add a -jpeg_like option

This option remaps internal parameters to better match
the expected compression curve of JPEG and produce output files
of similar size, but with better quality.

Change-Id: I96a1cbb480b1f6a0c6845a23c33dfd63f197b689

README

@@ -183,6 +183,7 @@ options:
   -progress .............. report encoding progress
 
 Experimental Options:
+  -jpeg_like ............. Roughly match expected JPEG size.
   -af .................... auto-adjust filter strength.
   -pre <int> ............. pre-processing filter
 

examples/cwebp.c

@@ -580,6 +580,7 @@ static void HelpLong(void) {
   printf("  -progress .............. report encoding progress\n");
   printf("\n");
   printf("Experimental Options:\n");
+  printf("  -jpeg_like ............. Roughly match expected JPEG size.\n");
   printf("  -af .................... auto-adjust filter strength.\n");
   printf("  -pre <int> ............. pre-processing filter\n");
   printf("\n");
@@ -719,6 +720,8 @@ int main(int argc, const char *argv[]) {
       config.filter_strength = strtol(argv[++c], NULL, 0);
     } else if (!strcmp(argv[c], "-af")) {
       config.autofilter = 1;
+    } else if (!strcmp(argv[c], "-jpeg_like")) {
+      config.emulate_jpeg_size = 1;
     } else if (!strcmp(argv[c], "-strong")) {
       config.filter_type = 1;
     } else if (!strcmp(argv[c], "-sharpness") && c < argc - 1) {

man/cwebp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH CWEBP 1 "February 01, 2013"
+.TH CWEBP 1 "February 6, 2013"
 .SH NAME
 cwebp \- compress an image file to a WebP file
 .SH SYNOPSIS
@@ -76,6 +76,12 @@ additional encoding possibilities and decide on the quality gain.
 Lower value can result is faster processing time at the expense of
 larger file size and lower compression quality.
 .TP
+.B \-jpeg_like
+Change the internal parameter mapping to better match the expected size
+of JPEG compression. This flag will generally produce an output file of
+similar size to its JPEG equivalent (for the same \fB\-q\fP setting), but
+with less visual distortion.
+.TP
 .B \-af
 Turns auto-filter on. This algorithm will spend additional time optimizing
 the filtering strength to reach a well-balanced quality.

src/enc/analysis.c

@@ -318,7 +318,8 @@ static int MBAnalyzeBestUVMode(VP8EncIterator* const it) {
 }
 
 static void MBAnalyze(VP8EncIterator* const it,
-                      int alphas[MAX_ALPHA + 1], int* const uv_alpha) {
+                      int alphas[MAX_ALPHA + 1],
+                      int* const alpha, int* const uv_alpha) {
   const VP8Encoder* const enc = it->enc_;
   int best_alpha, best_uv_alpha;
 
@@ -340,8 +341,11 @@ static void MBAnalyze(VP8EncIterator* const it,
   best_alpha = (3 * best_alpha + best_uv_alpha + 2) >> 2;
   best_alpha = FinalAlphaValue(best_alpha);
   alphas[best_alpha]++;
-  *uv_alpha += best_uv_alpha;
   it->mb_->alpha_ = best_alpha;   // for later remapping.
+
+  // Accumulate for later complexity analysis.
+  *alpha += best_alpha;   // mixed susceptibility (not just luma)
+  *uv_alpha += best_uv_alpha;
 }
 
 static void DefaultMBInfo(VP8MBInfo* const mb) {
@@ -362,35 +366,42 @@ static void DefaultMBInfo(VP8MBInfo* const mb) {
 // and decide intra4/intra16, but that's usually almost always a bad choice at
 // this stage.
 
+static void ResetAllMBInfo(VP8Encoder* const enc) {
+  int n;
+  for (n = 0; n < enc->mb_w_ * enc->mb_h_; ++n) {
+    DefaultMBInfo(&enc->mb_info_[n]);
+  }
+  // Default susceptibilities.
+  enc->dqm_[0].alpha_ = 0;
+  enc->dqm_[0].beta_ = 0;
+  // Note: we can't compute this alpha_ / uv_alpha_.
+  WebPReportProgress(enc->pic_, enc->percent_ + 20, &enc->percent_);
+}
+
 int VP8EncAnalyze(VP8Encoder* const enc) {
   int ok = 1;
   const int do_segments =
+      enc->config_->emulate_jpeg_size ||   // We need the complexity evaluation.
       (enc->segment_hdr_.num_segments_ > 1) ||
       (enc->method_ <= 2);  // for methods 0,1,2, we need preds_[] to be filled.
+  enc->alpha_ = 0;
+  enc->uv_alpha_ = 0;
   if (do_segments) {
     int alphas[MAX_ALPHA + 1] = { 0 };
     VP8EncIterator it;
 
     VP8IteratorInit(enc, &it);
-    enc->uv_alpha_ = 0;
     do {
       VP8IteratorImport(&it);
-      MBAnalyze(&it, alphas, &enc->uv_alpha_);
+      MBAnalyze(&it, alphas, &enc->alpha_, &enc->uv_alpha_);
       ok = VP8IteratorProgress(&it, 20);
       // Let's pretend we have perfect lossless reconstruction.
     } while (ok && VP8IteratorNext(&it, it.yuv_in_));
+    enc->alpha_ /= enc->mb_w_ * enc->mb_h_;
     enc->uv_alpha_ /= enc->mb_w_ * enc->mb_h_;
     if (ok) AssignSegments(enc, alphas);
   } else {   // Use only one default segment.
-    int n;
-    for (n = 0; n < enc->mb_w_ * enc->mb_h_; ++n) {
-      DefaultMBInfo(&enc->mb_info_[n]);
-    }
-    // Default susceptibilities.
-    enc->dqm_[0].alpha_ = 0;
-    enc->dqm_[0].beta_ = 0;
-    enc->uv_alpha_ = 0;   // we can't compute this one.
-    WebPReportProgress(enc->pic_, enc->percent_ + 20, &enc->percent_);
+    ResetAllMBInfo(enc);
   }
   return ok;
 }

src/enc/config.c

@@ -46,6 +46,7 @@ int WebPConfigInitInternal(WebPConfig* config,
   config->alpha_quality = 100;
   config->lossless = 0;
   config->image_hint = WEBP_HINT_DEFAULT;
+  config->emulate_jpeg_size = 0;
 
   // TODO(skal): tune.
   switch (preset) {
@@ -122,6 +123,8 @@ int WebPValidateConfig(const WebPConfig* config) {
     return 0;
   if (config->image_hint >= WEBP_HINT_LAST)
     return 0;
+  if (config->emulate_jpeg_size < 0 || config->emulate_jpeg_size > 1)
+    return 0;
   return 1;
 }
 

src/enc/quant.c

@@ -224,9 +224,35 @@ static void SetupFilterStrength(VP8Encoder* const enc) {
 // We want to emulate jpeg-like behaviour where the expected "good" quality
 // is around q=75. Internally, our "good" middle is around c=50. So we
 // map accordingly using linear piece-wise function
-static double QualityToCompression(double q) {
-  const double c = q / 100.;
-  return (c < 0.75) ? c * (2. / 3.) : 2. * c - 1.;
+static double QualityToCompression(double c) {
+  const double linear_c = (c < 0.75) ? c * (2. / 3.) : 2. * c - 1.;
+  // The file size roughly scales as pow(quantizer, 3.). Actually, the
+  // exponent is somewhere between 2.8 and 3.2, but we're mostly interested
+  // in the mid-quant range. So we scale the compressibility inversely to
+  // this power-law: quant ~= compression ^ 1/3. This law holds well for
+  // low quant. Finer modelling for high-quant would make use of kAcTable[]
+  // more explicitly.
+  const double v = pow(linear_c, 1 / 3.);
+  return v;
+}
+
+static double QualityToJPEGCompression(double c, double alpha) {
+  // We map the complexity 'alpha' and quality setting 'c' to a compression
+  // exponent empirically matched to the compression curve of libjpeg6b.
+  // On average, the WebP output size will be roughly similar to that of a
+  // JPEG file compressed with same quality factor.
+  const double amin = 0.30;
+  const double amax = 0.85;
+  const double exp_min = 0.4;
+  const double exp_max = 0.9;
+  const double slope = (exp_min - exp_max) / (amax - amin);
+  // Linearly interpolate 'expn' from exp_min to exp_max
+  // in the [amin, amax] range.
+  const double expn = (alpha > amax) ? exp_min
+                    : (alpha < amin) ? exp_max
+                    : exp_max + slope * (alpha - amin);
+  const double v = pow(c, expn);
+  return v;
 }
 
 static int SegmentsAreEquivalent(const VP8SegmentInfo* const S1,
@@ -274,18 +300,14 @@ void VP8SetSegmentParams(VP8Encoder* const enc, float quality) {
   int dq_uv_ac, dq_uv_dc;
   const int num_segments = enc->segment_hdr_.num_segments_;
   const double amp = SNS_TO_DQ * enc->config_->sns_strength / 100. / 128.;
-  const double c_base = QualityToCompression(quality);
+  const double Q = quality / 100.;
+  const double c_base = enc->config_->emulate_jpeg_size ?
+      QualityToJPEGCompression(Q, enc->alpha_ / 255.) :
+      QualityToCompression(Q);
   for (i = 0; i < num_segments; ++i) {
-    // The file size roughly scales as pow(quantizer, 3.). Actually, the
-    // exponent is somewhere between 2.8 and 3.2, but we're mostly interested
-    // in the mid-quant range. So we scale the compressibility inversely to
-    // this power-law: quant ~= compression ^ 1/3. This law holds well for
-    // low quant. Finer modelling for high-quant would make use of kAcTable[]
-    // more explicitely.
-    // Additionally, we modulate the base exponent 1/3 to accommodate for the
-    // quantization susceptibility and allow denser segments to be quantized
-    // more.
-    const double expn = (1. - amp * enc->dqm_[i].alpha_) / 3.;
+    // We modulate the base coefficient to accommodate for the quantization
+    // susceptibility and allow denser segments to be quantized more.
+    const double expn = 1. - amp * enc->dqm_[i].alpha_;
     const double c = pow(c_base, expn);
     const int q = (int)(127. * (1. - c));
     assert(expn > 0.);

src/enc/vp8enci.h

@@ -390,6 +390,7 @@ struct VP8Encoder {
   VP8SegmentInfo dqm_[NUM_MB_SEGMENTS];
   int base_quant_;                 // nominal quantizer value. Only used
                                    // for relative coding of segments' quant.
+  int alpha_;                      // global susceptibility (<=> complexity)
   int uv_alpha_;                   // U/V quantization susceptibility
   // global offset of quantizers, shared by all segments
   int dq_y1_dc_;

src/webp/encode.h

@@ -121,8 +121,12 @@ struct WebPConfig {
   int partition_limit;    // quality degradation allowed to fit the 512k limit
                           // on prediction modes coding (0: no degradation,
                           // 100: maximum possible degradation).
+  int emulate_jpeg_size;  // If true, compression parameters will be remapped
+                          // to better match the expected output size from
+                          // JPEG compression. Generally, the output size will
+                          // be similar but the degradation will be lower.
 
-  uint32_t pad[8];        // padding for later use
+  uint32_t pad[7];        // padding for later use
 };
 
 // Enumerate some predefined settings for WebPConfig, depending on the type