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LibGfx/JPEG2000/IDWT: Combine steps 1 and 2 for 9-7 _1D_FILTR

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Takes `Build/lagom/bin/image -o out.tiff ~/Downloads/balloon.jpf` from
531.4 ms ± 5.4 ms to 515.8 ms ± 4.7 ms ms under hyperfine on my system
(2.7% less). When run under `/usr/bin/time -l`, the changes are:

Before:

         13202665108  instructions retired
          2281080551  cycles elapsed

After:

         12821908788  instructions retired
          2213264986  cycles elapsed

Only affects lossy files.
This commit is contained in:
Nico Weber committed 2026-07-26 10:12:58 -04:00
1 parent fd743c7011
commit dc4f01f580
1 file changed
+6 -4
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Userland/Libraries/LibGfx/ImageFormats/JPEG2000InverseDiscreteWaveletTransform.h
+6 -4
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@@ -326,12 +326,14 @@ inline void _1D_FILTR(Transformation transformation, IDWTOutput& a, int start, i
// F.3.8.2 The 1D_FILTR_9-7I procedure
// "Firstly, step 1 is performed for all values of n such that..."
for (int n = N0 - 1; n < N1 + 2; ++n)
x[2 * n] = kappa * y_ext[2 * n]; // [STEP1]
// "and step 2 is performed for all values of n such that..."
for (int n = N0 - 2; n < N1 + 2; ++n)
// Steps 1 and 2 independently update even and odd samples, so do them in the same loop.
// Step 2 starts one sample earlier, do that one before the loop/
x[2 * (N0 - 2) + 1] = (1 / kappa) * y_ext[2 * (N0 - 2) + 1]; // [STEP2]
for (int n = N0 - 1; n < N1 + 2; ++n) {
x[2 * n] = kappa * y_ext[2 * n]; // [STEP1]
x[2 * n + 1] = (1 / kappa) * y_ext[2 * n + 1]; // [STEP2]
}
for (int n = N0 - 1; n < N1 + 2; ++n)
x[2 * n] = x[2 * n] - delta * (x[2 * n - 1] + x[2 * n + 1]); // [STEP3]