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Loading bitmap portions

Posted on 1998-09-01
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Last Modified: 2010-04-30
Lets say you have a really large bitmap file, one too large to completely load into the computer's memory.  Can you load and display only a selected area of the bitmap without loading the rest of the bitmap into the computer's memory?
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Question by:totalitary
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Expert Comment

by:MikeP090797
ID: 1432465
First, no bitmap is too large to fit in computer's memory. Even if it's 1600x1200x24 colors, it won't take more then 6MB. IF you just want to display part of it, you can load it to a hidden picture box, and then use PaintPicture to transfer part of it to another box or form. If you still want to read only part of it, you will have to do that manualy, there's no built in way for it
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Author Comment

by:totalitary
ID: 1432466
The image is 6ft by 4ft so its not possible to load it into memory without waiting 30 minutes.
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polygon earned 600 total points
ID: 1432467
You'll need to learn the BMP file structure. It is smth like:

BITMAPHEADER a
BITMAPINFOHEADER b
PALLETE c
THEBITMAP d

then you read just this part that you need.
Say, you get the headers to see how you bitmap is compressed and some other info to help you read the actual info you need. Then say read just bytes starting at position 1234 to 1334, 4567 to 5667, ... to get just a given rectangle of your bitmap.
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Author Comment

by:totalitary
ID: 1432468
Does anyone know the BMP header stucture and how would you tranfer the information from the chosen bytes to the screen?
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