{"id":154,"date":"2007-07-20T14:34:08","date_gmt":"2007-07-20T18:34:08","guid":{"rendered":"https:\/\/blogs.mathworks.com\/steve\/2007\/07\/20\/imoverlay-and-imagesc\/"},"modified":"2019-10-23T13:42:53","modified_gmt":"2019-10-23T17:42:53","slug":"imoverlay-and-imagesc","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/steve\/2007\/07\/20\/imoverlay-and-imagesc\/","title":{"rendered":"imoverlay and imagesc"},"content":{"rendered":"
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In March 2006<\/a> I wrote about how to overlay a specified color onto specified pixels of a grayscale or RGB image. I put a utility function\r\n (imoverlay<\/tt><\/a>) for performing this operation on the MATLAB Central File Exchange.\r\n <\/p>\r\n

Earlier this week, Matt W. posted a review of that submission, suggesting that additional documentation might be helpful for\r\n people displaying image data that's not in the range [0, 1]. MATLAB users often call imagesc<\/tt> to \"autoscale\" image data, and Image Processing Toolbox users can call imshow(I,[])<\/tt> to get a similar effect.\r\n <\/p>\r\n

Here's an example using imagesc<\/tt>.\r\n <\/p>\r\n <\/introduction>

L = membrane(1, 100);\r\nimagesc(L), axis equal<\/span>, colormap(hot)\r\naxis equal<\/span><\/pre> 
Note that L has negative values.<\/p>
min(L(:))<\/pre>
\r\nans =\r\n\r\n   -0.3338\r\n\r\n<\/pre>
max(L(:))<\/pre>
\r\nans =\r\n\r\n     1\r\n\r\n<\/pre>
imagesc<\/tt> automatically scales the values of L<\/tt> \"into\" the colormap, so that the minimum value is displaying using the first colormap color and the maximum value is displayed\r\n      using the last colormap color.  (See my post about scaled indexed images<\/a>.)\r\n   <\/p>\r\n   
Although Matt's specific suggestion was about getting imoverlay<\/tt> to work with this data, there's a more general concept here about how to save images like the one above to image file formats,\r\n      such as JPEG or PNG.  Image file formats have generally much less flexibility than MATLAB about displaying pixel values as\r\n      colors.  In particular, the most common formats don't have the notion of scaling an arbitrary range of data values into a\r\n      colormap.  In addition, the JPEG format doesn't even have the notion of using a colormap at all.\r\n   <\/p>\r\n   
We could try writing the L values directly into a JPEG file, like this:<\/p>
imwrite(L,'membrane1.jpg'<\/span>)\r\nimshow membrane1.jpg<\/span><\/pre> 
But we lost all the color information, and there's no automatic data scaling.  All the values of L<\/tt> less than 0 were simply clipped to black in the JPEG file.\r\n   <\/p>\r\n   
So how can we turn L<\/tt> and the hot colormap into something that can be stored in a JPEG file?\r\n   <\/p>\r\n   
We can do it in three steps:<\/p>\r\n   
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