setpixel,getpixel效率很低，尤其是在迴圈裡面去這麼做，速度慢的無法忍受，
用bitblt做了優化，**如下，
原函式如下：
void gdirectanglealpha(hdc hdc,const rect *rect,colorref color, unsigned char alpha)
if (!rect || !hdc)
return;
int xmin = rect->left;
int ymin = rect->top;
int xmax = rect->right;
int ymax = rect->bottom;
int x,y;
int r = getrvalue(color);
int g = getgvalue(color);
int b = getbvalue(color);
colorref clsrc;
int rsrc;
int gsrc;
int bsrc;
for (y = ymin; y < ymax; y++)
for (x = xmin; x < xmax; x++)
clsrc = getpixel(hdc,x,y);
rsrc = getrvalue(clsrc);
gsrc = getgvalue(clsrc);
bsrc = getbvalue(clsrc);
rsrc = (rsrc * alpha + r * (255 - alpha)) >>8;
gsrc = (gsrc * alpha + g * (255 - alpha)) >>8;
bsrc = (bsrc * alpha + b * (255 - alpha)) >>8;
setpixel(hdc,x,y,rgb(rsrc,gsrc,bsrc));
優化後的實現
void gdirectanglealpha(hdc hdc,const rect *rect,colorref color, unsigned char alpha)
byte * g_pbits;
hdc g_hmemdc;
hbitmap g_hbmp, g_holdbmp;
if (!rect || !hdc)
return;
int xmin = rect->left;
int ymin = rect->top;
int xmax = rect->right;
int ymax = rect->bottom;
int x,y;
byte r = getrvalue(color);
byte g = getgvalue(color);
byte b = getbvalue(color);
colorref clsrc;
unsigned char rsrc;
unsigned char gsrc;
unsigned char bsrc;
g_hmemdc = ::createcompatibledc(hdc);
if (!g_hmemdc)
::deletedc(hdc);
int iwidth = rect->right - rect->left;
int iheight = rect->bottom - rect->top;
byte bmibuf[sizeof(bitmapinfo) + 256 * sizeof(rgbquad)];
memset(bmibuf, 0, sizeof(bmibuf));
bitmapinfo* pbmi = (bitmapinfo*)bmibuf;
// bitmapinfo pbmi;
pbmi->bmiheader.bisize = sizeof(bitmapinfoheader);
pbmi->bmiheader.biwidth = iwidth;
pbmi->bmiheader.biheight = iheight;
pbmi->bmiheader.biplanes = 1;
pbmi->bmiheader.bibitcount = 24;
pbmi->bmiheader.bicompression = bi_rgb;
g_hbmp = ::createdibsection(g_hmemdc, pbmi, dib_rgb_colors, (void **)&g_pbits, 0, 0);
if (!g_hbmp)
::deletedc(g_hmemdc);
g_holdbmp = (hbitmap)::selectobject(g_hmemdc, g_hbmp);
bitblt(g_hmemdc,0,0,iwidth,iheight,hdc,0,0,srccopy);
// offset = y * (width * 24 / 8) + x * (24 / 8)
for (y = 0; y < iheight; y++)
for (x = 0; x < iwidth; x++)
rsrc = g_pbits[y * iwidth * 3 + x * 3 + 2];
gsrc = g_pbits[y * iwidth * 3 + x * 3 + 1];
bsrc = g_pbits[y * iwidth * 3 + x * 3];
rsrc = (rsrc * alpha + r * (255 - alpha)) >>8;
gsrc = (gsrc * alpha + g * (255 - alpha)) >>8;
bsrc = (bsrc * alpha + b * (255 - alpha)) >>8;
g_pbits[y * iwidth * 3 + x * 3 + 2] = rsrc;
g_pbits[y * iwidth * 3 + x * 3 + 1] = gsrc;
g_pbits[y * iwidth * 3 + x * 3] = bsrc;
bitblt(hdc, 0, 0, iwidth, iheight, g_hmemdc, 0, 0, srccopy);
selectobject(g_hmemdc, g_holdbmp);
deleteobject(g_hbmp);
deletedc(g_hmemdc);
releasedc(null, hdc);
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