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Approximate common gamma tables as single values

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This should improve performance of gamma conversion in
software (we can use vector math instead of table look
ups).

Additionally this allows us to quickly and easily
identify sRGB-like gammas.

On gpu, identifying sRGB gamma improves performance/memory,
because the hardware may support gamma conversion.

This will help us identify situations where gamma
conversion is not necessary.
Ex: sRGB input -> sRGB display

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1950183006

Review-Url: https://codereview.chromium.org/1950183006
This commit is contained in:
msarett 2016-05-09 14:27:48 -07:00 committed by Commit bot
parent 05c987cee3
commit 2c3eca38c9
2 changed files with 31 additions and 6 deletions
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35
src/core/SkColorSpace.cpp
View File

@ -324,13 +324,38 @@ bool SkColorSpace::LoadGammas(SkGammaCurve* gammas, uint32_t numGammas, const ui
// The table entry is the gamma (with a bias of 256).
uint16_t value = read_big_endian_short((const uint8_t*) table);
gammas[i].fValue = value / 256.0f;
SkColorSpacePrintf("gamma %d %g\n", value, *gamma);
SkColorSpacePrintf("gamma %d %g\n", value, gammas[i].fValue);
break;
}
// Fill in the interpolation table.
// FIXME (msarett):
// We should recognize commonly occurring tables and just set gamma to 2.2f.
// Check for frequently occurring curves and use a fast approximation.
// We do this by sampling a few values and see if they match our expectation.
// A more robust solution would be to compare each value in this curve against
// a 2.2f curve see if we remain below an error threshold. At this time,
// we haven't seen any images in the wild that make this kind of
// calculation necessary. We encounter identical gamma curves over and
// over again, but relatively few variations.
if (1024 == count) {
if (0 == read_big_endian_short((const uint8_t*) &table[0]) &&
3341 == read_big_endian_short((const uint8_t*) &table[256]) &&
14057 == read_big_endian_short((const uint8_t*) &table[512]) &&
34318 == read_big_endian_short((const uint8_t*) &table[768]) &&
65535 == read_big_endian_short((const uint8_t*) &table[1023])) {
gammas[i].fValue = 2.2f;
break;
}
} else if (26 == count) {
if (0 == read_big_endian_short((const uint8_t*) &table[0]) &&
3062 == read_big_endian_short((const uint8_t*) &table[6]) &&
12824 == read_big_endian_short((const uint8_t*) &table[12]) &&
31237 == read_big_endian_short((const uint8_t*) &table[18]) &&
65535 == read_big_endian_short((const uint8_t*) &table[25])) {
gammas[i].fValue = 2.2f;
break;
}
}
// Otherwise, fill in the interpolation table.
gammas[i].fTableSize = count;
gammas[i].fTable = std::unique_ptr<float[]>(new float[count]);
for (uint32_t j = 0; j < count; j++) {
@ -373,7 +398,7 @@ bool SkColorSpace::LoadGammas(SkGammaCurve* gammas, uint32_t numGammas, const ui
}
// Adjust src and len if there is another gamma curve to load.
if (0 != numGammas) {
if (i != numGammas - 1) {
// Each curve is padded to 4-byte alignment.
tagBytes = SkAlign4(tagBytes);
if (len < tagBytes) {

2
tests/ColorSpaceTest.cpp
View File

@ -74,7 +74,7 @@ DEF_TEST(ColorSpaceParsePngICCProfile, r) {
const float red[] = { 0.436066f, 0.222488f, 0.013916f };
const float green[] = { 0.385147f, 0.716873f, 0.0970764f };
const float blue[] = { 0.143066f, 0.0606079f, 0.714096f };
const float gamma[] = { 0, 0, 0 }; // table-based gamma returns 0 from this its float-getter
const float gamma[] = { 2.2f, 2.2f, 2.2f };
test_space(r, colorSpace, red, green, blue, gamma);
#endif
}