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Bug 323088 - RGB values slightly off in canvas after drawWindow().

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Use a table indexed by bit depth to provide a better "scale factor" when
  copying pixels from native surfaces to the canvas.
  Patch by roc, r=vladimir, sr=shaver.


git-svn-id: svn://10.0.0.236/trunk@188890 18797224-902f-48f8-a5cc-f745e15eee43
This commit is contained in:
mcs%pearlcrescent.com 2006-02-03 21:59:37 +00:00
parent f1df277163
commit 0a45e66e76
1 changed files with 30 additions and 6 deletions
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36
mozilla/content/canvas/src/nsCanvasRenderingContext2D.cpp
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@ -1935,6 +1935,25 @@ nsCanvasRenderingContext2D::DrawWindow(nsIDOMWindow* aWindow, PRInt32 aX, PRInt3
return rv;
}
/**
* Given aBits, the number of bits in a color channel, compute a number N
* such that for values v with aBits bits, floor((N*v)/256) is close to
* v*255.0/(2^aBits - 1) and in particular we need
* floor((N*(2^aBits - 1))/256) = 255.
* We'll just use a table that gives good results :-).
*/
static PRUint32 ComputeScaleFactor(PRUint32 aBits)
{
static PRUint32 table[9] = {
0, 255*256, 85*256, 9330, 17*256, 2110, 1038, 515, 256
};
NS_ASSERTION(aBits <= 8, "more than 8 bits in a color channel not supported");
NS_ASSERTION(((table[aBits]*((1 << aBits) - 1)) >> 8) == 255,
"Invalid table entry");
return table[aBits];
}
nsresult
nsCanvasRenderingContext2D::DrawNativeSurfaces(nsIDrawingSurface* aBlackSurface,
nsIDrawingSurface* aWhiteSurface,
@ -2072,6 +2091,11 @@ nsCanvasRenderingContext2D::DrawNativeSurfaces(nsIDrawingSurface* aBlackSurface,
// Convert the data
PRUint8* dest = tmpBuf;
PRInt32 index = 0;
PRUint32 RScale = ComputeScaleFactor(format.mRedCount);
PRUint32 GScale = ComputeScaleFactor(format.mGreenCount);
PRUint32 BScale = ComputeScaleFactor(format.mBlueCount);
for (PRInt32 i = 0; i < aSurfaceSize.height; ++i) {
PRUint8* src = data + i*rowSpan;
for (PRInt32 j = 0; j < aSurfaceSize.width; ++j) {
@ -2085,12 +2109,12 @@ nsCanvasRenderingContext2D::DrawNativeSurfaces(nsIDrawingSurface* aBlackSurface,
// Note that because aBlackSurface is the image rendered
// onto black, the channel values we get here have
// effectively been premultipled by the alpha value.
dest[BLUE_BYTE] = (PRUint8)(((v & format.mBlueMask) >> format.mBlueShift)
<< (8 - format.mBlueCount));
dest[GREEN_BYTE] = (PRUint8)(((v & format.mGreenMask) >> format.mGreenShift)
<< (8 - format.mGreenCount));
dest[RED_BYTE] = (PRUint8)(((v & format.mRedMask) >> format.mRedShift)
<< (8 - format.mRedCount));
dest[BLUE_BYTE] =
(PRUint8)((((v & format.mBlueMask) >> format.mBlueShift)*BScale) >> 8);
dest[GREEN_BYTE] =
(PRUint8)((((v & format.mGreenMask) >> format.mGreenShift)*GScale) >> 8);
dest[RED_BYTE] =
(PRUint8)((((v & format.mRedMask) >> format.mRedShift)*RScale) >> 8);
dest[ALPHA_BYTE] = alphas ? alphas[index++] : 0xFF;
src += bytesPerPix;
dest += 4;