zoom image interactively maintaining a square zoom field

I'd like to use the + button with e.g. some control key, or some other trick, in order to zoom interactively on an image while maintaining a square 1:1 aspect ratio so the zoomed image is not distorted. Is there a way to do this?

[Walter Roberson]

https://blogs.mathworks.com/community/2007/12/10/focused-on-zooming/

[Duncan Carlsmith]

A-ha. thanks Walter. Not exactly what I wanted but I wan't aware I could enable pan and zoom for a figure with the command line and use the keyboard per https://www.mathworks.com/help/matlab/ref/matlab.graphics.interaction.internal.pan.html and https://www.mathworks.com/help/matlab/ref/matlab.graphics.interaction.internal.zoom.html . So that is a work around. I am used to in interactive environment being able to say control-drag a zoom box open so its aspect ratio stays fixed, sort of thing.

[Walter Roberson]

You can use the PostAction callback to set the zoom limits back to what would fit with square aspect ratio.

Or you could turn on zoom for the x axis, and use the PostAction callback to make the same change to the y limits.

[Duncan Carlsmith]

Walter, OMG! I had no idea one could customize in this way - https://www.mathworks.com/help/matlab/creating_plots/create-callbacks-for-graphics-objects.html . That's awesome, have to try it out! Can you (cough cough) supply the code to do what I want? :)

Answer 1

[Image Analyst]

Try using a scroll panel. See attached demo.

[Duncan Carlsmith]

I wasn't familiar with Image Viewer app (https://www.mathworks.com/help/images/interact-with-images-using-image-viewer-app.html) which works like a charm for me in interactively scanning a square magnification field. I love dragging the view window around in the navigation view pane. Very cool demo you have there showing how one can roll one's own. Thank you!

This solution is suboptimal when I am viewing an image or plot just created in the Live Editor - I would have to open the plot in an external window, save the image, open the Image Viewer App, open the file... That use case is where I'd like a constrained Zoom mactive even in the Live Editor.

Answer 2

[Walter Roberson]

ax = gca;
hZoom = zoom(ax);
setAxesZoomConstraint(hZoom, 'x');
hZoom.ActionPostCallback = {@SquareZoom, ax.XLim(1), ax.YLim(1)};

function SquareZoom(hObj, evd, x0, y0)
   ax = evd.Axes;
   xwid = diff(ax.XLim);
   ax.XLim = [x0, x0 + xwid];
   ax.YLim = [y0, y0 + xwid];
end

This particular implementation anchors the x and y on the lower left corner of the axes limits as they exist at the time the action is started. Every time the callback is invoked, the limits will be re-anchored at that position.

There are, of course, other valid interpretations on what to do; for example you might have valid reason to want to support panning as well. Or at initialization time you might want to find the shorter of the current x and y and clamp the limits according to the shorter to keep the desired 1:1 ratio.

[DGM]

I'm not sure how to set this up, but it just gives me an error.

Error using matlab.graphics.interaction.internal.zoom/setAxesZoomConstraint (line 7)

Input must be an axes handle.

Error in AT_sandbox (line 17)

setAxesZoomConstraint(hZoom, 'x');

[Walter Roberson]

setAxesZoomConstraint(ax, 'x');

maybe.

[Duncan Carlsmith]

I tried and the 10 April suggestion failed. I can't find documentation on the setAxisZoomConstraint function but https://stackoverflow.com/questions/65901058/setting-one-instance-of-a-graph-to-zoom-in-the-x-direction-only suggested I try this mod, which failed.

setAxesZoomConstraint(hZoom,ax, 'xy');

hZoom.ActionPostCallback = {@SquareZoom, ax.XLim(1), ax.YLim(1)};

It runs but still does not constrain to square.

The best thing would be... You know how if you control click a fig, you get to choose uncontrained, x, or y zoom. Best if constrained were an option there.

[Walter Roberson]

setAxesZoomConstraint is listed under "object functions" in https://www.mathworks.com/help/matlab/ref/matlab.graphics.interaction.internal.zoom.html