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using StretchDIBits() can i draw transparent and semitransparent?

Apr 7, 2022 at 8:18pm
Cambalinho (1445)
using StretchDIBits() function, can i draw it without a color(backcolor transparent) and semi-transparent(opacy)?
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void New(unsigned int SizeWidth, unsigned int SizeHeight, COLORREF BackColor=RGB(0,0,0))
        {
            int BufferSize = SizeWidth * SizeHeight * sizeof(unsigned int);

            if(BufferMemory) VirtualFree(BufferMemory, 0,MEM_RELEASE);
            BufferMemory = VirtualAlloc(0, BufferSize,MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
            Pixels =static_cast<unsigned int *> (BufferMemory);
            std::fill(Pixels, Pixels + SizeWidth * SizeHeight, BackColor);

            Width = SizeWidth;
            Height = SizeHeight;
            BitInfo.bmiHeader.biSize = sizeof(BitInfo.bmiHeader);
            BitInfo.bmiHeader.biWidth = Width;
            BitInfo.bmiHeader.biHeight = -Height;
            BitInfo.bmiHeader.biPlanes = 1;
            BitInfo.bmiHeader.biBitCount = 32;
            BitInfo.bmiHeader.biCompression = BI_RGB;
        }
void Draw(HDC Destination, int PosY, int PosX)
        {
            if(BufferMemory)
            {
                StretchDIBits(Destination, PosX, PosY, Width, Height, 0,0, Width, Height,Pixels, &BitInfo,DIB_RGB_COLORS, SRCCOPY );
                
            }

        }
Last edited on Apr 7, 2022 at 8:31pm
Apr 7, 2022 at 9:08pm
jonnin (11466)
if everything is set up correctly and using transparency, yes.
the blts don't care. Its the canvas ... where you create compatible bmp or get a DC or whatever you are doing that needs to be alpha enabled.
Last edited on Apr 7, 2022 at 9:10pm
Apr 7, 2022 at 9:37pm
Cambalinho (1445)
how can i add or change the alpha?
Apr 7, 2022 at 11:27pm
jonnin (11466)
it probably already has it, but check stuff like
BitInfo.bmiHeader.biCompression = BI_RGB;
you may be turning it off there, maybe it wants an A on that...
or DIB_RGB_COLORS
Last edited on Apr 7, 2022 at 11:28pm
Apr 8, 2022 at 1:20am
Duthomhas (13235)
StretchDIBits() does not support alpha transparency. Never has, and probably never will.

Er, here is what recommendation I quickly googled: https://stackoverflow.com/a/55070258/2706707
Apr 8, 2022 at 11:22am
Cambalinho (1445)
What DIBits() functions can use alpha value?
Apr 8, 2022 at 1:49pm
jonnin (11466)
At this point, not sure. Ive only done really simple transparency, and about all I can say is that reading up on it, bitblt accidentally works and if you want more than that, you have to use gdiplus.
Apr 8, 2022 at 5:59pm
Cambalinho (1445)
Using Math, how combine the source and destination pixels with alpha value?
At least now i can use alpha.. the transparent is just not draw a pixel color ;)
Apr 8, 2022 at 6:23pm
jonnin (11466)
better look it up. I can make a guess, but there are probably details that require reading a bit more.
Apr 8, 2022 at 6:59pm
Cambalinho (1445)
i only found these:
RR = SR × SA + DR × (1 − SA)
RG = SG × SA + DG × (1 − SA)
RB = SB × SA + DB × (1 − SA)
on here: https://ciechanow.ski/alpha-compositing/
i will try it... thanks
so i did these function now:
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COLORREF AlphaBlendPixels(COLORREF SourcePixel, COLORREF DestinationPixel, BYTE AlphaValue)
{
    //Get Source RGB Colors:
    BYTE SR = GetRValue(SourcePixel);
    BYTE SG = GetGValue(SourcePixel);
    BYTE SB = GetBValue(SourcePixel);

    //Get Destination RGB Colors:
    BYTE DR = GetRValue(DestinationPixel);
    BYTE DG = GetGValue(DestinationPixel);
    BYTE DB = GetBValue(DestinationPixel);
    
    //Combine the 2 bit pixels with Alpha Value:
    BYTE RR = SR * AlphaValue + DR * (1-AlphaValue);
    BYTE RG = SG * AlphaValue + DG * (1-AlphaValue);
    BYTE RB = SB * AlphaValue + DB * (1-AlphaValue);

    return RGB(RR,RG,RB);
}

i'm confused: why '-1'?
Last edited on Apr 8, 2022 at 7:16pm
Apr 8, 2022 at 7:08pm
jonnin (11466)
good luck. It seems likely that you want SA = A/255 as a percent, and 1-(A/255) (the opposite percent, such that 25% and 75% are 100%.. 1.0 is 100% ..)
(I didn't read it, but just warning you that A here isnt 0-255 in case they didn't tell you).

it also seems likely that there is a more efficient way to do that math, but see if it works first
Last edited on Apr 8, 2022 at 7:10pm
Apr 9, 2022 at 6:53pm
Cambalinho (1445)
theres so many code.. but i choose the used class Bitmap functions:
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void Draw(Bitmap &BitPixels, int PosY, int PosX, COLORREF TransparentColor =0, BYTE AlphaBlendValue=0)
        {
            for(unsigned int Y = PosY, SourceY =0; Y < (Height+PosY); Y++, SourceY++)
            {
                for (unsigned int X= PosX, SourceX =0; X<(Width+PosX); X++, SourceX++)
                {
                    //if(TransparentColor!=0 && BitPixels.Pixels[Y*BitPixels.Width + X] == TransparentColor) continue;
                    BitPixels.Pixels[Y*BitPixels.Width + X] = AlphaBlendPixels ((COLORREF)Pixels[SourceY*Width + SourceX],(COLORREF) BitPixels.Pixels[Y*BitPixels.Width + X], AlphaBlendValue);
                }
            }
        }

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COLORREF AlphaBlendPixels(COLORREF SourcePixel, COLORREF DestinationPixel, BYTE AlphaValue)
{
    //Get Source RGB Colors:
    BYTE SR = GetRValue(SourcePixel);
    BYTE SG = GetGValue(SourcePixel);
    BYTE SB = GetBValue(SourcePixel);

    //Get Destination RGB Colors:
    BYTE DR = GetRValue(DestinationPixel);
    BYTE DG = GetGValue(DestinationPixel);
    BYTE DB = GetBValue(DestinationPixel);

    //Combine the 2 bit pixels with Alpha Value:
    BYTE RR = SR * AlphaValue + DR * (1-AlphaValue / 255);
    BYTE RG = SG * AlphaValue + DG * (1-AlphaValue / 255);
    BYTE RB = SB * AlphaValue + DB * (1-AlphaValue / 255);

    return RGB(RR,RG,RB);
}

now the main:
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int main()
{


    HWND HWNDConsoleWindow = GetConsoleWindow();
    HDC HDCConsoleWindow = GetDC(HWNDConsoleWindow);

    Bitmap Bit(500,500,RGB(255 ,0,0));
    Bitmap Bit2(100,100,RGB(0,0,255));
    do
    {
        //the Bit2 is just like a rectangle and is drawed on Bit:
        Bit2.Draw(Bit,20,20,0,250);
        //Draw the Bit on Window:
        Bit.Draw(HDCConsoleWindow,100,100);

    }while(!GetAsyncKeyState(VK_ESCAPE));
    return 0;
}

but i see 2 problems:
1 - the AlphaBlend() math seems wrong(i don't see the transparency correctly);
https://imgur.com/dFnhPnw

2 - why i get the flicker when i draw 'Bit' on HDC Window? too much for CPU?
Last edited on Apr 9, 2022 at 6:55pm
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