor

This function returns a rough estimation of the quality factor used
for compressing the WebP bitstream.

Simple command line: 'webp_quality [-quiet] in.webp'
should print the estimated quality factor.

Change-Id: Ifba3e489461f5a587003ac9f08cc7556e9b24ac2

extras/webp_quality.c

@@ -0,0 +1,49 @@
+// Simple tool to roughly evaluate the quality encoding of a webp bitstream
+//
+// Result is a *rough* estimation of the quality. You should just consider
+// the bucket it's in (q > 80? > 50? > 20?) and not take it for face value.
+/*
+ gcc -o webp_quality webp_quality.c -O3 -I../ -L. -L../imageio \
+    -lexample_util -limagedec -lwebpextras -lwebp -L/opt/local/lib \
+    -lpng -lz -ljpeg -ltiff -lm -lpthread
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "./extras.h"
+#include "../imageio/example_util.h"
+
+int main(int argc, const char *argv[]) {
+  int c;
+  int quiet = 0;
+  for (c = 1; c < argc; ++c) {
+    if (!strcmp(argv[c], "-quiet")) {
+      quiet = 1;
+    } else if (!strcmp(argv[c], "-help") || !strcmp(argv[c], "-h")) {
+      printf("webp_quality [-h][-quiet] webp_files...\n");
+      return 1;
+    } else {
+      const char* const filename = argv[c];
+      const uint8_t* data = NULL;
+      size_t data_size = 0;
+      int q;
+      const int ok = ExUtilReadFile(filename, &data, &data_size);
+      if (!ok) continue;
+      q = VP8EstimateQuality(data, data_size);
+      if (!quiet) printf("[%s] ", filename);
+      if (q < 0) {
+        fprintf(stderr, "Not a WebP file, or not a lossy WebP file.\n");
+      } else {
+        if (!quiet) {
+          printf("Estimated quality factor: %d\n", q);
+        } else {
+          printf("%d\n", q);   // just print the number
+        }
+      }
+      free((void*)data);
+    }
+  }
+  return 1;
+}