imread (repost)

[Thomas L. Scofield]

On Aug 5, 2008, at 12:53 AM, Søren Hauberg wrote:

> man, 04 08 2008 kl. 21:57 -0400, skrev Thomas L. Scofield
>
>> At least some png image files (BMP and PSD files as well, I believe)
>> use 4 channels: red, green, blue and alpha.  For those files, imread
>> returns an array A satisfying size (A, 3) = 4..  (Would it do so as
>> well for files using CMYK color space?)  As such, the matrix A in
>> which the image is stored cannot be sent off to imshow without
>> limiting its 3rd coordinate, as in imshow (A(:, :, 1:3)).  The  
>> changes
>> John Eaton made to imread.m as I submitted it were quite necessary,
>> but in the process code I had included for limiting this 3rd
>> coordinate in imread before ever returning the image was lost.   
>> Should
>> it be reinstated?  Or, should imshow be modified to handle the
>> occasional image like these png files?  I don't mind making the
>> changes, but lean towards altering imshow instead of imread if this
>> doesn't result in an unacceptable incompatability with Matlab.
>> Interestingly, when I read this same kind of png into Matlab, it
>> returns a 2D matrix, not 3 like in Octave and, subsequently, it
>> displays the image as grayscale when it was really a color image (the
>> Octave call imshow(A(:, :, 1:3)) gets the color right).  (Perhaps  
>> that
>> is remedied in Matlab with optional arguments to imread.)
>
> The matlab version of imread supports an extra output argument that
> contains the alpha channel of an image. I don't think it is trivial  
> (nor
> a good idea) to change imshow to make it possible to show images  
> with an
> alpha channel. I mean, what should be to visual result when showing an
> image that s 100% transparent? I'd stick with showing RGB only.
>
> Søren
>>


That's not the kind of modification I was suggesting for imshow.  I  
was suggesting this much simpler change (pseudocode):

   if (length (size (image)) == 3 && size (image, 3) == 4)
     show image(:, :, 1:3)
   else
     show image
   endif

Thomas L. Scofield
--------------------------------------------------------
Associate Professor
Department of Mathematics and Statistics
Calvin College
--------------------------------------------------------

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