Just for my understanding - I work with HDV (1440 X 1080, PAR 1.33). When I import this into a 1920 X 1080, PAR 1.0 comp, it fills the comp. I know that no new data is created. Looking at the process under the hood, I assume that the very first of the HDV pixels completely fills first and 1/3 of the comp's second pixel, and that the next HDV pixel fills the remaining 2/3 of the comp's second pixel and 1/2 of the third, and so forth. Thus, what is displayed in the comp's first pixel is the same as the HDV's first pixel, but what is displayed in the comp's second pixel is an average of 1/3 of the HDV's first pixel and 2/3 of its second pixel. Is this what is happening and if not, can anyone tell me? Thanks.
The pixels are not being recombined in any way here. They exist on a 1:1 level.
You will have as many pixels at the end of the process as you did at the beginning.
What's happening is the shape of pixels is actually changing. Your HDV codec uses non-square pixels (Par 1.3). This elongated pixel shape is what creates your nonstandard size (1440x1080) To be viewed properly in most circumstances this needs to be converted to square pixels (Par 1) Most editing applications do this automatically for you. After Effects will do it if you have the "toggle pixel aspect ratio" button selected. Once those rectangular pixels are squished back down to perfect squares your video size changes to the standard 1920x1080.