convi: ensure double sum precision when multiplying floats (#4418)

lovell · web-flow · commit fd1ec8a6502a · 2025-03-17T14:34:06.000Z
diff --git a/ChangeLog b/ChangeLog
@@ -12,6 +12,7 @@
 - svgload: add support for custom CSS via stylesheet option [lovell]
 - heifload: `unlimited` flag removes all limits (requires libheif 1.19.0+) [lovell]
 - heifsave: improve alpha channel detection [lovell]
+- convi: ensure double sum precision for floats [lovell]
 
 8.16.1
 
diff --git a/libvips/convolution/convi.c b/libvips/convolution/convi.c
@@ -728,7 +728,7 @@ vips_convi_gen_vector(VipsRegion *out_region,
 \
 			sum = 0; \
 			for (i = 0; i < nnz; i++) \
-				sum += t[i] * p[offsets[i]]; \
+				sum += (double) t[i] * p[offsets[i]]; \
 \
 			sum = (sum / scale) + offset; \
 \

Original file line number	Diff line number	Diff line change
`@@ -728,7 +728,7 @@ vips_convi_gen_vector(VipsRegion *out_region,`
`728`	`728`	`\`
`729`	`729`	`sum = 0; \`
`730`	`730`	`for (i = 0; i < nnz; i++) \`
`731`		`- sum += t[i] * p[offsets[i]]; \`
	`731`	`+ sum += (double) t[i] * p[offsets[i]]; \`
`732`	`732`	`\`
`733`	`733`	`sum = (sum / scale) + offset; \`
`734`	`734`	`\`