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Fix vcl alpha blending n#714787

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The way alpha compositing took place in vcl's alpha vdev was subtly
wrong - it was supposed to implement porter-duff 'over', but didn't.
This is now fixed also for cases where the source surface contains
alpha, and needs to blend correctly into fully-transparent
background.

For reference: http://en.wikipedia.org/wiki/Alpha_compositing
This commit is contained in:
Thorsten Behrens
2012-03-27 02:54:42 +02:00
parent f64d58e3e1
commit 06c16e1e26
1 changed files with 37 additions and 31 deletions
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68
vcl/source/gdi/outdev2.cxx
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@@ -1573,11 +1573,14 @@ void OutputDevice::DrawPixel( const Polygon& rPts, const Color& rColor )
namespace
{
sal_uInt8 lcl_calcColor( const sal_uInt8 nSourceColor, const sal_uInt8 nSourceOpaq, const sal_uInt8 nDestColor )
// Co = Cs + Cd*(1-As) premultiplied alpha -or-
// Co = (AsCs + AdCd*(1-As)) / Ao
inline sal_uInt8 lcl_calcColor( const sal_uInt8 nSourceColor, const sal_uInt8 nSourceAlpha,
const sal_uInt8 nDstAlpha, const sal_uInt8 nResAlpha, const sal_uInt8 nDestColor )
{
int c = ( (int)nDestColor * ( 255 - nSourceOpaq ) )
+ (int)nSourceOpaq * (int)nSourceColor;
return sal_uInt8( c / 255 );
int c = nResAlpha ? ( (int)nSourceAlpha*nSourceColor + (int)nDstAlpha*nDestColor -
(int)nDstAlpha*nDestColor*nSourceAlpha/255 ) / (int)nResAlpha : 0;
return sal_uInt8( c );
}
}
@@ -1631,27 +1634,29 @@ Bitmap OutputDevice::ImplBlendWithAlpha( Bitmap aBmp,
aSrcCol = pP->GetColor( nMapY, nMapX );
aDstCol = pB->GetColor( nY, nX );
const sal_uInt8 nSrcOpaq = 255 - pA->GetPixel( nMapY, nMapX ).GetBlueOrIndex();
const sal_uInt8 nDstOpaq = 255 - pAlphaW->GetPixel( nY, nX ).GetBlueOrIndex();
aDstCol.SetRed( lcl_calcColor( aSrcCol.GetRed(), nSrcOpaq, aDstCol.GetRed() ) );
aDstCol.SetBlue( lcl_calcColor( aSrcCol.GetBlue(), nSrcOpaq, aDstCol.GetBlue() ) );
aDstCol.SetGreen( lcl_calcColor( aSrcCol.GetGreen(), nSrcOpaq, aDstCol.GetGreen() ) );
// vcl stores transparency, not alpha - invert it
const sal_uInt8 nSrcAlpha = 255 - pA->GetPixel( nMapY, nMapX ).GetBlueOrIndex();
const sal_uInt8 nDstAlpha = 255 - pAlphaW->GetPixel( nY, nX ).GetBlueOrIndex();
// Perform porter-duff compositing 'over' operation
//
// Co = Cs + Cd*(1-As)
// Ad = As + Ad*(1-As)
const sal_uInt8 nResAlpha = (int)nSrcAlpha + (int)nDstAlpha - (int)nDstAlpha*nSrcAlpha/255;
aDstCol.SetRed( lcl_calcColor( aSrcCol.GetRed(), nSrcAlpha, nDstAlpha, nResAlpha, aDstCol.GetRed() ) );
aDstCol.SetBlue( lcl_calcColor( aSrcCol.GetBlue(), nSrcAlpha, nDstAlpha, nResAlpha, aDstCol.GetBlue() ) );
aDstCol.SetGreen( lcl_calcColor( aSrcCol.GetGreen(), nSrcAlpha, nDstAlpha, nResAlpha, aDstCol.GetGreen() ) );
aIndex.SetIndex( (sal_uInt8) ( nVCLRLut[ ( nVCLLut[ aDstCol.GetRed() ] + nD ) >> 16UL ] +
nVCLGLut[ ( nVCLLut[ aDstCol.GetGreen() ] + nD ) >> 16UL ] +
nVCLBLut[ ( nVCLLut[ aDstCol.GetBlue() ] + nD ) >> 16UL ] ) );
pW->SetPixel( nY, nX, aIndex );
// Have to perform the compositing 'algebra' in
// the inverse alpha space (with 255 meaning
// opaque), otherwise, transitivity is not
// achieved.
const sal_uInt8 nSrcAlpha = 255-COLOR_CHANNEL_MERGE( 255, (sal_uInt8)nDstOpaq, nSrcOpaq );
aIndex.SetIndex( (sal_uInt8) ( nVCLRLut[ ( nVCLLut[ nSrcAlpha ] + nD ) >> 16UL ] +
nVCLGLut[ ( nVCLLut[ nSrcAlpha ] + nD ) >> 16UL ] +
nVCLBLut[ ( nVCLLut[ nSrcAlpha ] + nD ) >> 16UL ] ) );
aIndex.SetIndex( (sal_uInt8) ( nVCLRLut[ ( nVCLLut[ 255-nResAlpha ] + nD ) >> 16UL ] +
nVCLGLut[ ( nVCLLut[ 255-nResAlpha ] + nD ) >> 16UL ] +
nVCLBLut[ ( nVCLLut[ 255-nResAlpha ] + nD ) >> 16UL ] ) );
pAlphaW->SetPixel( nY, nX, aIndex );
}
}
@@ -1676,22 +1681,23 @@ Bitmap OutputDevice::ImplBlendWithAlpha( Bitmap aBmp,
aSrcCol = pP->GetColor( nMapY, nMapX );
aDstCol = pB->GetColor( nY, nX );
const sal_uInt8 nSrcOpaq = 255 - pA->GetPixel( nMapY, nMapX ).GetBlueOrIndex();
const sal_uInt8 nDstOpaq = 255 - pAlphaW->GetPixel( nY, nX ).GetBlueOrIndex();
aDstCol.SetRed( lcl_calcColor( aSrcCol.GetRed(), nSrcOpaq, aDstCol.GetRed() ) );
aDstCol.SetBlue( lcl_calcColor( aSrcCol.GetBlue(), nSrcOpaq, aDstCol.GetBlue() ) );
aDstCol.SetGreen( lcl_calcColor( aSrcCol.GetGreen(), nSrcOpaq, aDstCol.GetGreen() ) );
// vcl stores transparency, not alpha - invert it
const sal_uInt8 nSrcAlpha = 255 - pA->GetPixel( nMapY, nMapX ).GetBlueOrIndex();
const sal_uInt8 nDstAlpha = 255 - pAlphaW->GetPixel( nY, nX ).GetBlueOrIndex();
// Perform porter-duff compositing 'over' operation
//
// Co = Cs + Cd*(1-As)
// Ad = As + Ad*(1-As)
const sal_uInt8 nResAlpha = (int)nSrcAlpha + (int)nDstAlpha - (int)nDstAlpha*nSrcAlpha/255;
aDstCol.SetRed( lcl_calcColor( aSrcCol.GetRed(), nSrcAlpha, nDstAlpha, nResAlpha, aDstCol.GetRed() ) );
aDstCol.SetBlue( lcl_calcColor( aSrcCol.GetBlue(), nSrcAlpha, nDstAlpha, nResAlpha, aDstCol.GetBlue() ) );
aDstCol.SetGreen( lcl_calcColor( aSrcCol.GetGreen(), nSrcAlpha, nDstAlpha, nResAlpha, aDstCol.GetGreen() ) );
pB->SetPixel( nY, nX, aDstCol );
// Have to perform the compositing 'algebra' in
// the inverse alpha space (with 255 meaning
// opaque), otherwise, transitivity is not
// achieved.
const sal_uInt8 nSrcAlpha = 255-COLOR_CHANNEL_MERGE( 255, (sal_uInt8)nDstOpaq, nSrcOpaq );
pAlphaW->SetPixel( nY, nX, Color(nSrcAlpha, nSrcAlpha, nSrcAlpha) );
pAlphaW->SetPixel( nY, nX, Color(255L-nResAlpha, 255L-nResAlpha, 255L-nResAlpha) );
}
}
}