edges with premultiplied alpha

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  04 April 2013
edges with premultiplied alpha

i have i common problem with premultiplied alpha. I just want to layer a cg graphic over a background but i always get a gray or white edge when using a merge node. iI tried the "Post-Multiply by Alpha" on or off in the forground layer. See attached files...

Can anybody help?!

Thanks a lot,
Attached Images
File Type: jpg pic_background.jpg (10.3 KB, 2 views)
File Type: jpg pic_post_multiply_by_alpha_OFF.jpg (12.8 KB, 2 views)
File Type: jpg pic_post_multiply_by_alpha_ON.jpg (12.2 KB, 1 views)
  04 April 2013
did you do any colorcorrection on the FG element? or does this happen when you just do an A over B merge?

on what color has the CG element as background? is it on black?
  05 May 2013
The background is black (0,0,0), no colorcorrection, it is just a simple merge forground over background. The cg is from 3dsmax 2013, rendered with vray, saved as png...
  05 May 2013
well, this looks really strange.
if you rendered on black and did nothing to the FG image, then it should work

can you maybe share the files? that i can take a look at them directly? from your discription everything seams to be okay
  05 May 2013
Hi Michael,

find attached the bg and the fg as original pngs. Sorry for the small sizes but the upload is limited.

Thanks in advance for your help!
Attached Images
File Type: png flower.png (25.2 KB, 2 views)
File Type: png BG_clean.png (21.9 KB, 1 views)
  05 May 2013
--spoken to soon--
  05 May 2013
okay, i can confirm that if you do a normal "a over b" merge you will see these edge.

but i can also give you a solution:

its one of the cases where you can see that a non linear workflow has errors. i assume your pngs are in sRGB colorspace and not in linear.

if you put a gamut node after each loader and set the source to "sRGB" and leave the output to "no change" fusion will remove the gamma encoding of the files and do the merge correct

to see you image correctly on your monitor you have to add another gamut tool at the end and set the output there to "sRGB". but then you will always have to look at this specific node to see the gamma corrected result. therefor you can use the viewer LUT function of fusion
set this to the "gamut view LUT" and edit its setting to output "sRGB" then you will always see the gamma correct result in the viewer and you need the last gamut node only before you render out

you can learn more about the linear workflow in fusion by the great tutorial from stefan ihringer
  05 May 2013
Thanks a lot for your help, i will check your hints!

  05 May 2013
Ok, everything is fine now! Thanks!

But does it mean that my workflow is wrong? How should i render / save cg? What are the correct settings and in wich format should a save renderings? And how does it work in photoshop...?

See attached my settings in vray...

Sorry for the bunch of questions.

Kind regards,
Attached Images
File Type: jpg png_settings.jpg (38.7 KB, 2 views)
File Type: jpg vray_settings.jpg (34.3 KB, 2 views)
  05 May 2013
best is you look for some tutorials on linear light workflow, there you will get all the settings for vray and how to do it properly inside your 3d app.

at the start this might seem a little bit confusing but it will bring your rendering to a better level
  05 May 2013
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