LockBits() not correct stride

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Hi to all, i'm in trouble with LockBits gdi+ function, it dosen't seems work correctly.

This is my very simple test code:

C++

BitmapData dataHigh;
BitmapData dataImg;
Bitmap *image = new Bitmap(480,520,PixelFormat24bppRGB);
Bitmap *imgSx = new Bitmap(480,480,PixelFormat24bppRGB);

imgSx->LockBits(Rect(10,10,10,10),ImageLockModeRead | ImageLockModeWrite,PixelFormat24bppRGB,&dataHigh);

image->LockBits(Rect(10,10,10,10),ImageLockModeRead | ImageLockModeWrite,PixelFormat24bppRGB,&dataImg);

memset(dataHigh.Scan0,0xff,320);
memcpy(dataImg.Scan0,dataHigh.Scan0,320);

imgSx->UnlockBits(&dataHigh);
image->UnlockBits(&dataImg);

I would fill with blank only the selection portion of image, but the result if a line not a square, Seem that the buffer pointed from Scan0 contains alla image data and not only portion selected with Rect.

I forced the number of byte in memset and memcpy because the Stride value of both BitmapData are always 1440 like entire image.

Can be a bug of LockBits function?

Thank you in advance!

Marco

Posted 7-Sep-11 21:17pm

Ruscoff

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Santhosh G_ · Answer 1 · 2011-09-08T00:45:00

Stride is 1440, it means one row of Scan0 consists of 480( 1440 / 3) pixels information.

To memset a portion of bitmap, I suggest to do like this,

C#

BYTE* pbyFirstLine = (BYTE*) dataHigh.Scan0;

    int nX_i = 10;
    int nY_i = 10;
    int nW_i = 10; // Width of Bitmap
    int nH_i = 10; // Height of bitmap

    for(int nY = nY_i; nY < nY_i + nH_i; nY++)
    {
        // Seek in X and Y direction.
        BYTE* pbySeek = pbyFirstLine + ( nX_i * 3 ) + nY * dataHigh.Stride;
        memset(pbySeek, 0xFF, 3 * nW_i );
    }

Scan0 always return first pixel of Bitmap, This may be a bug of Gdi :)
According to msdn documentation,

Scan0: Gets or sets the address of the first pixel data in the bitmap. This can also be thought of as the first scan line in the bitmap.