Lightroom cannot crop on export, you have to do that first. So if the exported photo is not exactly square, then you did not crop it to an exactly square format.
Well, I just tried that and it gives me a perfectly square exported image. Perhaps you unlocked the aspect ratio and changed the crop just a little without realising it?
I don’t see how this could be a rounding error. When both sides are equal, they will round off to the same value when you resize the image. No matter how large the rounding error. This sounds more like a bug, or like some user error after all. What is the size of the original image, and what settings do you use on export?
I'm a software engineer, so I can give a (very rough and simplified) explanation how this could be a rounding error.
Let's say the order of operations when cropping in lightroom is:
- Apply cropped scale
- Apply aspect ratio
The original file is 5472x3648.
Let's say we crop the photo by 30%, resulting in 3830,4x2553,6.
Now apply aspect ratio, cutting off the difference. The difference is 1276,8. If for some reason this value is floored or rounded before usage, you will end up with rounding errors under the right circumstances.
I’m sorry, but I don’t buy that. That is not how the crop tool works. The crop tool does not give you an option to crop by a certain percentage. You crop manually, with or without a fixed aspect ratio. And the crop tool does not size steplessly, it sizes in steps of one pixel. At least that is the only thing that makes sense, and so you can’t crop half a pixel. That means that an aspect ratio of 1:1 should give you an image with perfectly equal pixel dimensions for width and height. If it does not, then that is a bug.
When in cropping mode, it is stepless, and you can indeed crop half a pixel. When you exit crop mode it is applied and rounded to whole pixels. If you go back to crop mode though, you'll see the half pixel crop again, thus the crop is stored as a stepless (float) value.
In any case, yes, this is a bug. Should this be reported somewhere else?
xinux s right. Crops in Lightroom are done in fractional values. They are stored as floats and you can indeed crop in the middle of a pixel which could result in a 1-pixel difference between the horizontal and vertical dimension if you for example crop horizontally at pixel 100.25 to 1100.65 (which would give 1002 pixels) and vertically to pixels 200 to 1200.4 (rounding to 1001 pixels) even though the horizontal and vertical crop dimensions are identical. Lightroom should recognize this though and normally you shouldn't be able to make crops this way but there are cases in which it can happen such as syncing settings or applying presets..
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