prevent SkiaSalBitmap::Scale() from breaking indexed bitmaps (tdf#134574)

Since the actual scaling is done later at some unknown time,
the palette mustn't change, but scaling can change colors.

Change-Id: Ie254c8b31993d9d509c32a730dd8c8b5d3cb2256
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/98258
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>
(cherry picked from commit 3769d01791e54be0fbfc6d706596283213700ad0)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/98330
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/vcl/skia/salbmp.cxx b/vcl/skia/salbmp.cxx
index 439baca41b6f..9b4e0b3324b4 100644
--- a/vcl/skia/salbmp.cxx
+++ b/vcl/skia/salbmp.cxx
@@ -293,8 +293,8 @@ bool SkiaSalBitmap::Scale(const double& rScaleX, const double& rScaleY, BmpScale
     if (mSize == newSize)
         return true;
 
-    SAL_INFO("vcl.skia.trace", "scale(" << this << "): " << mSize << "->" << newSize << ":"
-                                        << static_cast<int>(nScaleFlag));
+    SAL_INFO("vcl.skia.trace", "scale(" << this << "): " << mSize << "/" << mBitCount << "->"
+                                        << newSize << ":" << static_cast<int>(nScaleFlag));
 
     // The idea here is that the actual scaling will be delayed until the result
     // is actually needed. Usually the scaled bitmap will be drawn somewhere,
@@ -316,8 +316,18 @@ bool SkiaSalBitmap::Scale(const double& rScaleX, const double& rScaleY, BmpScale
             currentQuality = kHigh_SkFilterQuality;
             break;
         default:
+            SAL_INFO("vcl.skia.trace", "scale(" << this << "): unsupported scale algorithm");
             return false;
     }
+    if (mBitCount < 24 && !mPalette.IsGreyPalette8Bit())
+    {
+        // Scaling can introduce additional colors not present in the original
+        // bitmap (e.g. when smoothing). If the bitmap is indexed (has non-trivial palette),
+        // this would break the bitmap, because the actual scaling is done only somewhen later.
+        // Linear 8bit palette (grey) is ok, since there we use directly the values as colors.
+        SAL_INFO("vcl.skia.trace", "scale(" << this << "): indexed bitmap");
+        return false;
+    }
     // if there is already one scale() pending, use the lowest quality of all requested
     static_assert(kMedium_SkFilterQuality < kHigh_SkFilterQuality);
     mScaleQuality = std::min(mScaleQuality, currentQuality);