LR 5 - Image Sizing Dimensions - wrong image size

Question

Hi,
Image Sizing Dimensions produces wrong image sizes upon export for certain images.
Example, I have export set up for iPad resolution, 2048x1536px at 264ppi. If I now export an 3264x4928px image it should be resized to 1356x2048px. What I get is 1356x2047 instead, one pixel too short on the long side. This doesn't happen for all images source resolutions and ratios though. I haven't figured out the pattern yet.
Here a link to an image which exports at wrong size (export dimension set to 2048x1536px).
http://www.bobtronic.com/files/IMG_0289.JPG
I would appreciate a speedy fix for this problem. Thanks
cheers,
Matthias

Answer 1

The image has a resolution of 3264x4928. Exporting using Dimension resizing of 2048x1536 results in an 2047x1536 image.
The scaling factor 4928 down to 2048, is 0.4155844155844156...
Assuming that LR is working to only 3 decimal places here, which indications discussed in another thread would suggest, then there are several ways to achieve that:
truncate down to 0.415 by discarding the subsequent digits
"quick-rounding" of the third digit by inspecting the fourth-place digit only, and incrementing the third-place digit when the fourth-place digit is larger than 5, which it is not in this case, resulting in 0.415
"math-rounding" by inspecting all of the following digit pairs in turn, until you reach a pair that is lower than 50, and then working back again to simplify. This procedure rounds 0.41558 up to 0.4156, then after doing that, rounds 0,4156 up to 0.416
Speculation: the difference between 0.415 and 0.416, is the difference between a result with 2047, and one with 2048, when executed.
Most of the time the "quick-round" procedure will work out fine, also truncating will usually be good enough, assuming the above has anything to do with the reality (which it may not). Just occasionally, though, there'll be an error in the final digit, compared with what the theoretically accurate procedure would have given. So if I am right this is not strictly speaking a bug (something failing to execute as it has been designed to do), but the result of a software design choice. "Works as designed", iow .
It seems that LR, perhaps differently between versions, is working this through by simple calculation (from the front-end), without comparing the back-end outcome against what you have requested. How much this (intermittent) issue actually matters, is a question of judgement, and of particular requirement.
[ alternative method: I find the Print module, outputting to JPG, is completely reliable when it comes to making a standardised JPG as to the dimensions, as well as offering some different technical and design options, than what you get with Export. For one thing, you get the option of on-the-fly cropping to shape, which you can then adjust interactively where (as here) the image is a different aspect than the Retina display. So if you want it, you can fill the screen instead of getting letterbox, all without having altered your main (Develop) crop for that image. Alternatively, if you don't want the Zoom to Fill option (e.g. with landscape images to be shown in portrait orientation as part of a sequence) you can still make all your images to the same dimension, but controlling the appearance of the borders that are left (which become part of the image) ]
RP
ps: AFAICT one would expect simple truncation to produce the appropriate answer half of the time; one would expect simple rounding to just one further digit, to be wrong half of the times when the following digit happened to be 5 - or to put in another way, to be right in 95% of cases; and proper rounding, examinng as many places as it takes, to be right 100% (within precision limits).