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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
Download Patch File Download Diff File Expand all files Collapse all files

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);