As shown in the previous article, drawing in a fixed position on a scroll view rises some problems. We thought at tree possible solutions:

1. Use layered windows. This is quite good but relatively complicated and still rises problems, especially in MDI applications. By curiosity or just for fun, you can download a demo here: Overlay text in MDI using layered windows.
2. Get rid of CScrollView and directly derive from CView. This may be a good/professional solution but implies a lot of work, “reinventing the wheel”.
3. Derive from CScrollView, and override the necessary virtual methods in order to modify the default behavior according to our needs.

Finally, I chose the option #3, by implemeting an MFC-extension class, CImageView, derived from CScrollView. Additonally, I made a class CImageDoc to be used togheter with CImageView in a document-view architecture (MDI/SDI MFC project). To make a simple image viewer with overlay drawing capability, you simply have to derive from these classes. Note that, CImageView has additional built-in features like image zoom-in, zoom-out, and zoom to selection, dragging image with the mouse, and so on.
Next is a brief list of public and virtual methods, to be used according to your needs.

CImageView class

• CImageView::SetBackPatternBrush – sets a background brush based on a bitmap.
• CImageView::SetBackSolidBrush – sets a background brush based on a solid color.
• CImageView::SetHandCursor – sets “hand” cursor, used when user begins dragging the image.
• CImageView::SetHandDragCursor – sets “hand drag” cursor, used when the user is dragging the image.
• CImageView::ZoomIn – increments the image zoom.
• CImageView::ZoomOut – decrements the image zoom.
• CImageView::ZoomToOriginalSize – displays the image in its original size.
• CImageView::ZoomToWindowWidth – fits image to the view’s width.
• CImageView::ZoomToWindow – fits the image in the view.
• CImageView::ZoomToRectangle – (virtual) can override the default type of “zoom to selection”.
• CImageView::DrawBackgound – (virtual) can override the backgound filling mode.
• CImageView::DrawImage – (virtual) can override the default image drawing mode.
• CImageView::DrawOverlay – (pure virtual) override is mandatory; it is designed for “overlay drawing”, according to each application requirements.

CImageDoc class

• CImageDoc::GetImage – returns a reference to contained source bitmap.
• CImageDoc::IsValidImage – returns TRUE if the source bitmap is valid.
• CImageDoc::GetImageSize – returns the source bitmap sizes.
• CImageDoc::LoadFromFile – (virtual) load the bitmap from a BMP file. Can be overridden if for example, instead of LoadImage WinAPI function we are using something else, whichallows loading more image file formats (JPEG, GIF, TIFF, PNG, etc).

Demo project

The demo project is a simple image viewer with overlay drawing capability. For demo purpose, it draws over the source image a text and an animated image.

To make something similar, you just have to:

1. Download the demo project.
2. Add to your project the definition and implementation files (.h and .cpp) for CImageView, CImageDoc, and CMemDC classes.
3. Derive your own classes form CImageView and CImageDoc (as in demo project are CDemoView and CDemoDoc).
4. Override CImageView::DrawOverlay to perform your own custom overlay drawings.
5. Do anything else as shown in the demo project. .

If something is unclear, as well as for any remark or recommendation, please do not hesitate to leave a reply

See also

• Codexpert blog: Overlay Drawing in CScrollView – Part 1
• Codexpert forum: Draw overlay discussion (RO)

Downloads

• Image viewer with overlay drawing - Demo project (2561 downloads)