Difference between revisions of "StretchBlt"

Revision as of 13:01, 10 December 2005

What is StretchBlt?

StretchBlt is a GDI API that is very similar to BitBlt in its operation, except that it not only copies part of a DC to another DC, but it will also stretch it.

Simple StretchBlt Use

The following is a simple example of how to use stretchblt (along with a previous code sample - [1]) in order to stretch an image to the form's size.

 Private Sub Form_Paint()
     Dim p As StdPicture
     Dim mDC As Long
     
     Set p = LoadPicture("some path")
     
     mDC = CreateCompatibleDC(Me.hdc)
     DeleteObject SelectObject(mDC, p.Handle)
     
     StretchBlt _
         Me.hdc, _
         0, 0, _
         ScaleX(Me.ScaleWidth, vbTwips, vbPixels), ScaleY(Me.ScaleHeight, vbTwips, vbPixels), _
         mDC, _
         0, 0, _
         ScaleX(p.Width, vbHimetric, vbPixels), ScaleY(p.Height, vbHimetric, vbPixels), _
         vbSrcCopy
     
     DeleteDC mDC
     
     Set p = Nothing
 End Sub

NOTE: You will need to replace "some path" in the LoadPicture call with an actual image path for this code to work.

Caveats and refinements of this method

You may have noticed, while playing with this code, that when you are stretching the image to a size smaller than original, you begin to get horrible drawing artifacts (especially with photos). But don't despair! MS thought of this and included two StretchBlt modes that eliminate this. To change the StretchBlt mode for a DC, you have to call SetStretchBltMode with one of the available modes. The two modes that eliminate this horrid effect are: COLORONCOLOR and HALFTONE. HALFTONE is higher quality but takes longer.

So here's the code:

 Private Sub Form_Paint()
     Dim p As StdPicture
     Dim mDC As Long
     
     Set p = LoadPicture("some path")
     
     mDC = CreateCompatibleDC(Me.hdc)
     DeleteObject SelectObject(mDC, p.Handle)
     
     SetStretchBltMode Me.hdc, HALFTONE
     StretchBlt _
         Me.hdc, _
         0, 0, _
         ScaleX(Me.ScaleWidth, vbTwips, vbPixels), ScaleY(Me.ScaleHeight, vbTwips, vbPixels), _
         mDC, _
         0, 0, _
         ScaleX(p.Width, vbHimetric, vbPixels), ScaleY(p.Height, vbHimetric, vbPixels), _
         vbSrcCopy
     
     DeleteDC mDC
     
     Set p = Nothing
 End Sub