Alpha problem

[Patrick Spadrille]

i had started this thread inside another thread (which was not a good idea) so i copy paste it here :

I have 2 video cues. One is a background in full screen and the second one is a title in ProRes 4444 with alpha. There seems to be a kind of problem with the way Qlab handles the alpha channel.The title should fade out at the end for 28 frames. Instead it changes color to black in 14 frames then stays black for some frames and then fades out for the rest. I have tried to superpose those 2 clips in Premiere and there everythings works fine BUT in Final Cut Pro 7, it is exactly the same issue as in QLab. So it seems that QLab and Final Cut Pro 7 shares the same (bad) way to handle alpha. I have put in attachment the video from Final Cut Pro 7. Does it seems like something you can solve?

answer from micpool :

Hi Patrick

[You should probably have started a new thread for this.]

I can't replicate this problem.

 

I have tried 422LT  444+alpha and Jpeg still as the background codecs

I have tried straight and premultiplied alphas at 8 bit and 16 and 32 bit depth on the foreground  text in 4444with alpha 

I get white text fading out to the background colours all the way to 0%

What software are you using to encode? What plugin are you using for the spherize effect?

Mic

answer from me :

You're right, i should have started a new thread. I will put the result on the forum so anybody can use it. I've found the problem. When i set the alpha on straight it works alright. I still don't know what cause the problem with premultiplied since it works for you with premultiplied. I have tried to make only a fade (no spherize) and i had the same issue. The background is in H264 encoded in Compressor 4 and the title is made and encode in After effects CC.

Anyway, don't loose your time investigate on this since it works alright with straight alpha.

[micpool]

Hi

Sorry I had misunderstood your original post. I was testing fading the title within Qlab with an animation cue, not playing an alpha clip with the fade rendered.

I can confirm your findings. 

The solution is to use a straight alpha not  premultipled  (presumably with black)

I don't know if this is a bug as such. There is a huge variation in how video editing and compositing programs handle alphas.

Your example of white text with a fade even plays differently in QuickTime Player and in the finder quick view.

With the premultiplied version you see the fade correctly

With the straight the text is visible at 100 percent through to the end of the fade

So it's worth knowing that straight alphas are best for Qlab but if you open clips encoded this way in quicktime player you might no see the true result.

Unless you put the alpha type in the filename it's pretty difficult to see which type it is as it's not reported in most get info panes.

Mic

[Patrick Spadrille]

Good to know the behaviour is consistent. So, straight alpha it is! Thank you for your help.