add additional 0-1 alpha argument to DrawTransformedBitmap()

This allows the VCL backends the apply the extra alpha transformation
as it sees fit, rather than it being done manually elsewhere (and
even if the backend doesn't implement it, at least do it in one
place in the function).
With the document from tdf#136223, going from slide 2 to slide 3,
this easily saves 10-30% of CPU cycles. As an additional bonus,
using AlphaMask::BlendWith() rather than AlphaMask::Replace()
makes edges of shapes noticeably more smooth.

Change-Id: I036dc9b887d6def0c7cdad3982becabdc7cd5206
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/111247
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

1 files changed, 21 insertions, 3 deletions

diff --git a/vcl/skia/gdiimpl.cxx b/vcl/skia/gdiimpl.cxx
index d8077df48a68..950010183cb1 100644
--- a/vcl/skia/gdiimpl.cxx
+++ b/vcl/skia/gdiimpl.cxx
@@ -1762,7 +1762,7 @@ bool SkiaSalGraphicsImpl::drawTransformedBitmap(const basegfx::B2DPoint& rNull,
                                                 const basegfx::B2DPoint& rX,
                                                 const basegfx::B2DPoint& rY,
                                                 const SalBitmap& rSourceBitmap,
-                                                const SalBitmap* pAlphaBitmap)
+                                                const SalBitmap* pAlphaBitmap, double fAlpha)
 {
     assert(dynamic_cast<const SkiaSalBitmap*>(&rSourceBitmap));
     assert(!pAlphaBitmap || dynamic_cast<const SkiaSalBitmap*>(pAlphaBitmap));
@@ -1787,6 +1787,8 @@ bool SkiaSalGraphicsImpl::drawTransformedBitmap(const basegfx::B2DPoint& rNull,
     // so use mergeCacheBitmaps(), which will cache the result if useful.
     // It is better to use SkShader if in GPU mode, if the operation is simple or if the temporary
     // image would be very large.
+    // The extra fAlpha blending is not cached, with the assumption that it usually gradually changes
+    // for each invocation.
     sk_sp<SkImage> imageToDraw = mergeCacheBitmaps(
         rSkiaBitmap, pSkiaAlphaBitmap, Size(round(aXRel.getLength()), round(aYRel.getLength())));
     if (imageToDraw)
@@ -1808,7 +1810,15 @@ bool SkiaSalGraphicsImpl::drawTransformedBitmap(const basegfx::B2DPoint& rNull,
         SkPaint paint;
         if (!matrix.isTranslate())
             paint.setFilterQuality(kHigh_SkFilterQuality);
-        canvas->drawImage(imageToDraw, 0, 0, &paint);
+        if (fAlpha == 1.0)
+            canvas->drawImage(imageToDraw, 0, 0, &paint);
+        else
+        {
+            paint.setShader(
+                SkShaders::Blend(SkBlendMode::kDstIn, imageToDraw->makeShader(),
+                                 SkShaders::Color(SkColorSetARGB(fAlpha * 255, 0, 0, 0))));
+            canvas->drawRect(SkRect::MakeWH(imageToDraw->width(), imageToDraw->height()), paint);
+        }
     }
     else
     {
@@ -1831,11 +1841,19 @@ bool SkiaSalGraphicsImpl::drawTransformedBitmap(const basegfx::B2DPoint& rNull,
             paint.setShader(SkShaders::Blend(SkBlendMode::kDstOut, // VCL alpha is one-minus-alpha.
                                              rSkiaBitmap.GetSkShader(),
                                              pSkiaAlphaBitmap->GetAlphaSkShader()));
+            if (fAlpha != 1.0)
+                paint.setShader(
+                    SkShaders::Blend(SkBlendMode::kDstIn, paint.refShader(),
+                                     SkShaders::Color(SkColorSetARGB(fAlpha * 255, 0, 0, 0))));
             canvas->drawRect(SkRect::MakeWH(aSize.Width(), aSize.Height()), paint);
         }
-        else if (rSkiaBitmap.PreferSkShader())
+        else if (rSkiaBitmap.PreferSkShader() || fAlpha != 1.0)
         {
             paint.setShader(rSkiaBitmap.GetSkShader());
+            if (fAlpha != 1.0)
+                paint.setShader(
+                    SkShaders::Blend(SkBlendMode::kDstIn, paint.refShader(),
+                                     SkShaders::Color(SkColorSetARGB(fAlpha * 255, 0, 0, 0))));
             canvas->drawRect(SkRect::MakeWH(aSize.Width(), aSize.Height()), paint);
         }
         else