Re: changing brightness and contrast in an nsimage
Re: changing brightness and contrast in an nsimage
- Subject: Re: changing brightness and contrast in an nsimage
- From: Eric Smith <email@hidden>
- Date: Sat, 3 Dec 2005 19:46:12 -0800
OK. I have NSImages that I've created by filling an NSBitmapRep's
data buffer with information that I get from an image sensor. When I
create the rep, I scale all the data from the sensor so that I get a
grayscale image spanning 0-255 in each color channel... so I start
with maximum contrast.
So, part one of the problem is, what's the most efficient way of
changing the brightness and contrast of this image? vImage uses
Altivec, and was my first choice for speed. vImage does have a
function for *increasing* the contrast... works just fine. Problem
is, I start with maximum contrast, so any contrast stretching causes
information near the edges of the histogram to be lost... ie,
everything from, say, 200 - 255 gets mapped to 255. Question: how
do people typically deal with this problem? Keep an original bitmap
around in case one wants to reduce the contrast (get back all the
original information) ?
Part two of the problem: vImage has no brightness control (unless
it's called something else, and I just don't recognize it... that's
possible). Does it exist somewhere else? Question: does Apple
provide a set of routines that operate on either and NSImage, or the
bitmap rep, that will change the brightness and contrast for me?
I'd also be interested in general brightness and contrast algorithms,
if anyone has info they'd like to pass along.
Thanks,
Eric
On Dec 3, 2005, at 7:30 PM, Dave Camp wrote:
I think the problem is it isn't clear what you are asking for...
Are you asking for pointers to general brightness and contrast
algorithms?
Are you asking how to get at the NSImage bitmap data to manipulate
it yourself?
Or are you asking if there are other Apple supplied frameworks that
will perform brightness and contrast adjustments for you?
Dave
On Dec 3, 2005, at 8:04 PM, Eric Smith wrote:
CIImage wasn't even around until 10.4... there must be a way to
handle this from an NSImage. Anyone out there use brightness and
contrast controls on an NSImage, or the bitmap data associated
with the NSImage?
Thanks,
Eric
On Dec 2, 2005, at 9:34 PM, John C. Randolph wrote:
On Dec 2, 2005, at 9:29 PM, Eric Smith wrote:
I've been going around in circles with this one...
What is a good way to change the brightness and contrast in an
NSImage?
I've been playing with the vImage library for changing contrast,
using the vImageEndsInContrastStretch. This works, but how does
one return the contrast to it's original value? Once you
specify that > 0% of the pixels lie at the histogram extreme,
you can't get the original values back. I know of the ImagApp
example showing how to use CIImage, but using that would require
a re-write of quite a bit of code... I'd rather not go there.
Too bad, because that's how to do it.
-jcr
John C. Randolph <email@hidden> (408) 914-0013
Roaming Cocoa Engineer,
Available for your projects at great Expense and Inconvenience.
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