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/Makefile.am

@@ -5,15 +5,21 @@ noinst_HEADERS =
 noinst_HEADERS += ../src/webp/types.h
 
 libwebpextras_la_SOURCES =
-libwebpextras_la_SOURCES += extras.c extras.h
+libwebpextras_la_SOURCES += extras.c extras.h quality_estimate.c
 
 libwebpextras_la_CPPFLAGS = $(AM_CPPFLAGS)
 libwebpextras_la_LIBADD = ../src/libwebp.la
 
-noinst_PROGRAMS = get_disto
+noinst_PROGRAMS = get_disto webp_quality
 
 get_disto_SOURCES = get_disto.c
 get_disto_CPPFLAGS  = $(AM_CPPFLAGS)
 get_disto_LDADD = ../imageio/libexample_util.la ../imageio/libimagedec.la
 get_disto_LDADD += ../src/libwebp.la
 get_disto_LDADD += $(PNG_LIBS) $(JPEG_LIBS) $(TIFF_LIBS)
+
+webp_quality_SOURCES  = webp_quality.c
+webp_quality_CPPFLAGS = $(AM_CPPFLAGS) $(USE_EXPERIMENTAL_CODE)
+webp_quality_LDADD  = ../imageio/libexample_util.la ../imageio/libimagedec.la
+webp_quality_LDADD += ./libwebpextras.la
+webp_quality_LDADD += ../src/libwebp.la $(JPEG_LIBS) $(PNG_LIBS) $(TIFF_LIBS)

extras/extras.h

@@ -44,6 +44,14 @@ WEBP_EXTERN(int) WebPImportRGB4444(const uint8_t* rgb4444, WebPPicture* pic);
 
 //------------------------------------------------------------------------------
 
+// Parse a bitstream, search for VP8 (lossy) header and report a
+// rough estimation of the quality factor used for compressing the bitstream.
+// Any error (invalid bitstream, lossless compression, incomplete header, etc.)
+// will return a value of -1.
+WEBP_EXTERN(int) VP8EstimateQuality(const uint8_t* const data, size_t size);
+
+//------------------------------------------------------------------------------
+
 #ifdef __cplusplus
 }    // extern "C"
 #endif

extras/quality_estimate.c

@@ -0,0 +1,121 @@
+// Copyright 2016 Google Inc. All Rights Reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the COPYING file in the root of the source
+// tree. An additional intellectual property rights grant can be found
+// in the file PATENTS. All contributing project authors may
+// be found in the AUTHORS file in the root of the source tree.
+// -----------------------------------------------------------------------------
+//
+//  VP8EstimateQuality(): rough encoding quality estimate
+//
+// Author: Skal (pascal.massimino@gmail.com)
+
+#include "./extras.h"
+
+#include <math.h>
+
+//------------------------------------------------------------------------------
+
+#define INVALID_BIT_POS (1ull << 63)
+
+// In most cases, we don't need to use a full arithmetic decoder, since
+// all the header's bits are written using a uniform probability of 128.
+// We can just parse the header as if it was bits (works in 99.999% cases).
+static WEBP_INLINE uint32_t GetBit(const uint8_t* const data, size_t nb,
+                                   uint64_t max_size, uint64_t* const bit_pos) {
+  uint32_t val = 0;
+  if (*bit_pos + nb <= 8 * max_size) {
+    while (nb-- > 0) {
+      const uint64_t p = (*bit_pos)++;
+      const int bit = !!(data[p >> 3] & (128 >> ((p & 7))));
+      val = (val << 1) | bit;
+    }
+  } else {
+    *bit_pos = INVALID_BIT_POS;
+  }
+  return val;
+}
+
+#define GET_BIT(n) GetBit(data, (n), size, &bit_pos)
+#define CONDITIONAL_SKIP(n) (GET_BIT(1) ? GET_BIT((n)) : 0)
+
+int VP8EstimateQuality(const uint8_t* const data, size_t size) {
+  size_t pos = 0;
+  uint64_t bit_pos;
+  uint64_t sig = 0x00;
+  int ok = 0;
+  int Q = -1;
+
+  if (data == NULL) return -1;
+
+  while (pos < size) {
+    sig = (sig >> 8) | ((uint64_t)data[pos++] << 40);
+    if ((sig >> 24) == 0x2a019dull) {
+      ok = 1;
+      break;
+    }
+  }
+  if (!ok) return -1;
+  if (pos + 4 > size) return -1;
+
+  // Skip main Header
+  // width  = (data[pos + 0] | (data[pos + 1] << 8)) & 0x3fff;
+  // height = (data[pos + 2] | (data[pos + 3] << 8)) & 0x3fff;
+  pos += 4;
+  bit_pos = pos * 8;
+
+  GET_BIT(2);  // color_space + clamp type
+
+  // Segment header
+  if (GET_BIT(1)) {       // use_segment_
+    int s;
+    const int update_map = GET_BIT(1);
+    if (GET_BIT(1)) {     // update data
+      const int absolute_delta = GET_BIT(1);
+      int q[4]  = { 0, 0, 0, 0 };
+      for (s = 0; s < 4; ++s) {
+        if (GET_BIT(1)) {
+          q[s] = GET_BIT(7);
+          if (GET_BIT(1)) q[s] = -q[s];   // sign
+        }
+      }
+      if (absolute_delta) Q = q[0];  // just use the first segment's quantizer
+      for (s = 0; s < 4; ++s) CONDITIONAL_SKIP(7);   //  filter strength
+    }
+    if (update_map) {
+      for (s = 0; s < 3; ++s) CONDITIONAL_SKIP(8);
+    }
+  }
+  // Filter header
+  GET_BIT(1 + 6 + 3);     // simple + level + sharpness
+  if (GET_BIT(1)) {       // use_lf_delta
+    if (GET_BIT(1)) {     // update lf_delta?
+      int n;
+      for (n = 0; n < 4 + 4; ++n) CONDITIONAL_SKIP(6);
+    }
+  }
+  // num partitions
+  GET_BIT(2);
+
+  // ParseQuant
+  {
+    const int base_q = GET_BIT(7);
+    /* dqy1_dc = */ CONDITIONAL_SKIP(5);
+    /* dqy2_dc = */ CONDITIONAL_SKIP(5);
+    /* dqy2_ac = */ CONDITIONAL_SKIP(5);
+    /* dquv_dc = */ CONDITIONAL_SKIP(5);
+    /* dquv_ac = */ CONDITIONAL_SKIP(5);
+
+    if (Q < 0) Q = base_q;
+  }
+  if (bit_pos == INVALID_BIT_POS) return -1;
+
+  // base mapping
+  Q = (127 - Q) * 100 / 127;
+  // correction for power-law behavior in low range
+  if (Q < 80) {
+    Q = (int)(pow(Q / 80., 1. / 0.38) * 80);
+  }
+  return Q;
+}

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;
+}

makefile.unix

@@ -252,6 +252,7 @@ UTILS_ENC_OBJS = \
 
 EXTRA_OBJS = \
     extras/extras.o \
+    extras/quality_estimate.o \
 
 LIBWEBPDECODER_OBJS = $(DEC_OBJS) $(DSP_DEC_OBJS) $(UTILS_DEC_OBJS)
 LIBWEBP_OBJS = $(LIBWEBPDECODER_OBJS) $(ENC_OBJS) $(DSP_ENC_OBJS) \
@@ -311,17 +312,18 @@ OUT_LIBS += src/libwebp.a
 EXTRA_LIB = extras/libwebpextras.a
 OUT_EXAMPLES = examples/cwebp examples/dwebp
 EXTRA_EXAMPLES = examples/gif2webp examples/vwebp examples/webpmux \
-                 examples/anim_diff extras/get_disto
+                 examples/anim_diff
+OTHER_EXAMPLES = extras/get_disto extras/webp_quality
 
 OUTPUT = $(OUT_LIBS) $(OUT_EXAMPLES)
 ifeq ($(MAKECMDGOALS),clean)
-  OUTPUT += $(EXTRA_EXAMPLES)
+  OUTPUT += $(EXTRA_EXAMPLES) $(OTHER_EXAMPLES)
   OUTPUT += src/demux/libwebpdemux.a src/mux/libwebpmux.a $(EXTRA_LIB)
   OUTPUT += examples/libgifdec.a examples/libanim_util.a
 endif
 
 ex: $(OUT_EXAMPLES)
-all: ex $(EXTRA_EXAMPLES)
+all: ex $(EXTRA_EXAMPLES) $(OTHER_EXAMPLES)
 extras: $(EXTRA_LIB)
 
 $(EX_FORMAT_DEC_OBJS): %.o: %.h
@@ -379,11 +381,18 @@ examples/webpmux: imageio/libexample_util.a src/mux/libwebpmux.a
 examples/webpmux: src/libwebpdecoder.a
 
 extras/get_disto: extras/get_disto.o
-extras/get_disto: imageio/libimagedec.a imageio/libexampledec.a
+extras/get_disto: imageio/libimagedec.a
 extras/get_disto: imageio/libexample_util.a
-extras/get_disto: src/libwebp.a $(CWEBP_LIBS)
+extras/get_disto: src/libwebp.a $(EXTRA_LIB)
+extras/get_disto: EXTRA_LIBS += $(CWEBP_LIBS)
 
-$(OUT_EXAMPLES) $(EXTRA_EXAMPLES):
+extras/webp_quality: extras/webp_quality.o
+extras/webp_quality: imageio/libimagedec.a
+extras/webp_quality: imageio/libexample_util.a
+extras/webp_quality: src/libwebp.a $(EXTRA_LIB)
+extras/get_disto: EXTRA_LIBS += $(CWEBP_LIBS)
+
+$(OUT_EXAMPLES) $(EXTRA_EXAMPLES) $(OTHER_EXAMPLES):
 	$(CC) -o $@ $^ $(LDFLAGS)
 
 dist: DESTDIR := dist