Hi Shiu - I'm moving the looping topic I started at CC2D to here - seems a more reasonable place to discuss...
You can still loop even if you don't set start/end, then it will just look at the amount of keys you have in your animations. Just enable looping without setting the range. Also you don't need to remove a frame if you want perfect looping, only if you export to a sequence of images will you have to remove the last frame after export.
So... we are definitely on the same page about the looping thing - I'm understanding what the program does at this point. And I still strongly suggest the user be able to set loop points per animation. Just as a workflow thing it makes much more sense. Even on the simple shots I posted, the loops were different lengths - every time I switched to an animation, I had to reset the in and out, and every time I restarted the app I had to reset the in and out. I'm not able to test the output until I get a code after kickstarter (congrats btw!) but exporting all keys might not always be what I want. I'm assuming I can change this programaticly (is that even a word???), but again much better for a visual person (such as myself) to define it in Spine. So a default of all key export (like you have now) would be great, with a way to set in/out per animation if I choose to... Your current loop in/loop out buttons are perfect, they just need to be made "sticky".
As for my rotation example - it works the same whether you're outputting keys or frames. Your first and last frames will always be the same - they run twice creating a subtle hiccup. So the way to create a perfect loop is to move the timeline end 1 frame back. This works whether your frame ends at 10, 34, 100, whatever... The walk loop on the beaver I made is incorrect for this reason http://vimeo.com/collidernyc/beaver. You could move the keys one frame out, then move back and key the previous frame, then delete the last keys, but that's a pretty kludgy way to do it, and makes it hard to retime animation (if I move my now end keys, the math is off).
One more quick question, while we're on the subject of retiming... is there a way to scale (retime) multiple keys? In AE you select keys and hold down option. In Maya, you use the scale tool etc...