Image manipulation: bitmap rotation (C++/GDI+) - c++

Question

I spend much time trying to find solution but cannot. I hope you can help me. The code is bit longer so I give here just the part where I have the problem. My code captures bitmap from window and its saved in HBitmap. I need to do rotation of the bitmap. So I start GDI+ and create bitmap pBitmap from HBitmap:
// INIT GDI
ULONG_PTR gdiplusToken;
GdiplusStartupInput gdiplusStartupInput;
GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);
if (!gdiplusToken) return 3;
// Gdip_GetRotatedDimensions:
GpBitmap* pBitmap;
int result = Gdiplus::DllExports::GdipCreateBitmapFromHBITMAP(HBitmap, 0, &pBitmap);
Then I calculate the variables needed for rotation. Then I create graphics object and tried to rotate the image:
GpGraphics * pG;
result = Gdiplus::DllExports::GdipGetImageGraphicsContext(pBitmap, &pG);
Gdiplus::SmoothingMode smooth = SmoothingModeHighQuality;
result = Gdiplus::DllExports::GdipSetSmoothingMode(pG, smooth);
Gdiplus::InterpolationMode interpolation = InterpolationModeNearestNeighbor;
result = Gdiplus::DllExports::GdipSetInterpolationMode(pG, interpolation);
MatrixOrder MatrixOrder_ = MatrixOrderPrepend;
result = Gdiplus::DllExports::GdipTranslateWorldTransform(pG, xTranslation, yTranslation, MatrixOrder_);
MatrixOrder_ = MatrixOrderPrepend;
result = Gdiplus::DllExports::GdipRotateWorldTransform(pG, ROTATION_ANGLE, MatrixOrder_);
GpImageAttributes * ImgAttributes;
result = Gdiplus::DllExports::GdipCreateImageAttributes(&ImgAttributes); // create an ImageAttribute object
result = Gdiplus::DllExports::GdipDrawImageRectRect(pG,pBitmap,0,0,w,h,0,0,w,h,UnitPixel,ImgAttributes,0,0); // Draw the original image onto the new bitmap
result = Gdiplus::DllExports::GdipDisposeImageAttributes(ImgAttributes);
Finally I wanted to check the image so I added:
CLSID pngClsid;
GetEncoderClsid(L"image/png", &pngClsid);
result = Gdiplus::DllExports::GdipCreateBitmapFromGraphics(w, h, pG, &pBitmap);
result = Gdiplus::DllExports::GdipSaveImageToFile(pBitmap, L"justest.png", &pngClsid, NULL); // last voluntary? GDIPCONST EncoderParameters* encoderParams
But my image is blank. I found out that GdipCreateBitmapFromGraphics creates blank image, but how should I finish it to check what drawings I have done? Are these steps correct (not just here but above, near GdipCreateBitmapFromHBITMAP() and GdipGetImageGraphicsContext() or I need to add something? How to get it working?
PS: I am sure that HBitmap contains picture of window, I already checked it.

Answer 1

To my eyes, you have some things backwards in your approach.
What you need to do is the following:
Read in your Image (src)
Find the minimum bounding rectangle that will contain the rotated image (ie, rotate the corners and the distance between min and max x and y are the dimensions.
Create a new Image object with these dimensions and the pixel format you want (likely the same as src, but maybe you want an alpha channel) and background color you want (dst)
Create a graphics based on dst (new Graphics(dst))
Set the appropriate transform on the graphics
Draw src onto dst
Export dst
The good news is that to make sure you're doing things right, you can isolate steps out.
For example, you can just make an image and a graphics and draw a line on it with no transform (or better a box with an X) and save that. If you have what you expect, then you're on the right path. Next add a transform to the box. In your case you'll need both a rotation and a translation. Next get the dimensions of the dest image right for that rotation (protip: don't use a square to test). Finally, do it with your actual image.
This will get you step-by-step to the correct output instead of trying to get the whole thing in one shot.