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Lossy encoding: smoothen transparent areas to improve compression

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If "exact" is false, we can modify the luma samples in fully transparent
areas to facilitate lossy compression. Experiments on some PNG images
show compression improvement of more than 20%.

Change-Id: I1a728cfa920a6652bc1f600d87c01f7f648c4942
This commit is contained in:
hui su 2017-07-05 10:03:01 -07:00
parent e50650c77f
commit 7d67a1646d
3 changed files with 86 additions and 39 deletions
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113
src/enc/picture_tools_enc.c
View File

@ -25,20 +25,7 @@ static WEBP_INLINE uint32_t MakeARGB32(int r, int g, int b) {
#define SIZE 8
#define SIZE2 (SIZE / 2)
static int is_transparent_area(const uint8_t* ptr, int stride, int size) {
int y, x;
for (y = 0; y < size; ++y) {
for (x = 0; x < size; ++x) {
if (ptr[x]) {
return 0;
}
}
ptr += stride;
}
return 1;
}
static int is_transparent_argb_area(const uint32_t* ptr, int stride, int size) {
static int IsTransparentARGBArea(const uint32_t* ptr, int stride, int size) {
int y, x;
for (y = 0; y < size; ++y) {
for (x = 0; x < size; ++x) {
@ -51,7 +38,7 @@ static int is_transparent_argb_area(const uint32_t* ptr, int stride, int size) {
return 1;
}
static void flatten(uint8_t* ptr, int v, int stride, int size) {
static void Flatten(uint8_t* ptr, int v, int stride, int size) {
int y;
for (y = 0; y < size; ++y) {
memset(ptr, v, size);
@ -59,7 +46,7 @@ static void flatten(uint8_t* ptr, int v, int stride, int size) {
}
}
static void flatten_argb(uint32_t* ptr, uint32_t v, int stride, int size) {
static void FlattenARGB(uint32_t* ptr, uint32_t v, int stride, int size) {
int x, y;
for (y = 0; y < size; ++y) {
for (x = 0; x < size; ++x) ptr[x] = v;
@ -67,54 +54,114 @@ static void flatten_argb(uint32_t* ptr, uint32_t v, int stride, int size) {
}
}
// Smoothen the luma components of transparent pixels. Return true if the whole
// block is transparent.
static int SmoothenBlock(const uint8_t* a_ptr, int a_stride, uint8_t* y_ptr,
int y_stride, int width, int height) {
int sum = 0, count = 0;
int x, y;
const uint8_t* alpha_ptr = a_ptr;
uint8_t* luma_ptr = y_ptr;
for (y = 0; y < height; ++y) {
for (x = 0; x < width; ++x) {
if (alpha_ptr[x] != 0) {
++count;
sum += luma_ptr[x];
}
}
alpha_ptr += a_stride;
luma_ptr += y_stride;
}
if (count > 0 && count < width * height) {
const uint8_t avg_u8 = (uint8_t)(sum / count);
alpha_ptr = a_ptr;
luma_ptr = y_ptr;
for (y = 0; y < height; ++y) {
for (x = 0; x < width; ++x) {
if (alpha_ptr[x] == 0) luma_ptr[x] = avg_u8;
}
alpha_ptr += a_stride;
luma_ptr += y_stride;
}
}
return (count == 0);
}
void WebPCleanupTransparentArea(WebPPicture* pic) {
int x, y, w, h;
if (pic == NULL) return;
w = pic->width / SIZE;
h = pic->height / SIZE;
// note: we ignore the left-overs on right/bottom
// note: we ignore the left-overs on right/bottom, except for SmoothenBlock().
if (pic->use_argb) {
uint32_t argb_value = 0;
for (y = 0; y < h; ++y) {
int need_reset = 1;
for (x = 0; x < w; ++x) {
const int off = (y * pic->argb_stride + x) * SIZE;
if (is_transparent_argb_area(pic->argb + off, pic->argb_stride, SIZE)) {
if (IsTransparentARGBArea(pic->argb + off, pic->argb_stride, SIZE)) {
if (need_reset) {
argb_value = pic->argb[off];
need_reset = 0;
}
flatten_argb(pic->argb + off, argb_value, pic->argb_stride, SIZE);
FlattenARGB(pic->argb + off, argb_value, pic->argb_stride, SIZE);
} else {
need_reset = 1;
}
}
}
} else {
const uint8_t* const a_ptr = pic->a;
const int width = pic->width;
const int height = pic->height;
const int y_stride = pic->y_stride;
const int uv_stride = pic->uv_stride;
const int a_stride = pic->a_stride;
uint8_t* y_ptr = pic->y;
uint8_t* u_ptr = pic->u;
uint8_t* v_ptr = pic->v;
const uint8_t* a_ptr = pic->a;
int values[3] = { 0 };
if (a_ptr == NULL) return; // nothing to do
for (y = 0; y < h; ++y) {
if (a_ptr == NULL || y_ptr == NULL || u_ptr == NULL || v_ptr == NULL) {
return;
}
for (y = 0; y + SIZE <= height; y += SIZE) {
int need_reset = 1;
for (x = 0; x < w; ++x) {
const int off_a = (y * pic->a_stride + x) * SIZE;
const int off_y = (y * pic->y_stride + x) * SIZE;
const int off_uv = (y * pic->uv_stride + x) * SIZE2;
if (is_transparent_area(a_ptr + off_a, pic->a_stride, SIZE)) {
for (x = 0; x + SIZE <= width; x += SIZE) {
if (SmoothenBlock(a_ptr + x, a_stride, y_ptr + x, y_stride,
SIZE, SIZE)) {
if (need_reset) {
values[0] = pic->y[off_y];
values[1] = pic->u[off_uv];
values[2] = pic->v[off_uv];
values[0] = y_ptr[x];
values[1] = u_ptr[x >> 1];
values[2] = v_ptr[x >> 1];
need_reset = 0;
}
flatten(pic->y + off_y, values[0], pic->y_stride, SIZE);
flatten(pic->u + off_uv, values[1], pic->uv_stride, SIZE2);
flatten(pic->v + off_uv, values[2], pic->uv_stride, SIZE2);
Flatten(y_ptr + x, values[0], y_stride, SIZE);
Flatten(u_ptr + (x >> 1), values[1], uv_stride, SIZE2);
Flatten(v_ptr + (x >> 1), values[2], uv_stride, SIZE2);
} else {
need_reset = 1;
}
}
if (x < width) {
SmoothenBlock(a_ptr + x, a_stride, y_ptr + x, y_stride,
width - x, SIZE);
}
a_ptr += SIZE * a_stride;
y_ptr += SIZE * y_stride;
u_ptr += SIZE2 * uv_stride;
v_ptr += SIZE2 * uv_stride;
}
if (y < height) {
const int sub_height = height - y;
for (x = 0; x + SIZE <= width; x += SIZE) {
SmoothenBlock(a_ptr + x, a_stride, y_ptr + x, y_stride,
SIZE, sub_height);
}
if (x < width) {
SmoothenBlock(a_ptr + x, a_stride, y_ptr + x, y_stride,
width - x, sub_height);
}
}
}
}

8
src/enc/webp_enc.c
View File

@ -336,10 +336,6 @@ int WebPEncode(const WebPConfig* config, WebPPicture* pic) {
if (!config->lossless) {
VP8Encoder* enc = NULL;
if (!config->exact) {
WebPCleanupTransparentArea(pic);
}
if (pic->use_argb || pic->y == NULL || pic->u == NULL || pic->v == NULL) {
// Make sure we have YUVA samples.
if (config->use_sharp_yuv || (config->preprocessing & 4)) {
@ -361,6 +357,10 @@ int WebPEncode(const WebPConfig* config, WebPPicture* pic) {
}
}
if (!config->exact) {
WebPCleanupTransparentArea(pic);
}
enc = InitVP8Encoder(config, pic);
if (enc == NULL) return 0; // pic->error is already set.
// Note: each of the tasks below account for 20% in the progress report.

4
src/webp/encode.h
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@ -508,8 +508,8 @@ WEBP_EXTERN(int) WebPPictureSmartARGBToYUVA(WebPPicture* picture);
WEBP_EXTERN(int) WebPPictureYUVAToARGB(WebPPicture* picture);
// Helper function: given a width x height plane of RGBA or YUV(A) samples
// clean-up the YUV or RGB samples under fully transparent area, to help
// compressibility (no guarantee, though).
// clean-up or smoothen the YUV or RGB samples under fully transparent area,
// to help compressibility (no guarantee, though).
WEBP_EXTERN(void) WebPCleanupTransparentArea(WebPPicture* picture);
// Scan the picture 'picture' for the presence of non fully opaque alpha values.