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View Full Version : My alpha masks my render!



Otterman
04-07-2009, 09:49 AM
Argh...for ages ive output sequential images as 32bit pngs. Stuff like skies in the alpha always appear when i open them in either photoshop or quicktime. Now however, its just a missing black hole!

Whats occuring? Is there a setting ive mucked around with accidently?

The left image is preview-alpha mssed up
The middle image is lightwave render progress (see sky)
The right image is quick time-alpha messed up again.

What figures?

biliousfrog
04-07-2009, 11:11 AM
Nice image!

I always render seperate RGB + Alpha, it saves a lot of those hassles and sometimes proves useful in compositing and post

Dexter2999
04-07-2009, 12:08 PM
Is the Alpha reversed? Check your options in PS.

toby
04-07-2009, 07:08 PM
PNG will always do that, it's a premultiplied format. If you haven't had this before it's probably because you had a solid alpha background, which you could probably fix this with. Something like a 100% transparent poly with it's alpha option set to 255. Or you could open the image in a compositing app, most don't delete premultiplied data like Photosnob does.

Otterman
04-08-2009, 01:38 AM
Thanks guys



PNG will always do that, it's a premultiplied format. If you haven't had this before it's probably because you had a solid alpha background, which you could probably fix this with. Something like a 100% transparent poly with it's alpha option set to 255. Or you could open the image in a compositing app, most don't delete premultiplied data like Photosnob does.

Toby your right, when dealing with alphas in the past they have indeed been a transparent poly set to alpha. Hum interesting. Perhaps il just output as 24bit with seperate alpha instead. Thanks for the pearls!

Hector Pascalid
04-08-2009, 03:24 PM
How do you set a polygon to alpha and what is that for?.

My use of alpha is limited to those cases when there is a transparent object (like a window) and the background is directly behind. The rendered image will register reflections on the glass but not the background, so that I can overlap my rendered image over a different background in photoshop keeping my rendered reflections. Is this somehow related to what you are talking about?.

I usually save as Tiff24 or Tiff32 depending on if I want to keep the background or not. What's the advantage of the PNG ?. I only do stills by the way.

regards,

toby
04-08-2009, 06:13 PM
How do you set a polygon to alpha and what is that for?.
In the surface editor/advanced tab you have some alpha options - you can also alter it per object. If you wanted to keep the background and save an alpha too you'd have to do something like that. Kinda defeats the purpose, but you never know.


My use of alpha is limited to those cases when there is a transparent object (like a window) and the background is directly behind. The rendered image will register reflections on the glass but not the background, so that I can overlap my rendered image over a different background in photoshop keeping my rendered reflections. Is this somehow related to what you are talking about?.

I usually save as Tiff24 or Tiff32 depending on if I want to keep the background or not. What's the advantage of the PNG ?. I only do stills by the way.

regards,
The only advantage in rendering to .png (that I know of) is file size, it's lossless compressed. But since LW supports .exr now I prefer that. Never worry about clipping again!