Here's a quick explanation of the ppi thingee:
There are 3 ways to look at size of a picture: physical size in inches/mm (e.g. 4x6), size in pixels (640x480) and pixels per inch (72, 96, 300). Pick any two and the third is determined.
For printing a picture, you usually have a specific size you are worried about (11x 17 for framing, 8x10 for a calendar page). In order for a print to look good, you would like 300 pixels per inch, although 150 is a reasonable minimum. Once you pick those two, the total pixels is multiplied out. You need between 1200x1500 and 2400x3000 for a great looking 8x10 print.
For displaying on the web, your computer or a TV, you have a fixed maximum number of pixels: 1680x1050 for a 17" MacBook Pro or 1920x1080 on 1080p HDTV. Depending on the size of your screen (17", 40", 60") you will end up with a fixed PPI. My MacBook Pro is 120 ppi. My 40" Sony is about 55 ppi.
Therefore when you send a picture to a screen, the number of pixels is all that matters. Your AppleTV and TV will scale up anything smaller (with some degradation) or scale down anything larger (with no serious degradation). Always better to be too big. But for storage sake, not TOO big.
Because of Ken Burns Effect, AppleTV will display your image BIGGER than the TV size, so for OPTIMUM results, I would make the images about 10 or 20% bigger than the resolution of the TV. Then the Ken Burns will be at native or scaled down, but never scaling up.
Therefore, for a 1080p, the screen size is 1920x1080: to allow for Ken Burns, make your images 2176x1224. The ppi won't matter, but put in 72 for good measure. If you want to be anal, measure the width of your TV and divide that into 1980 and use that. But, as pointed out before, the TV won't actually look at that number. It displays all the pixels.
By the way, once you have a 1920x1080 picture, the small upscaling Ken Burns Effect does won't look bad at all, but heck, a few extra pixels and you've got it perfect.
Jim
