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fix rescaling-with-alpha inaccuracy

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(still missing YUVA decoding case for now)

https://code.google.com/p/webp/issues/detail?id=160

Change-Id: If723b4a5c0a303d0853ec9d839f995adce056095
This commit is contained in:
Pascal Massimino
2013-07-26 12:05:42 -07:00
committed by skal
parent 11249abfc7
commit 8dcae8b3cf
10 changed files with 342 additions and 37 deletions
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43
src/enc/picture.c
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@ -16,6 +16,7 @@
#include <math.h>
#include "./vp8enci.h"
#include "../utils/alpha_processing.h"
#include "../utils/rescaler.h"
#include "../utils/utils.h"
#include "../dsp/dsp.h"
@ -399,6 +400,28 @@ static void RescalePlane(const uint8_t* src,
}
}
static void AlphaMultiplyARGB(WebPPicture* const pic, int inverse) {
uint32_t* ptr = pic->argb;
int y;
for (y = 0; y < pic->height; ++y) {
WebPMultARGBRow(ptr, pic->width, inverse);
ptr += pic->argb_stride;
}
}
static void AlphaMultiplyY(WebPPicture* const pic, int inverse) {
const uint8_t* ptr_a = pic->a;
if (ptr_a != NULL) {
uint8_t* ptr_y = pic->y;
int y;
for (y = 0; y < pic->height; ++y) {
WebPMultRow(ptr_y, ptr_a, pic->width, inverse);
ptr_y += pic->y_stride;
ptr_a += pic->a_stride;
}
}
}
int WebPPictureRescale(WebPPicture* pic, int width, int height) {
WebPPicture tmp;
int prev_width, prev_height;
@ -429,9 +452,19 @@ int WebPPictureRescale(WebPPicture* pic, int width, int height) {
WebPPictureFree(&tmp);
return 0;
}
// If present, we need to rescale alpha first (for AlphaMultiplyY).
if (pic->a != NULL) {
RescalePlane(pic->a, prev_width, prev_height, pic->a_stride,
tmp.a, width, height, tmp.a_stride, work, 1);
}
// We take transparency into account on the luma plane only. That's not
// totally exact blending, but still is a good approximation.
AlphaMultiplyY(pic, 0);
RescalePlane(pic->y, prev_width, prev_height, pic->y_stride,
tmp.y, width, height, tmp.y_stride, work, 1);
AlphaMultiplyY(&tmp, 1);
RescalePlane(pic->u,
HALVE(prev_width), HALVE(prev_height), pic->uv_stride,
tmp.u,
@ -441,10 +474,6 @@ int WebPPictureRescale(WebPPicture* pic, int width, int height) {
tmp.v,
HALVE(width), HALVE(height), tmp.uv_stride, work, 1);
if (tmp.a != NULL) {
RescalePlane(pic->a, prev_width, prev_height, pic->a_stride,
tmp.a, width, height, tmp.a_stride, work, 1);
}
#ifdef WEBP_EXPERIMENTAL_FEATURES
if (tmp.u0 != NULL) {
const int s = IS_YUV_CSP(tmp.colorspace, WEBP_YUV422) ? 2 : 1;
@ -462,12 +491,16 @@ int WebPPictureRescale(WebPPicture* pic, int width, int height) {
WebPPictureFree(&tmp);
return 0;
}
// In order to correctly interpolate colors, we need to apply the alpha
// weighting first (black-matting), scale the RGB values, and remove
// the premultiplication afterward (while preserving the alpha channel).
AlphaMultiplyARGB(pic, 0);
RescalePlane((const uint8_t*)pic->argb, prev_width, prev_height,
pic->argb_stride * 4,
(uint8_t*)tmp.argb, width, height,
tmp.argb_stride * 4,
work, 4);
AlphaMultiplyARGB(&tmp, 1);
}
WebPPictureFree(pic);
free(work);