Skip to content

Remove the +8192 rounding-bias offset from kLinearToGammaTab #182

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
skal65535 merged 1 commit into from
Sep 1, 2026
Merged

Remove the +8192 rounding-bias offset from kLinearToGammaTab #182

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
12 changes: 3 additions & 9 deletions src/yuv_convert.cc
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -143,10 +143,7 @@ static void InitGammaTablesF() {
} else {
value = (1. + a) * pow(g, 1. / gamma) - a;
}
// we already incorporate the 1/2 rounding constant here
kLinearToGammaTab[v] =
static_cast<uint32_t>(MAX_Y_T * value)
+ (1 << GAMMA_TO_LINEAR_BITS >> 1);
kLinearToGammaTab[v] = static_cast<uint32_t>(MAX_Y_T * value);
}
// to prevent small rounding errors to cause read-overflow:
kLinearToGammaTab[GAMMA_TABLE_SIZE + 1] =
Expand All @@ -158,10 +155,7 @@ static void InitGammaTablesF() {
// (used by yuv_convert_avx2.cc too)
uint32_t GammaToLinear(int v) { return kGammaToLinearTab[v]; }

// TODO(skal): return range is currently [8192, 9215], and not [0, MAX_Y_T]!
// => kLinearToGammaTab[] bakes the rounding bias into the entries.
// It's ok since we're only using diffs of these, but it's a ticking bomb.
// TODO(skal): remove the +8192 offset.
// return value is in [0, MAX_Y_T]
uint32_t LinearToGamma(uint32_t value) {
// 'value' is in GAMMA_TO_LINEAR_BITS fractional precision
const uint32_t v = value * GAMMA_TABLE_SIZE;
Expand All @@ -171,7 +165,7 @@ uint32_t LinearToGamma(uint32_t value) {
// v0 / v1 are in GAMMA_TO_LINEAR_BITS fixed-point precision (range [0..1])
const uint32_t v0 = kLinearToGammaTab[tab_pos + 0];
const uint32_t v1 = kLinearToGammaTab[tab_pos + 1];
// Final interpolation. Note that rounding is already included.
// Final interpolation.
const uint32_t v2 = (v1 - v0) * x; // note: v1 >= v0.
const uint32_t result = v0 + (v2 >> GAMMA_TO_LINEAR_BITS);
return result;
Expand Down
Loading