I recently encountered a tricky situation with a png graphic imported in to FCP 7 that had transparent sections. The graphic was a rectangular shape with rounded edges and had initially been against a white background. I removed the excess white from the corners where the rounded edges did not meet with image's frame boundaries, leaving transparency behind.
I imported the png in to final cut and laid it on the timeline. It was way too big for my project frame size so I scaled it down, everything looked fine. The graphic was intended to remain on screen throughout the sequence. As I scrubbed along I noticed the white edges would occasionally pop back up again despite the fact that it was the same image file stretched across the timeline duration and which at certain points appeared fine with no additional white corners. I tried resetting final cut, and my machine, and clearing the render cache and rendering the timeline and checking composite mode and the alpha settings and everything was correct and it makes sense that it should be since at times the graphic looks fine on the timeline. I also tried going back to the viewer and laying in the source png clip once again as a separate edit on to the offending timeline sections, I initially tried pasting the attributes of the previous timeline instance of the image and experienced the same issues despite the full unscaled image not exhibiting the problem, so then I laid it down fresh again and manually scaled down and noticed that as I approached the 45% scaling I was trying to reach, the white corners problem appeared when the image scaled below 165%, I also realised at that point that whenever I placed the image unaltered on to the timeline it was by default scaled to around 500% which is odd.
The only way I was able to fix the problem was to load the scaled version of the graphic from a section of the timeline where it looked okay in to the viewer and to then make a freeze frame of that graphic. I could then place the graphic on to the sections where there was problematic behaviour and it fixed it.
Though I've fixed the issue, this was strange and something I'd never experienced before. Anyone know what was going on? The sequence settings seemed to be fine and as I'd expect at 1920x1080 ProRes422 square pixels. Admittedly, I did not create this project and there may have been something awry with it's setup that I'd not spotted because I was in a hurry but it all seemed to check out to me.
Most of the time PNGs play well with FCP, but occasionally you get one that does not want to get along with the program. This usually shows up when there is a transition involved. The best solution I've found is to take the graphic into PhotoShop and make TIFF file with an alpha channel.