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Sent: Thursday, July 28, 2011 12:44 PM

To:

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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

browser.

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

point

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

manual):

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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https://mailman.cae.wisc.edu/listinfo/help-octaveImagesc 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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