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RE: imagesc

From: William Krekeler
Subject: RE: imagesc
Date: Thu, 28 Jul 2011 19:07:28 +0000

-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of pathematica
Sent: Thursday, July 28, 2011 12:44 PM
To: address@hidden
Subject: imagesc

octave 3.23/ubuntu 10.04 

I would be grateful if someone could explain how to use imagesc(). If it is
clear to someone who knows how to use it that I do not have the necessary
insight and remedial education is not a practical option in this forum,
please do not be afraid to say so in this forum - I will not be offended! 

I was hoping (I might have misunderstood) that it allows scaling of images
so that a suitable sized raster image might be plotted for rendering in a

Following the instructions in the manual, I have installed xloadimage (into
ubuntu) using the Synaptic Package Manager. 

% the following is just a random plot (not important) to illustrate the
tx = ty = linspace (0, 3, 31)'; 
[xx, yy] = meshgrid (tx, ty); 
tz = exp(-xx); 
mesh (tx, ty, tz); 
box("off"); hold("on"); 
% end of plot 

% then, intending to scale by 0.5, I have tried the following (I am
embarrassed to admit I am not sure how to interpret the 'x' given in the

imagesc(x, 0.5); 

% with the hope of printing a scaled png image with eg 
print -dpng filename.png 

This yields the following error messages 
error: `x' undefined near line () column () 
error: evaluating argument list element number 1 
error: evaluating argument list element number 1 

I apologise if my reading of the function is naive and this looks stupid!
Still, I have no pride and I will not be offended if someone merely lets me
know that I do not have the necessary skills or insight to use the function

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Imagesc works great and creates its own scaling if you just give it a matrix as 
input. I generally don't try to scale be a specific value though you could 
provide limits over the range of which to scale.

% examples
X = rand(100,100).*(2^14);      % you can't plot this 14 bit image in 8 bit 
image space
figure,imagesc(X); colorbar     % but you can plot it scaled
or you could apply limits
figure, imagesc( X, [ 6000 10000 ] ); colorbar; % or limit the range of 
presented data to be scaled to 6000 to 10000

% start specific explanations
Note often times the documentation for Matlab found online closely links or 
directly matches the Octave functions.

imagesc(x,y,C) displays C as an image and specifies the bounds of the x- and 
y-axis with vectors x and y. If x(1) > x(2) or y(1) > y(2), the image is 
flipped left-right or up-down, respectively
imagesc(...,clims) normalizes the values in C to the range specified by clims 
and displays C as an image. clims is a two-element vector that limits the range 
of data values in C. These values map to the full range of values in the 
current colormap.

Hope that helps.

Bill Krekeler

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