building an image from Mat object in OpenCV

Question

Im using OpenCV and I have a Mat object of size 1024*1024(extracted from a photo and manipulated) and the values are in the range [1..25].for example:

Mat g;
g=[1,5,2,14,13,5,22,24,5,13....;
21,12,...;
..
.];

I want to represent these values as an image.It is only an illustration image to show the different areas,each area with a color. For example: all the values that equals 1=red, all the values that equals 14=blue, and so on..

and then construct and display this photo.

Anybody have an idea how should i proceed?

Thanks!

Bull · Accepted Answer · 2013-09-04 12:12:42Z

1

If you are not too fussed what colors you get, you can scale your data (so it almost fills the 0 to 255 range) then use an inbuilt colormap. e.g.

cv::Mat g = ...
cv::Mat image;
cv::applyColorMap(g * 10, image, COLORMAP_RAINBOW);

See the applyColorMap() doco

answered Sep 4, 2013 at 12:12

Bull

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3 Comments

triple13 Over a year ago

I tried it but i get a black image when i try to use colormaps i don't know why..thanks anyway :)

Bull Over a year ago

@triple13 The rainbow colormap doesn't even contain black so you have done something really wrong.

triple13 Over a year ago

Yes i know. The problem i think is the type of my Mat, because if i load a normal image using imread and then apply colormaps it works fine

berak · Accepted Answer · 2013-09-04 10:27:24Z

0

there are colormaps , but they won't help if your data is in the [0..25] range only. so you probably ned to roll your own version of that:

   Vec3b lut[26] = { 
        Vec3b(0,0,255),
        Vec3b(13,255,11),
        Vec3b(255,22,1),
        // all the way down, you get the picture, no ?
   };

   Mat color(w,h,CV_8UC3);
   for ( int y=0; y<h; y++ ) {   
       for ( int x=0; x<w; x++ ) {
           color.at<Vec3b>(y,x) = lut[ g.at<uchar>(y,x) ];   
          // check the type of "g" please, i assumed CV_8UC1 here. 
          // if it's CV_32S, use g.at<int>  , i.e, you need the right type here 
       }
   }

edited Sep 4, 2013 at 10:27

answered Sep 4, 2013 at 9:06

berak

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1 Comment

triple13 Over a year ago

@berak..thanks so much this code works perfectly. Although i had to choose the elements values of 'lut' manually using suitable pixels values, but it is ok as long as it works fine. When i tried to use colormaps i had a black image and i don't know why.maybe it is because of the type of Mat or something else but no problem i used the other way anyway. Thanks again :)

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