Re: imread image sizing

[Daniel J Sebald]

Allow me to elaborate on what JD said...   I think I see what the issue 
is.  I just looked inside the "image.m" file.  There is a silly little 
unsuspecting variable called "zoom".  Maybe a "help image" will describe 
what that is.  Anyway, it looks like a parameter that if NOT specified 
will be chosen so that it always attempts a resolution of 350 x 600, but 
it uses rounding to the nearest integer.  So in your case, Ross, the 
result is probably 1/2, i.e., the 320x240 you describe.

My preference would be to simply set the zoom equal to 1.0 if the 
variable is not specified.  Anyway...  Since you cannot specify the zoom 
parameter in imshow() and pass it to image(), which imshow() uses, I 
suggest just using image() instead.  Here are the three commands that 
imshow() uses if there are three arguments, i.e., R G B.

   [a1, a2] = rgb2ind (a1, a2, a3);
   colormap (a2);
 image (a1);

So do those directly and change the last command so that it reads

image([],[],a1,1);

Hopefully that will force the 'x' and 'y' to be generated by the routine 
and force the resolution to stay just as it is.

I'm not sure this truely allows RGB images.  (Unless the colormap 
happens to be huge.)  But, give that a try.  Glad you pointed this out, 
because I too probably would have been bitten by this along the way.

Dan


JD Cole wrote:

> Hi Ross,
>    I've come across this before too. If you follow the trail of 
> functions that 'imshow' actually uses, you'd see that after it creates 
> a colormap and an indexed image from the r/g/b parameters, it uses the 
> 'image' command to display them. 'image' takes  a scale parameter, but 
> in this case, imshow doesn't pass one, in the case of no scale 
> parameter, 'image' decides to make a scaled image, hence, your 
> 320x240. You got a couple of choices, you could create you're own 
> version of 'imshow' and add a scale parameter, something like this
>
> imshow (a1, a2, a3, scale)
> ....
> ....
> image (a1, scale)
> ....
> ....
> or at the very least
>
> ....
> image (a1, 1)
> ....
>
> This problem bothers me often too, but I'm not sure what the 
> mainstream solution would be since changing the definition for 
> 'imshow', as distributed with octave, would break from compatibility 
> with other numerical programs which shall remain un-named.
>
> Maintainers have any thoughts?
>
> JD
> Ross Vandegrift wrote:
>
> > Hello everyone,
> >
> >     I'm working on an image analysis project, and am using imread to
> > read in a color 640x480 JPEG file and sample some points.
> >
> >     Oddly though, when read in and displayed, the graphic is
> > displayed as 320x240.  If I manually run "display eye.jpg", the image is
> > loaded as 640x480.  I've tried setting "-size 640x480" and "-sample
> > 640x480" in the options, neither one helps.
> >
> >     Here's how I'm loading/displaying my image:
> >
> > [red, green, blue] = imread('jpg:eye.jpg', ["-size 640x480";]);
> > red = red / 255;
> > blue = blue / 255;
> > green = green / 255;
> > imshow(red, green, blue)
> >
> >     It's not a critical problem or anything - I'm still able to work
> > on the project at 320x240, but is there something I can do to get
> > 640x480?
> >
> > Thanks!
> >  

--

Dan Sebald
email: daniel DOT sebald AT ieee DOT org
URL: http://acer-access DOT com/~dsebald AT acer-access DOT com/