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Q: c++ bitmap search ( No Answer,   1 Comment )
Subject: c++ bitmap search
Category: Computers > Algorithms
Asked by: chilltown-ga
List Price: $10.00
Posted: 08 Nov 2005 13:08 PST
Expires: 08 Dec 2005 13:08 PST
Question ID: 590667
This question is regarding bitmaps in a win32 application.  I am
looking for an efficient way to search an active window for a given
24bit bitmap.  For example, does the current webpage contain bitmap X
anywhere in the window?  Currently I am creating a compatible DC to
the window to be searched, then creating compatable bitmap and loading
the bitmap to be searched for into it.  Then I am literally searching
the target window pixel by pixel for one that matches pixel 0,0 of the
created source bitmap.  Once I find a match I move to the second pixel
etc until a match is found or the entire window has been searched. 
This works fine when, say, searching for a 20x20 pixel bitmap in maybe
only the top-left quad of a window, but when searching for larger
bitmaps or larger areas of the window, this process is taking too long
for my application needs.  Are there any suggestions to improve?  Are
there any libraries developed for this type of search?
There is no answer at this time.

Subject: Re: c++ bitmap search
From: doopdoop-ga on 08 Nov 2005 14:24 PST
If you are only looking for EXACT matches, you can use an appropriate
pattern matching algorithm like Rabin-Karp


Usually, only the one-dimensional case is shown, but it can be adapted
to 2D, if the hash update procedure is changed. It should not be that
hard to do.
For a small (constant) number of possible results, this algorithm is
linear on the image size.
If you have only few colors (e.g. black and white) you might consider
adapting Boyer-Moore. If you have a relative small pattern and a
fairly large image, Knuth-Morris-Pratt might be for you. Both only
work for 1D pattern, but you could start with searching for a line
from the pattern first, and then test, whether the whole pattern match
at this position. Google will help.

The problem of finding ALMOST exact patterns in images is know as
"template matching" in computer vision. Usually,
cross-correlation-based methods are used. Look e.g. for the free
OpenCV library, which implements such methods.

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