Rotating an image using Borland C++ Builder and Windows API functions - c++

Question

I built this example to quickly rotate images 90 degrees but I always get a cut of the image on the sides. After many tests, unfortunately I still don't understand the cause of the problem.
void rotate()
{
Graphics::TBitmap *SrcBitmap = new Graphics::TBitmap;
Graphics::TBitmap *DestBitmap = new Graphics::TBitmap;
SrcBitmap->LoadFromFile("Crayon.bmp");
DestBitmap->Width=SrcBitmap->Width;
DestBitmap->Height=SrcBitmap->Height;
SetGraphicsMode(DestBitmap->Canvas->Handle, GM_ADVANCED);
double myangle = (double)(90.0 / 180.0) * 3.1415926;
int x0=SrcBitmap->Width/2;
int y0=SrcBitmap->Height/2;
double cx=x0 - cos(myangle)*x0 + sin(myangle)*y0;
double cy=y0 - cos(myangle)*y0 - sin(myangle)*x0;
xForm.eM11 = (FLOAT) cos(myangle);
xForm.eM12 = (FLOAT) sin(myangle);
xForm.eM21 = (FLOAT) -sin(myangle);
xForm.eM22 = (FLOAT) cos(myangle);
xForm.eDx = (FLOAT) cx;
xForm.eDy = (FLOAT) cy;
SetWorldTransform(DestBitmap->Canvas->Handle, &xForm);
BitBlt(DestBitmap->Canvas->Handle,
0,
0,
SrcBitmap->Width,
SrcBitmap->Height,
SrcBitmap->Canvas->Handle,
0,
0,
SRCCOPY);
DestBitmap->SaveToFile("Crayon2.bmp");
delete DestBitmap;
delete SrcBitmap;
}

Answer 1

If rotating the whole image, the width and height for destination image should be flipped:
DestBitmap->Width = SrcBitmap->Height;
DestBitmap->Height = SrcBitmap->Width;
The transform routine was centering the image based on original width/height. We want to adjust x/y position to push the starting point to left/top for BitBlt
int offset = (SrcBitmap->Width - SrcBitmap->Height) / 2;
BitBlt(DestBitmap->Canvas->Handle, offset, offset, SrcBitmap->Width, SrcBitmap->Height,
SrcBitmap->Canvas->Handle, 0, 0, SRCCOPY);

Answer 2

Once I had a similar problem.
I'm wanted to rotate two images around a common rotation point. But I couldn't do it with the standard function, because it doesn't allow a rotation point decentralized to the center.
Nevertheless I had made notes to the standard function at that time. Maybe they help you.
I'm remember that it was important that the size of the target image is correct! If a portrait image becomes a landscape image, the image becomes wider, therefore the BitBlt function must also specify the size of the target image.
Here my note to standard function.
Filling the xForm parameters was not quite the same for me as in your code snippet.
This was then the function I used to rotate around any center.