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Use deterministic random-dithering during RGB->YUV conversion

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-> helps debanding (sky, gradients, etc.)

This dithering can only be triggered when using -preset photo
or -pre 2 (as a preprocessing). Everything is unchanged otherwise.

Note that this change is likely to make the perceived PSNR/SSIM drop
since we're altering the input internally.

Change-Id: Id8d4326245d9b828141de162c94ba381b1fa5813
This commit is contained in:
skal 2013-10-17 22:36:49 +02:00
parent 8a2fa099cc
commit 0b2b05049f
7 changed files with 136 additions and 43 deletions
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7
man/cwebp.1
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@ -1,5 +1,5 @@
.\" Hey, EMACS: -*- nroff -*-
.TH CWEBP 1 "March 28, 2013"
.TH CWEBP 1 "October 17, 2013"
.SH NAME
cwebp \- compress an image file to a WebP file
.SH SYNOPSIS
@ -168,8 +168,9 @@ Output additional ASCII-map of encoding information. Possible map values
range from 1 to 6. This is only meant to help debugging.
.TP
.BI \-pre " int
Specify a pre-processing filter. This option is a placeholder
and has currently no effect.
Specify some pre-processing steps. Using a value of '2' will trigger
quality-dependent pseudo-random dithering during RGBA->YUVA conversion
(lossy compression only).
.TP
.BI \-alpha_filter " string
Specify the predictive filtering method for the alpha plane. One of 'none',

19
src/dec/vp8l.c
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@ -481,7 +481,8 @@ static void ConvertToYUVA(const uint32_t* const src, int width, int y_pos,
uint8_t* const y = buf->y + y_pos * buf->y_stride;
for (i = 0; i < width; ++i) {
const uint32_t p = src[i];
y[i] = VP8RGBToY((p >> 16) & 0xff, (p >> 8) & 0xff, (p >> 0) & 0xff);
y[i] = VP8RGBToY((p >> 16) & 0xff, (p >> 8) & 0xff, (p >> 0) & 0xff,
YUV_HALF);
}
}
@ -500,11 +501,11 @@ static void ConvertToYUVA(const uint32_t* const src, int width, int y_pos,
const int g = ((v0 >> 7) & 0x1fe) + ((v1 >> 7) & 0x1fe);
const int b = ((v0 << 1) & 0x1fe) + ((v1 << 1) & 0x1fe);
if (!(y_pos & 1)) { // even lines: store values
u[i] = VP8RGBToU(r, g, b);
v[i] = VP8RGBToV(r, g, b);
u[i] = VP8RGBToU(r, g, b, YUV_HALF << 2);
v[i] = VP8RGBToV(r, g, b, YUV_HALF << 2);
} else { // odd lines: average with previous values
const int tmp_u = VP8RGBToU(r, g, b);
const int tmp_v = VP8RGBToV(r, g, b);
const int tmp_u = VP8RGBToU(r, g, b, YUV_HALF << 2);
const int tmp_v = VP8RGBToV(r, g, b, YUV_HALF << 2);
// Approximated average-of-four. But it's an acceptable diff.
u[i] = (u[i] + tmp_u + 1) >> 1;
v[i] = (v[i] + tmp_v + 1) >> 1;
@ -516,11 +517,11 @@ static void ConvertToYUVA(const uint32_t* const src, int width, int y_pos,
const int g = (v0 >> 6) & 0x3fc;
const int b = (v0 << 2) & 0x3fc;
if (!(y_pos & 1)) { // even lines
u[i] = VP8RGBToU(r, g, b);
v[i] = VP8RGBToV(r, g, b);
u[i] = VP8RGBToU(r, g, b, YUV_HALF << 2);
v[i] = VP8RGBToV(r, g, b, YUV_HALF << 2);
} else { // odd lines (note: we could just skip this)
const int tmp_u = VP8RGBToU(r, g, b);
const int tmp_v = VP8RGBToV(r, g, b);
const int tmp_u = VP8RGBToU(r, g, b, YUV_HALF << 2);
const int tmp_v = VP8RGBToV(r, g, b, YUV_HALF << 2);
u[i] = (u[i] + tmp_u + 1) >> 1;
v[i] = (v[i] + tmp_v + 1) >> 1;
}

33
src/dsp/yuv.h
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@ -265,27 +265,27 @@ extern void VP8YUVInitSSE2(void);
//------------------------------------------------------------------------------
// RGB -> YUV conversion
static WEBP_INLINE int VP8ClipUV(int v) {
v = (v + (257 << (YUV_FIX + 2 - 1))) >> (YUV_FIX + 2);
return ((v & ~0xff) == 0) ? v : (v < 0) ? 0 : 255;
// Stub functions that can be called with various rounding values:
static WEBP_INLINE int VP8ClipUV(int uv, int rounding) {
uv = (uv + rounding + (128 << (YUV_FIX + 2))) >> (YUV_FIX + 2);
return ((uv & ~0xff) == 0) ? uv : (uv < 0) ? 0 : 255;
}
#ifndef USE_YUVj
static WEBP_INLINE int VP8RGBToY(int r, int g, int b) {
const int kRound = (1 << (YUV_FIX - 1)) + (16 << YUV_FIX);
static WEBP_INLINE int VP8RGBToY(int r, int g, int b, int rounding) {
const int luma = 16839 * r + 33059 * g + 6420 * b;
return (luma + kRound) >> YUV_FIX; // no need to clip
return (luma + rounding + (16 << YUV_FIX)) >> YUV_FIX; // no need to clip
}
static WEBP_INLINE int VP8RGBToU(int r, int g, int b) {
static WEBP_INLINE int VP8RGBToU(int r, int g, int b, int rounding) {
const int u = -9719 * r - 19081 * g + 28800 * b;
return VP8ClipUV(u);
return VP8ClipUV(u, rounding);
}
static WEBP_INLINE int VP8RGBToV(int r, int g, int b) {
static WEBP_INLINE int VP8RGBToV(int r, int g, int b, int rounding) {
const int v = +28800 * r - 24116 * g - 4684 * b;
return VP8ClipUV(v);
return VP8ClipUV(v, rounding);
}
#else
@ -293,20 +293,19 @@ static WEBP_INLINE int VP8RGBToV(int r, int g, int b) {
// This JPEG-YUV colorspace, only for comparison!
// These are also 16bit precision coefficients from Rec.601, but with full
// [0..255] output range.
static WEBP_INLINE int VP8RGBToY(int r, int g, int b) {
const int kRound = (1 << (YUV_FIX - 1));
static WEBP_INLINE int VP8RGBToY(int r, int g, int b, int rounding) {
const int luma = 19595 * r + 38470 * g + 7471 * b;
return (luma + kRound) >> YUV_FIX; // no need to clip
return (luma + rounding) >> YUV_FIX; // no need to clip
}
static WEBP_INLINE int VP8RGBToU(int r, int g, int b) {
static WEBP_INLINE int VP8_RGB_TO_U(int r, int g, int b, int rounding) {
const int u = -11058 * r - 21710 * g + 32768 * b;
return VP8ClipUV(u);
return VP8ClipUV(u, rounding);
}
static WEBP_INLINE int VP8RGBToV(int r, int g, int b) {
static WEBP_INLINE int VP8_RGB_TO_V(int r, int g, int b, int rounding) {
const int v = 32768 * r - 27439 * g - 5329 * b;
return VP8ClipUV(v);
return VP8ClipUV(v, rounding);
}
#endif // USE_YUVj

6
src/enc/config.c
View File

@ -58,11 +58,13 @@ int WebPConfigInitInternal(WebPConfig* config,
config->sns_strength = 80;
config->filter_sharpness = 4;
config->filter_strength = 35;
config->preprocessing &= ~2; // no dithering
break;
case WEBP_PRESET_PHOTO:
config->sns_strength = 80;
config->filter_sharpness = 3;
config->filter_strength = 30;
config->preprocessing |= 2;
break;
case WEBP_PRESET_DRAWING:
config->sns_strength = 25;
@ -72,10 +74,12 @@ int WebPConfigInitInternal(WebPConfig* config,
case WEBP_PRESET_ICON:
config->sns_strength = 0;
config->filter_strength = 0; // disable filtering to retain sharpness
config->preprocessing &= ~2; // no dithering
break;
case WEBP_PRESET_TEXT:
config->sns_strength = 0;
config->filter_strength = 0; // disable filtering to retain sharpness
config->preprocessing &= ~2; // no dithering
config->segments = 2;
break;
case WEBP_PRESET_DEFAULT:
@ -111,7 +115,7 @@ int WebPValidateConfig(const WebPConfig* config) {
return 0;
if (config->show_compressed < 0 || config->show_compressed > 1)
return 0;
if (config->preprocessing < 0 || config->preprocessing > 1)
if (config->preprocessing < 0 || config->preprocessing > 3)
return 0;
if (config->partitions < 0 || config->partitions > 3)
return 0;

92
src/enc/picture.c
View File

@ -590,6 +590,64 @@ int WebPPictureHasTransparency(const WebPPicture* picture) {
//------------------------------------------------------------------------------
// RGB -> YUV conversion
#define DITHER_FIX 8 // fixed-point precision for dithering
#define kRandomTableSize 55
static const uint32_t kRandomTable[kRandomTableSize] = { // 31b-range values
0x0de15230, 0x03b31886, 0x775faccb, 0x1c88626a, 0x68385c55, 0x14b3b828,
0x4a85fef8, 0x49ddb84b, 0x64fcf397, 0x5c550289, 0x4a290000, 0x0d7ec1da,
0x5940b7ab, 0x5492577d, 0x4e19ca72, 0x38d38c69, 0x0c01ee65, 0x32a1755f,
0x5437f652, 0x5abb2c32, 0x0faa57b1, 0x73f533e7, 0x685feeda, 0x7563cce2,
0x6e990e83, 0x4730a7ed, 0x4fc0d9c6, 0x496b153c, 0x4f1403fa, 0x541afb0c,
0x73990b32, 0x26d7cb1c, 0x6fcc3706, 0x2cbb77d8, 0x75762f2a, 0x6425ccdd,
0x24b35461, 0x0a7d8715, 0x220414a8, 0x141ebf67, 0x56b41583, 0x73e502e3,
0x44cab16f, 0x28264d42, 0x73baaefb, 0x0a50ebed, 0x1d6ab6fb, 0x0d3ad40b,
0x35db3b68, 0x2b081e83, 0x77ce6b95, 0x5181e5f0, 0x78853bbc, 0x009f9494,
0x27e5ed3c
};
typedef struct {
int index1_, index2_;
uint32_t tab_[kRandomTableSize];
int amp_;
} VP8Random;
static void InitRandom(VP8Random* const rg, float dithering) {
memcpy(rg->tab_, kRandomTable, sizeof(rg->tab_));
rg->index1_ = 0;
rg->index2_ = 31;
rg->amp_ = (dithering < 0.0) ? 0
: (dithering > 1.0) ? (1 << DITHER_FIX)
: (uint32_t)((1 << DITHER_FIX) * dithering);
}
// D.Knuth's Difference-based random generator.
static WEBP_INLINE int Random(VP8Random* const rg, int num_bits) {
int diff;
assert(num_bits + DITHER_FIX <= 31);
diff = rg->tab_[rg->index1_] - rg->tab_[rg->index2_];
if (diff < 0) diff += (1u << 31);
rg->tab_[rg->index1_] = diff;
if (++rg->index1_ == kRandomTableSize) rg->index1_ = 0;
if (++rg->index2_ == kRandomTableSize) rg->index2_ = 0;
diff = (diff << 1) >> (32 - num_bits); // sign-extend, 0-center
diff = (diff * rg->amp_) >> DITHER_FIX; // restrict range
diff += 1 << (num_bits - 1); // shift back to 0.5-center
return diff;
}
static int RGBToY(int r, int g, int b, VP8Random* const rg) {
return VP8RGBToY(r, g, b, Random(rg, YUV_FIX));
}
static int RGBToU(int r, int g, int b, VP8Random* const rg) {
return VP8RGBToU(r, g, b, Random(rg, YUV_FIX + 2));
}
static int RGBToV(int r, int g, int b, VP8Random* const rg) {
return VP8RGBToV(r, g, b, Random(rg, YUV_FIX + 2));
}
// TODO: we can do better than simply 2x2 averaging on U/V samples.
#define SUM4(ptr) ((ptr)[0] + (ptr)[step] + \
(ptr)[rgb_stride] + (ptr)[rgb_stride + step])
@ -602,8 +660,8 @@ int WebPPictureHasTransparency(const WebPPicture* picture) {
const int r = SUM(r_ptr + src); \
const int g = SUM(g_ptr + src); \
const int b = SUM(b_ptr + src); \
picture->u[dst] = VP8RGBToU(r, g, b); \
picture->v[dst] = VP8RGBToV(r, g, b); \
picture->u[dst] = RGBToU(r, g, b, &rg); \
picture->v[dst] = RGBToV(r, g, b, &rg); \
}
#define RGB_TO_UV0(x_in, x_out, y, SUM) { \
@ -612,8 +670,8 @@ int WebPPictureHasTransparency(const WebPPicture* picture) {
const int r = SUM(r_ptr + src); \
const int g = SUM(g_ptr + src); \
const int b = SUM(b_ptr + src); \
picture->u0[dst] = VP8RGBToU(r, g, b); \
picture->v0[dst] = VP8RGBToV(r, g, b); \
picture->u0[dst] = RGBToU(r, g, b, &rg); \
picture->v0[dst] = RGBToV(r, g, b, &rg); \
}
static void MakeGray(WebPPicture* const picture) {
@ -632,12 +690,14 @@ static int ImportYUVAFromRGBA(const uint8_t* const r_ptr,
const uint8_t* const a_ptr,
int step, // bytes per pixel
int rgb_stride, // bytes per scanline
float dithering,
WebPPicture* const picture) {
const WebPEncCSP uv_csp = picture->colorspace & WEBP_CSP_UV_MASK;
int x, y;
const int width = picture->width;
const int height = picture->height;
const int has_alpha = CheckNonOpaque(a_ptr, width, height, step, rgb_stride);
VP8Random rg;
picture->colorspace = uv_csp;
picture->use_argb = 0;
@ -646,12 +706,14 @@ static int ImportYUVAFromRGBA(const uint8_t* const r_ptr,
}
if (!WebPPictureAlloc(picture)) return 0;
InitRandom(&rg, dithering);
// Import luma plane
for (y = 0; y < height; ++y) {
for (x = 0; x < width; ++x) {
const int offset = step * x + y * rgb_stride;
picture->y[x + y * picture->y_stride] =
VP8RGBToY(r_ptr[offset], g_ptr[offset], b_ptr[offset]);
RGBToY(r_ptr[offset], g_ptr[offset], b_ptr[offset], &rg);
}
}
@ -722,7 +784,7 @@ static int Import(WebPPicture* const picture,
if (!picture->use_argb) {
return ImportYUVAFromRGBA(r_ptr, g_ptr, b_ptr, a_ptr, step, rgb_stride,
picture);
0.f /* no dithering */, picture);
}
if (import_alpha) {
picture->colorspace |= WEBP_CSP_ALPHA_BIT;
@ -855,7 +917,8 @@ int WebPPictureYUVAToARGB(WebPPicture* picture) {
return 1;
}
int WebPPictureARGBToYUVA(WebPPicture* picture, WebPEncCSP colorspace) {
int WebPPictureARGBToYUVADithered(WebPPicture* picture, WebPEncCSP colorspace,
float dithering) {
if (picture == NULL) return 0;
if (picture->argb == NULL) {
return WebPEncodingSetError(picture, VP8_ENC_ERROR_NULL_PARAMETER);
@ -871,7 +934,8 @@ int WebPPictureARGBToYUVA(WebPPicture* picture, WebPEncCSP colorspace) {
PictureResetARGB(&tmp); // reset ARGB buffer so that it's not free()'d.
tmp.use_argb = 0;
tmp.colorspace = colorspace & WEBP_CSP_UV_MASK;
if (!ImportYUVAFromRGBA(r, g, b, a, 4, 4 * picture->argb_stride, &tmp)) {
if (!ImportYUVAFromRGBA(r, g, b, a, 4, 4 * picture->argb_stride, dithering,
&tmp)) {
return WebPEncodingSetError(picture, VP8_ENC_ERROR_OUT_OF_MEMORY);
}
// Copy back the YUV specs into 'picture'.
@ -883,6 +947,10 @@ int WebPPictureARGBToYUVA(WebPPicture* picture, WebPEncCSP colorspace) {
return 1;
}
int WebPPictureARGBToYUVA(WebPPicture* picture, WebPEncCSP colorspace) {
return WebPPictureARGBToYUVADithered(picture, colorspace, 0.f);
}
//------------------------------------------------------------------------------
// Helper: clean up fully transparent area to help compressibility.
@ -960,14 +1028,16 @@ void WebPBlendAlpha(WebPPicture* pic, uint32_t background_rgb) {
const int red = (background_rgb >> 16) & 0xff;
const int green = (background_rgb >> 8) & 0xff;
const int blue = (background_rgb >> 0) & 0xff;
VP8Random rg;
int x, y;
if (pic == NULL) return;
InitRandom(&rg, 0.f);
if (!pic->use_argb) {
const int uv_width = (pic->width >> 1); // omit last pixel during u/v loop
const int Y0 = VP8RGBToY(red, green, blue);
const int Y0 = RGBToY(red, green, blue, &rg);
// VP8RGBToU/V expects the u/v values summed over four pixels
const int U0 = VP8RGBToU(4 * red, 4 * green, 4 * blue);
const int V0 = VP8RGBToV(4 * red, 4 * green, 4 * blue);
const int U0 = RGBToU(4 * red, 4 * green, 4 * blue, &rg);
const int V0 = RGBToV(4 * red, 4 * green, 4 * blue, &rg);
const int has_alpha = pic->colorspace & WEBP_CSP_ALPHA_BIT;
if (!has_alpha || pic->a == NULL) return; // nothing to do
for (y = 0; y < pic->height; ++y) {

12
src/enc/webpenc.c
View File

@ -358,7 +358,17 @@ int WebPEncode(const WebPConfig* config, WebPPicture* pic) {
VP8Encoder* enc = NULL;
if (pic->y == NULL || pic->u == NULL || pic->v == NULL) {
// Make sure we have YUVA samples.
if (!WebPPictureARGBToYUVA(pic, WEBP_YUV420)) return 0;
float dithering = 0.f;
if (config->preprocessing & 2) {
const float x = config->quality / 100.f;
const float x2 = x * x;
// slowly decreasing from max dithering at low quality (q->0)
// to 0.5 dithering amplitude at high quality (q->100)
dithering = 1.0f + (0.5f - 1.0f) * x2 * x2;
}
if (!WebPPictureARGBToYUVADithered(pic, WEBP_YUV420, dithering)) {
return 0;
}
}
enc = InitVP8Encoder(config, pic);

10
src/webp/encode.h
View File

@ -117,7 +117,8 @@ struct WebPConfig {
int show_compressed; // if true, export the compressed picture back.
// In-loop filtering is not applied.
int preprocessing; // preprocessing filter (0=none, 1=segment-smooth)
int preprocessing; // preprocessing filter:
// 0=none, 1=segment-smooth, 2=pseudo-random dithering
int partitions; // log2(number of token partitions) in [0..3]. Default
// is set to 0 for easier progressive decoding.
int partition_limit; // quality degradation allowed to fit the 512k limit
@ -443,6 +444,13 @@ WEBP_EXTERN(int) WebPPictureImportBGRX(
WEBP_EXTERN(int) WebPPictureARGBToYUVA(WebPPicture* picture,
WebPEncCSP colorspace);
// Same as WebPPictureARGBToYUVA(), but the conversion is done using
// pseudo-random dithering with a strength 'dithering' between
// 0.0 (no dithering) and 1.0 (maximum dithering). This is useful
// for photographic picture.
WEBP_EXTERN(int) WebPPictureARGBToYUVADithered(
WebPPicture* picture, WebPEncCSP colorspace, float dithering);
// Converts picture->yuv to picture->argb and sets picture->use_argb to true.
// The input format must be YUV_420 or YUV_420A.
// Note that the use of this method is discouraged if one has access to the