I have a batch of images of varying aspect ratios. For example,
I want to export them so that they are ALL exactly 800 x 1200 pixels (or 1200 x 800), with white/black space added to the "canvas" as needed. For example,
If "Don't Enlarge" is checked, image would be 500 x 500, with 300 pixels of white space added one way, and 700 the other to get to 800 x 1200If "Don't Enlarge" is unchecked, image would be 800 x 800, with 400 pixels of white space added either way to get 800 x 1200
Image would be 1200 x 600 (2:1), with 200 pixels of white to get 1200 x 800
Image would be 400 x 1200 (1:3), with 400 pixels of white to get 800 x 1200
Image would be 800 x 1200, no extra processing needed
I've tried every combination of LR export option, and while it seems to get one dimension or the other correct, none of them seem to ever add extra pixels to increase the canvas size.
The problem, of course, is that online print processors have different rules for handling images when you specify a 4" x 6" print and the image isn't a 2:3 ratio. Most (all?) will allow you to crop to exclude part of your image (and they all seem to have a default, automatic crop), but few offer an option to ensure the entire image is included on the print, with borders added as needed. And the ones that do (e.g. Shutterfly with "constrain to crop" option) won't do it automatically.
My life would be SO much easier if I could have LR handle the heavy lifting, and just always export an image that's exactly a 2:3 ratio.
A plug-in would be fine.
What you want to do is crop the images (or add blank pixels). You can't do that in the Export dialog. In the Export dialog you can only resize, but that maintains the aspect ratio. I don't know if there is a plugin that can do this.
There is another way you could do this in Lightroom however: via the print module. Because you can 'print to jpeg', you could setup a 4" x 6" page with a borderless image cell filling that whole page. Check 'Rotate to Fit' in the Image Settings, so images are fitted to one side (and a white border is added if needed), not cropped. In order to get 800 x 1200 pixels, you would have to set the print resolution to 1200 pixels / 6 inches = 200 ppi.
That actually worked really well!!
One important setting was "Zoom to Fill". When checked, LR enlarged the image to fit the 4x6 size, which meant that some of the picture was lost when the source image as not a 2:3 ratio. When unchecked, it did exactly what I wanted: every picture was exactly 4x6 (well, more precisely, exactly 1200x1800 @ 300dpi), and every picture contained the entire source image with added blank pixels as needed for non-2:3 images. Perfect!
Seems odd this is not part of Export > Resize, but I'm glad I found a working solution because this is something that has popped up from time to time.