how can I keep track of changes in a Bitmap in view

Iam facing a problem regarding Bitmap. Actually iam displaying a bitmap on my view. If bitmap's height and width exceeds my view size then, scrollbar will appear. Ok. Now iam providing a pencil using which the user can edit/draw something on the bitmap.

Now, for the bitmaps of bigger size(ie: in which case scrollbar is coming), when user scrolls down and draw something on the bitmap area, how do i keep track of that change in the original bitmap. [iam not able
to keep track of that change, and when redraws that change goes off].
Hope now u got the problem. If u have any idea regarding this, please let me know.

Jayaraj_JAsked:
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chensuCommented:
You should use a off-screen bitmap, which is held in a memory DC. The off-screen bitmap is full-size. Whenver the users make some changes, it should be recorded on the off-screen bitmap. The bitmap you display on your view is just a part of the off-screen bitmap.

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Jayaraj_JAuthor Commented:
I am using MSDN January-98. Is there any sample programs which demonstrates this off-screen bitmap handling. Or can you give me an idea how to create and handle an off-screen bitmaps? please.
chensuCommented:
The idea is:
1. Create a memory DC;
2. Load the bitmap;
3. Selete the bitmap into the memory DC;
4. Always BitBlt from the memory DC to the screen DC when you draw it;
5. Whenever the users draw on the screen, draw it on the memory DC and update the screen.

Because the screen view size may be smaller than the bitmap, you need to keep a rectangle which you can use for coordinates translation between the screen view and the off-screen bitmap.
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