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add a -jpeg_like option

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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
This commit is contained in:
skal
2013-02-05 19:40:18 +01:00
parent 1f803f645d
commit e895059a05
8 changed files with 80 additions and 29 deletions
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50
src/enc/quant.c
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@ -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.);