C++ builder transparent bitmaps cover the canvas

Question

In the OnPaint event of a TForm, I want to paint bitmaps that do not cover the background or other painted object because they have transparent parts.

If I draw an image over an image, it works.

But when I draw on the Form's Canvas, it does not work: the white part of the image, which is supposed to be transparent covers the other objects of the Canvas by a white square color.

Canvas->CopyMode = cmMergePaint ;
Graphics::TBitmap * Image=new Graphics::TBitmap();
Image->Transparent = true;
MainForm->Images->GetBitmap(14, Image);

Canvas->Draw(10,10,Image;
MainForm->Images->GetBitmap(0, Image);
Canvas->Draw(15,15,Image);

Update

When I draw on the Image using MainForm->Images->Draw(Image->Canvas...), I get a transparent square with nothing inside, that I can move over the other components.

When I draw using MainForm->Images->GetBitmap(ImgIndex[HisType]+Rotation, Image), I get the correct streched image on the Form, but without transparencies i.e. its white parts cover the other components.

While the MainForm->Images->Draw(Canvas, X, Y, ImgIndex[HisType]+Rotation, dsTransparent, itImage); does the job, I need to strech it for this component depending on a Size variable.

TRect DstRect(X,Y, X+32 + ( 1 - Rotation ) * 32 * Size, Y+32 + Rotation * 32 * Size);
Graphics::TBitmap * Image=new Graphics::TBitmap();
Image->Transparent=true;
//MainForm->Images->GetBitmap(ImgIndex[HisType]+Rotation, Image);
MainForm->Images->Draw(Image->Canvas, 0, 0, ImgIndex[HisType]+Rotation, dsTransparent, itImage);
Canvas->StretchDraw(DstRect, Image);
delete Image;

//MainForm->Images->Draw(Canvas, X, Y, ImgIndex[HisType]+Rotation, dsTransparent, itImage);

Answer 1

Found the solution, Thank to Remy. We must first fill the newly created bitmap with one color and not let it empty in order to transparency to work...

Size=1; //debug
TRect DstRect(X,Y, X+32 + ( 1 - Rotation ) * 32 * Size, Y+32 + Rotation * 32 * Size);
Graphics::TBitmap * Image=new Graphics::TBitmap();
Image->Width = 32;
Image->Height = 32;
Image->Canvas->FillRect(Rect(0,0,32,32));
MainForm->Images->GetBitmap(ImgIndex[HisType]+Rotation, Image);
//MainForm->Images->Draw(Image->Canvas, 0, 0, ImgIndex[HisType]+Rotation, dsTransparent, itImage);
Image->Canvas->Pen->Color = clRed;
Image->Canvas->MoveTo( 3, 3 );
Image->Canvas->LineTo( 29, 29 );
Image->Transparent=true;
Canvas->StretchDraw(DstRect, Image);
delete Image;

Answer 2

Use Images->Draw() instead, let the TImageList handle the drawing for you:

MainForm->Images->Draw(Canvas, 10, 10, 14, dsTransparent, itImage);
MainForm->Images->Draw(Canvas, 15, 15, 0, dsTransparent, itImage);