Re^2: Perl Image Analysis

You can get just the first channel (your grey value) with the getsamples() method:

# get all the samples for row $y
my @samples = $image->getsamples(y => $y, channels => [ 0 ]);
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If your image does have only one channel (check $image->getchannels) the channels argument is unneeded so you have:

my @samples = $image->getsamples(y => $y);
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You can also get the samples as a string of bytes:

my $samples = $image->getsamples(y => $y);
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You don't mention the precision of your data, but Imager currently always reads TIFFs at 8-bits/sample, even if the TIFF has higher precision.

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Re^3: Perl Image Analysis by lparsons42 (Novice) on Oct 05, 2006 at 13:49 UTC
Yes, this is looking more promising. The triplet values were not as useful as hoped for. I tried to use the getsamples() method to sample just one pixel, however, and the results were like: `巷褘『檄噙橙姍~蟋禿w U=9dy菽苠I*\ZGp諞` [download] I'm not sure how to compare those values to each other... My code to do this was as follows: `### #it appears there may actually be a way to read the grey channel #using the "getsamples" routine my $ycoord = 5; my $xcoord = 5; my $sample = $img->getsamples(y => $ycoord, x => $xcoord); print "we sampled 5,5 to have a value of $sample\n";` [download] I tried sampling two different points that I knew to be quite different by this method, which is how I got those two odd strings above. I'll have to look further into the Perldocs for Imager, it appears :)	[reply] [d/l] [select]
Re^4: Perl Image Analysis by tonyc (Hermit) on Oct 06, 2006 at 01:10 UTC
You're running into 2 problems here: getsamples() uses the context you call it in to decide whether to produce samples packed into a string or a list of samples if you don't supply a width getsamples() returns samples from the given x position all the way to the right side of the image. So to get just the sample for one pixel as an integer you'd do: `my ($sample) = $image->getsamples(x => $x, y => $y, width => 1);` [download] But if you're looking for speed you'll probably find that working a row at a time is faster: `for my $y ( 0 .. $image->getheight() - 1 ) { for my $sample ($image->getsamples(y => $y)) { # do something with the sample } }` [download]	[reply] [d/l] [select]
Re^5: Perl Image Analysis by lparsons42 (Novice) on Oct 06, 2006 at 18:58 UTC
Ok, I used that to determine the intensity of a single pixel in greyscale. However, when I tried a simple test of it, I got unexpected results. I made a test image, that was 24 pixels wide by 12 pixels tall. This is a jpeg that I made in kolourpaint and set to greyscale. It has a white background and I used only black color on it. I made two boxes on this image, which are each 8x with 2 white pixels all the way around and 4 white pixels in the center between the two boxes. The truly curious can see this da vinci - quality 834 byte jpeg at this site However, when I analyze this file, I get varied results for what should not be varied intensities. Specifically, the spots (5,5) and (5,17) should both be intensity 0 (since they are 100% black). However, this is not the case. Instead, (5,5) registers 15 and (5,17) registers 0. Any insight as to why this is would be great!	[reply]
Re^6: Perl Image Analysis by BrowserUk (Patriarch) on Oct 06, 2006 at 20:05 UTC
Re^7: Perl Image Analysis by lparsons42 (Novice) on Oct 06, 2006 at 21:14 UTC
Some notes below your chosen depth have not been shown here
Re^6: Perl Image Analysis by grep (Monsignor) on Oct 06, 2006 at 20:22 UTC
Re^4: Perl Image Analysis by rustyjuggler (Initiate) on Oct 06, 2006 at 06:41 UTC
If you have the RGB-values you can easily convert them to grayscale through the formula on Wikipedia where Y (luminance) is the grayscale intensity: Y = 0.299 * R + 0.587 * G + 0.114 * B Here is the link: http://en.wikipedia.org/wiki/YUV Anders, Norway	[reply]