From 2dea3087b392a8c9ba80a98b66a7dcc39ee280e3 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Thu, 9 Oct 2025 16:52:13 +0530
Subject: [PATCH] Faster is_opaque implementation for paletted images

---
 tools/utils/images/opaque.go | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/tools/utils/images/opaque.go b/tools/utils/images/opaque.go
index d0e72c605..6d62181dd 100644
--- a/tools/utils/images/opaque.go
+++ b/tools/utils/images/opaque.go
@@ -9,6 +9,35 @@ import (
 
 var _ = fmt.Print
 
+func paletted_is_opaque(p *image.Paletted) bool {
+	if len(p.Palette) > 256 {
+		return p.Opaque()
+	}
+	var is_alpha [256]bool
+	has_alpha := false
+	for i, c := range p.Palette {
+		_, _, _, a := c.RGBA()
+		if a != 0xffff {
+			is_alpha[i] = true
+			has_alpha = true
+		}
+	}
+	if !has_alpha {
+		return true
+	}
+	i0, i1 := 0, p.Rect.Dx()
+	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
+		for _, c := range p.Pix[i0:i1] {
+			if is_alpha[c] {
+				return false
+			}
+		}
+		i0 += p.Stride
+		i1 += p.Stride
+	}
+	return true
+}
+
 func IsOpaque(img image.Image) bool {
 	switch i := img.(type) {
 	case *image.RGBA:
@@ -30,7 +59,7 @@ func IsOpaque(img image.Image) bool {
 	case *image.CMYK:
 		return i.Opaque()
 	case *image.Paletted:
-		return i.Opaque()
+		return paletted_is_opaque(i)
 	case *image.Uniform:
 		return i.Opaque()
 	case *image.YCbCr: