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segmentation: fix mask upsampling to pixel-centre convention #21764

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Merged
TurboGit merged 1 commit into from
Aug 10, 2026
+18 −7
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segmentation: fix mask upsampling to pixel-centre convention #21764

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25 changes: 18 additions & 7 deletions src/common/ai/segmentation.c
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Original file line number Diff line number Diff line change
Expand Up @@ -225,17 +225,26 @@ static void _crop_resize_mask(const float *const restrict src,
}
}

// inverse of the sampling mapping below, hoisted out of the pixel loops
const float inv_scale = (scale > 1e-6f) ? 1.0f / scale : 0.0f;

DT_OMP_FOR(shared(range_lut))
for(int y = 0; y < dst_h; y++)
{
const float sy = (dst_h > 1) ? (float)y * (float)(valid_h - 1) / (float)(dst_h - 1) : 0.0f;
// pixel-centre convention: output pixel y covers [y, y+1[, whose
// centre y + 0.5 maps to (y + 0.5) * scale in mask space, i.e.
// sample index (y + 0.5) * scale - 0.5. the previous align-corners
// mapping was not the inverse of that and stretched the mask
// outwards; see the commit message for figures. the clamp is
// load-bearing: it keeps fy >= 0 and makes (int)sy a floor
const float sy = MAX(((float)y + 0.5f) * scale - 0.5f, 0.0f);
const int y0 = MIN((int)sy, valid_h - 1);
const int y1 = MIN(y0 + 1, valid_h - 1);
const float fy = sy - (float)y0;

for(int x = 0; x < dst_w; x++)
{
const float sx = (dst_w > 1) ? (float)x * (float)(valid_w - 1) / (float)(dst_w - 1) : 0.0f;
const float sx = MAX(((float)x + 0.5f) * scale - 0.5f, 0.0f);
const int x0 = MIN((int)sx, valid_w - 1);
const int x1 = MIN(x0 + 1, valid_w - 1);
const float fx = sx - (float)x0;
Expand All @@ -255,11 +264,13 @@ static void _crop_resize_mask(const float *const restrict src,
// reference luma at the output pixel (guide is at dst resolution)
const int luma_ref = _luma709(&guide_rgb[(y * guide_w + x) * 3]);

// map the 4 source-grid corners back into guide coords
const float gx0 = (valid_w > 1) ? (float)x0 * (float)(dst_w - 1) / (float)(valid_w - 1) : 0.0f;
const float gx1 = (valid_w > 1) ? (float)x1 * (float)(dst_w - 1) / (float)(valid_w - 1) : 0.0f;
const float gy0 = (valid_h > 1) ? (float)y0 * (float)(dst_h - 1) / (float)(valid_h - 1) : 0.0f;
const float gy1 = (valid_h > 1) ? (float)y1 * (float)(dst_h - 1) / (float)(valid_h - 1) : 0.0f;
// map the 4 source-grid corners back into guide coords -- exact
// inverse of the sampling mapping above, so that the range weights
// are read at the guide pixels the mask values actually come from
const float gx0 = ((float)x0 + 0.5f) * inv_scale - 0.5f;
const float gx1 = ((float)x1 + 0.5f) * inv_scale - 0.5f;
const float gy0 = ((float)y0 + 0.5f) * inv_scale - 0.5f;
const float gy1 = ((float)y1 + 0.5f) * inv_scale - 0.5f;

const int ix00 = MIN(MAX((int)(gx0 + 0.5f), 0), guide_w - 1);
const int ix01 = MIN(MAX((int)(gx1 + 0.5f), 0), guide_w - 1);
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