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POTroubles reading pixel values in C#, .Net 4.0
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copied!<p>I am learning C# and I am trying this making my homeworks on image processing examples. I am trying to read the pixel values along an horizontal line in a gray levels jpeg image. Some prints I made inform me that I am reading a Format8bppIndexed image (this was a kind of surprise to me, because my belief was that jpeg images do not use a palette at all). The code to load the image in the proper control in the windows form is something like this:</p> <pre><code> try { myImage = ( Bitmap ) Image.FromFile(imageName); } catch { MessageBox .Show("Unable to load the image" , Text, MessageBoxButtons .OK, MessageBoxIcon .Hand); } pictureBox1.Image = myImage ; </code></pre> <p>Then I try to read the pixel along an arbitrary straight path in the image listing them in a csv file when pressing a button. I assume that, being the image a gray level one, reading the red color is enough (is it? other color components are equal; using the getBrightness seems to me an overkill):</p> <pre><code> cursorStartx = 0; cursorStarty = 256; cursorEndx = myImage.Width; cursorEndy = 256; Color pixel; StreamWriter fs = new StreamWriter( "pixels.csv" , true ); for (var i = 0; i < cursorEndx; i++) { pixel = myImage.GetPixel(i, cursorStarty); fs.WriteLine( String .Format("{0}; {1}; {2}" , i, cursorStarty, pixel.R)); } fs.Close(); </code></pre> <p>When reading the cross section in the file I see values that make no sense at all: they are all multiple of 17 (???):</p> <pre><code>0; 256; 17 1; 256; 0 2; 256; 17 3; 256; 0 4; 256; 17 5; 256; 0 6; 256; 17 7; 256; 0 8; 256; 17 9; 256; 0 10; 256; 17 ............ 66; 256; 17 67; 256; 34 68; 256; 51 69; 256; 68 70; 256; 85 71; 256; 85 72; 256; 102 73; 256; 85 </code></pre> <p>An histogram made in the same program shows clearly distinct count peaks at multiple of 17 (????)</p> <p>Just to perform a reality check, I made something similar in Perl (I have a lot more experience in perl, I am just learning C#) using the module GD and, reading the pixel on the very same path in the same image, I get very different values.</p> <p>First rows of the histogram drwan by imageJ are the following:</p> <pre><code>0 0 1 0 2 1 3 1 4 15 5 81 6 304 7 984 8 2362 9 206144 10 2582 11 1408 12 653 13 451 14 345 15 321 16 277 17 288 </code></pre> <p>a quite different story.</p> <p>Someone could possibly explain to me what horribly stupid kind of error I am doing?</p> <p>Thanks a lot</p> <p>----------------------------------- EDITED LATER --------------------------------------</p> <p>Things are becoming intriguing. Reading the same image using <strong>Aforge</strong> image lab, written in C#, I got the same kind of error: the histogram shows discrete count at pixel values that are multiple of 17. Reading the image with <strong>ImageJ</strong>, written in Java, I get a correct histogram. Now I am trying to figure out if there is a way, using C#, to manage the image reading somehow, in order to be able to get the correct values, or this is a major issue and the alternatives are: to give up or to perform some sort of low level image reading (hard stuff for jpeg format...) </p>

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