This is likely happening because you are using software OpenGL.
When using software OpenGL, MATLAB is used without hardware-accelerated graphics rendering of your graphics card. For this reason, images can appear more blocky when displayed.
It is possible that your installation of MATLAB is set up to use the software version of OpenGL. You can check by executing the following in the MATLAB Command Window:
Then, check if the 'Software' parameter of the output is 'true'. To use hardware OpenGL, you can execute the following:
>> opengl('save','hardware')
Then, restart MATLAB and these changes should take effect. After zooming in when hardware graphics are enabled, the zoomed-in image should look the same as the image where the bounds were specified.
If you would ever like to switch back to using software OpenGL, you can execute:
>> opengl('save','sofwtare')
and restart MATLAB.
This should resolve the issue.
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