Can't Display Bitmap of Higher Resolution than CDC area

Question

Hi there dear gurus and expert coders.

i am not gonna start with im a newbie and don't know much about image programming but unfortunately those are the facts :(

I am trying to display an image from a bitmap pointer *ImageData which is of resolution 1392x1032. I am trying to draw that at an area of resolution or size 627x474.

However, repeated trying doesnt seem to work. It works when I change the bitmap image I created from *ImageData width and height to resolution or size of around 627x474

I really do not know how to solve this after trying all possible solutions from various forums and google.

pDC is a CDC* and memDC is a CDC initialized in an earlier method Anything uninitialized here was initialized in other methods.

Here is my code dear humble gurus. Do provide me with guidance Yoda and Obi-Wan provided to Luke Skywalker.

void DemoControl::ShowImage( void *ImageData )
{


    int Width; //Width of Image From Camera
    int Height; //Height of Image From Camera

    int m_DisplayWidth = 627 ;//width of rectangle area to display
    int m_DisplayHeight = 474;//height of rectangle area to display

    GetImageSize( &Width, &Height ) ; //this will return Width = 1392, Height 1032

    CBitmap bitmap;

    bitmap.CreateBitmap(Width,Height,32,1,ImageData);

    CBitmap* pOldBitmap = memDC.SelectObject((CBitmap*)&bitmap);

    pDC->BitBlt(22, 24, 627, 474, &memDC, 0, 0, SRCCOPY);

    memDC.SelectObject((CBitmap*)pOldBitmap);

    ReleaseDC(pDC);

}

---

Ok heres some additional parts

I think i should explain how the flow goes.

(a) A class (lets say DemoTestingDlg class) will pass the CDC as below to another class (lets say DemoControl class)

m_Demo = new DemoControl ; 

m_Demo->Initialisation( this, this->GetDC() ) ; 

(b) At the DemoControl class

bool DemoControl::Initialisation( CDemoTestingDlg *m_FormControl, CDC* m_StaticDisplay ) {

          pDC = m_StaticDisplay ; 
          memDC.CreateCompatibleDC(pDC); 

}

pDC and memDC is as such in the header:

CDC* pDC ; CDC memDC; 

(c) If lets say an image is captured, the image pointer is passed to the DemoTestingDlg class which will subsequently call a showImage method in the Demo Control Class which is the method I wrote in the question. Am i doing it right?

Note: It did show an image if lets say they are of the same size (by they i mean the CDC and bitmap) so i was under the impression that my CDC pointer was passed correctly

Answer 1

StretchBlt is your friend :)

Edit: OK how do you get pDC? When is your function called? Form OnPaint or DrawItem?

This is a StretchBlt I do from a DrawItem call in an overriden CStatic.

HDC hBitmapDC   = CreateCompatibleDC( pDrawItemStruct->hDC );

HBITMAP hBitmap = GetBitmap();
HGDIOBJ hOld    = SelectObject( hBitmapDC, (HGDIOBJ)hBitmap );

StretchBlt( pDrawItemStruct->hDC, pDrawItemStruct->rcItem.left, pDrawItemStruct->rcItem.top, pDrawItemStruct->rcItem.right, pDrawItemStruct->rcItem.bottom,
            hBitmapDC, 0, 0, 4, 4, SRCCOPY );

SelectObject( hBitmapDC, hOld );
DeleteObject( hBitmapDC );

Its not using the MFC classes to stretch a 4x4 bitmap into a larger space but works perfectly. My guess is that you aren't doing it in response to a WM_PAINT/WM_DRAWITEM and/or are using the wrong DC.

Edit re your edit: Do you then call DrawImage from inside an OnPaint or DrawItem call?

I would have thought you are better off not cacheing that CDC and passing the CDC in each time you wish to draw it.

Answer 2

"from a bitmap pointer *ImageData which is of resolution 1392x1032"

No it isn't, it's of size 1392x1032. Resolution is the amount of discrete visual units per distance.

Anyway as was mentioned above, you need to post more code. Show at least OnPaint(). Where are you instantiating the CPaintDC? Make a new project and put all your code in there, so that you have a minimal test set that exhibits the problem. You seem to be roughly on the right track, if you use StretchBlt() in place of the BitBlt() you're using now.