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    <p>You are making two mistakes here.</p> <p>While reading the image, you are specifying 1 as input arguments. As explained for <a href="http://docs.opencv.org/doc/tutorials/introduction/display_image/display_image.html#explanation" rel="nofollow">imread</a>, the function can read image in three different formats.</p> <blockquote> <p>CV_LOAD_IMAGE_UNCHANGED (&lt;0) loads the image as is (including the alpha channel if present) CV_LOAD_IMAGE_GRAYSCALE ( 0) loads the image as an intensity one<br> CV_LOAD_IMAGE_COLOR (>0) loads the image in the RGB format</p> </blockquote> <p>for your case you will have to use <code>CV_LOAD_IMAGE_GRAYSCALE</code> as the second argument</p> <p>In the next step where you are using: <code>image.at&lt;unsigned&gt;(0,1);</code> which does not correspond to anything. you use <code>&lt;unsigned&gt;</code> but the compiler says "UNSIGNED? UNSIGNED what???"</p> <p>I believe a better way to do this is to use correct labels like <code>CV_LOAD_IMAGE_COLOR</code> or <code>CV_LOAD_IMAGE_GRAYSCALE</code>. In the example below, I am reading the image into one channel (<code>CV_LOAD_IMAGE_GRAYSCALE</code>), which automatically converts it to grayscale.</p> <pre><code>#include &lt;cv.h&gt; #include &lt;highgui.h&gt; #include &lt;iostream&gt; using namespace std; using namespace cv; int main() { Mat gImg = imread("img.png", CV_LOAD_IMAGE_GRAYSCALE); uchar val; val = gImg.at&lt;uchar&gt;(0,0); cout &lt;&lt; (int)val &lt;&lt; endl; imshow("showImg",gImg); cvWaitKey(0); return 1; } </code></pre>
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    1. CORight, ok, this works, but have you tried what this actually outputs? In my case I get -89 for pixel (0,0). How is this possible when the image is 8bit unsigned? next I'd like to ask why do you use uchar?
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    2. COofcourse I have tried it. I dont know why you are not getting the output correctly. I have used UCHAR exactly for that same reason which is explained in the comment above by @count0, i.e. it can represent a value between 0 to 255
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    3. COYou cannot use imread this way. The second argument is a flag that is either 0, >0 or <0 depending on if you want to read the image grayscale, 3-channel color or as is. You cannot specify the type exactly. It just runs in the specific case because CV_8U==0
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