How can I rescale an image while not changing the dimension of the image?

How can I rescale an image while not changing the dimension of the image? When I scale it up, it just crops the image to fit within the original size. When I scale it down, it pads the edges of the image with zeros. I want to zoom in and out about the center of the image. When using "imresize", it changes the dimensions of the image to preserve all the content. I have the image processing toolbox.

Answer 1

I'm not sure what you're talking about. How are you performing the scaling? Are you using the zoom() function? Are you using 'InitialMagnification' property of imshow()?

For example, I have a 640x512 image. I want to rescale it up or down, but I still want the processed image to be 640x512. If I scale it up, it crops the rescaled image to fit within 640x512. If I scale it down, it pads the image with zeros to fit within 640x512. Similar to a digital zoom on a video camera.

Assuming you're doing your scaling with zoom(), this does not affect what you process. It only affects what is displayed. The display region on your monitor is the same number of pixels of course but calling zoom() will replicate pixels for display, but only for display. You can click and drag the image to move it around once it's zoomed. If you zoom way out, so that your image gets tiny on the screen, I don't know what it does - if it just shrinks the axes control, or if it keeps the axes the same size and just makes it black outside the tiny image. But it doesn't really matter - your image is still the same variable and all this is just for display only. It won't affect what parts of the image that you process.

This is not exact what I need. The zoom function assumes I am displaying the image. I need to do further processing of the scaled image. In addition, it can be larger than the monitor as well as have different bit depths. I am looking for something similar to imresize which outputs a processed image.

imresize() does output the processed image ?

Like Walter says, imresize() does output a processed (i.e. resized) image. You can then cast that to a different bit depth, if you want. Look:

smallImage = imread('pout.tif');
whos smallImage
% Enlarge the image to 4000 by 6000, bigger than the monitor.
bigImage = uint16(imresize(smallImage, [4000,6000]));
whos bigImage

    Name              Size             Bytes  Class    Attributes
    smallImage      291x240            69840  uint8 
    Name             Size                 Bytes  Class     Attributes
    bigImage      4000x6000            48000000  uint16  

No zooming or displaying was done at all. Please tell us why this does not meet your needs.

Richard - are you still there?????

Answer 2

I think you want to look into imtransform, specifically the Xdata and Ydata properties. E.g. you can rotate around the center--keeping original size. Or rotate around the left corner (image moves out of view if you keep original dataspace). Etcetera for other 'focus' points and transform types.

imrotate() can also crop or enlarge the canvass when you rotate. But he didn't say anything about rotation, just scaling. I'm still trying to figure out what he wants though.

e.g. means for example, or exempli gratia according to wiki--since we all know what is meant by rotate. Based on his wording, richard may not differentiate properly between scaling and cropping, so I chose rotate as example; imtransform can resize and crop too.

I think what he means is what some photo-viewing applications do when you scroll: the display window size remains constant and the image is scaled and cropped(or padded) around the center to the windowsize.

Should also be possible with imresize, but after the resize the user would need to manually crop or padd, e.g. using padarray or imcrop.