Yes and no. When you stabilize a clip, movement is required to keep, say, the horizon level. That will reveal some corners or much worse. You compensate by scaling up or cropping out.
There are many ways to do this including AE's various stability options and external software and they all have benefits or limitations. I tend to do our GoPro stabilization at native size, allowing no scaling or cropping. Then I bring that movie (or file with data) back into an editing format. If you just put 2k into 1080 comps you may need to scale down or shift the image around to keep it centered and cropped but there will be no discernible quality loss. However, before you start assembling in 1080, be sure your target distribution benefits from the format; you may want to use 740 just because it's far less intensive and, frankly, most people can't tell the difference.
Better advice may be available from the thousands of gopro users who share tips and techniques on the company's user forms.
.Drop your original gopro footage in the new comp icon then stabilize then nest your stabilized comp in a 1080 comp. Scale and position to fit.
Thx for all the advice!
Rick: That's exactly the method I have been using.
But I was asking myself: If i stabilize the footage and at the same time resize it inside the 2,7k comp and then bring it down to a smaller size ( by nesting etc. ) isnt the footage therefore loosing information?
When I upscale say a 640x480 picture to 800x600 e.g. in Photoshop save it and then bring it back to vga size it will be not the same quality any more as the original.
That's why I am trying to understand how AE handles resizing internally. Is it lossless or not ?
Any time you resample information you are loosing information. Scaling down is visually more pleasing than scaling up, but if you want to optimize the look of the video you should add some kind of sharpening. You cannot judge sharpening, like unsharp mask, until you render a full resolution preview and look at it at 100% because too much detail in a video is worse than too little. You'll get all kinds of unpleasant ringing in high detail areas. The same thing goes for upscaling images. To get the best results you have to apply the right kind of scaling algorithm and do some filtering to hide the upscaling artifacts and make the image more pleasing to look at. There is a limit on how far you can scale up and still get a pleasing image, there is also a limit on how far you can scale down and still see detail. For example, if you had 4K footage of a house made of bricks and you scaled down the image to 100 pixels wide it is very unlikely that you would still be able to see individual bricks.
Here's the deal with manipulating pixels. Everything is built on a grid. If you move your original pixels in any way so that they do not line up perfectly with the grid the pixel values are then interpreted and detail is lost. Draw a vertical one pixel wide white line on a black background then move the line 1/2 pixel to the left or right and you don't have a white line any more, you have a 2 pixel wide gray line. Just the act of stabilizing your footage causes a slight quality loss because it is moved in sub pixel increments and the pixels have to be re-interpreted.
If your final product is standard HD, then that's all the quality you can get. There are fewer pixels so there is less information and less resolution, but an HD sized comp will easily fill a 60' wide screen in your local cinema and be extremely pleasing to look at as long as you are not standing 6 feet from the screen. You're obsessing too much about this whole process. Build your comp, do your work, then check to see how it looks. Make a single frame look as good as it can and render a test to see if it is good enough when it moves. That's the only sure fire test of quality. Then compress for delivery using the highest data rate allowed with multi pass rendering and see if that quality is also good enough to pass your quality control standards. At the end of the day you must realize that nobody watching a movie is going to analyze every pixel on every frame. You would be surprised at what some still frames from big budget Hollywood blockbusters look like. When the image is moving, you can get away with things that would never pass in a still image.