Nice clean alpha?

[k-arts]

Hi there,

This might have been discussed a few times, which i almost all read, oh and for those yes i used the "search button." When rendering out an image i use the exr format, set the compression to none, standard options to Float - 32bit per channel, RGB ticked and no alpha. The alpha is rendered out as an element being a png file as 16 bit grayscale. The background is white and the object is black. In Fusion i add a black background, use a ChannelBoolean with Luminance BG set. And i do get a white line around my black object. I am able to remove this "aura" with a MattControl but thats actually not what i wanna do. My first though and still is that i am doing something wrong in vray.For IS i use A DMC as well as AA with Area(tried a few ) Min and Max in the A DMC IS is set to 4 as well the threshold is set to 0,01. Colormapping is set to Linear Multiply. This is, where i believe, the bug is sitting, not sure though. Using Vray SP2 and max2008. If you need any more information,files let me know

Thx in advance

Ralf

[vlado]

You probably need to render against a black background or something. Otherwise, the white background is burned into the RGB color channel.

Best regards,
Vlado

[dlparisi]

Vlado: this reminds me - the "Don't antialias against background" checkbox in the Preferences doesn't seem to have any effect, does it? Can this be implemented?

[k-arts]

Thx

Well the checkbox thing you are talking about is checked but doesnt seem to do any difference. The thing that Vlado talks about is a solution but imagine the black object is in an animation where colors change frequently in the background. It is a test i am doing here, so far we were able to remove the "aura" with compositing tricks but i would like to know if there is a solution for this or a different approach.

cheers

Ralf

[k-arts]

Again?

What if i render the alpha channel as an element in scanline with the option ticked "dont antialias against background". would that be a solution? This drives me nuts How do big studios like Mackevision do it?
thx


ralf

[dlparisi]

This won't affect the alpha, just the RGB channel, so that won't work and the alpha also won't align exactly due to the different edge antialiasing.

[k-arts]

So there is no solution?

Well so it seems there is no way of getting a clean alpha then? As soon as i find something i let you guys now

Ralf

[rivoli]

actually yes, there is. as vlado said you usually want to render against a black background when you need to comp later on, and, since EXRs are premultiplied by default, just make sure your comp application treats the images accordingly.

[dans]

Sounds like your alpha is actually clean. It's just your object that is anti-aliased against the background. So converting from premultiplied alpha to straight alpha is all you can do about it.

A different approach would be to extract your object from the G-Buffer of the image. Because of multiple layers in the G-Buffer it is possible to extract an object in a way so that it is not anti-aliased against any background or other objects behind it. You can do this using Combustion (via RPF files) or psd-manager.

[mike_kennedy]

Most apps for compositing default to pre mutlipied with black, so renderign out with a black back ground helps. Now if you are using After Effects you can sample the background color to anti-alias against, not sure if that can be done in Fusion though. Turning off AA Against background will give you jaggies.
Cheers
Mike K

[dlparisi]

Turning off AA Against background will give you jaggies.

I believe maya has a "don't pre-multiply RGB" check box (or something that functions like that-it's been a while) in the render globals and I wrongly assumed the Max checkbox worked the same way since the end result is visually similar. Basically you get full value pixels along object/background edges and an AA'd alpha channel so you can multiply them afterwords if necessary. While most compositing apps can assume you are premult on black, photoshop cannot and you end up with black halos. Is there any way to render out a non-premultiplied (i.e. full edges) image out of Vray?

[rivoli]

Is there any way to render out a non-premultiplied (i.e. full edges) image out of Vray?

it depends on the format you're saving to. when saving to exr you can, as you can with tgas. other formats that can store rgba such as tif or png, don't give you this option afaik.

[dlparisi]

it depends on the format you're saving to. when saving to exr you can, as you can with tgas. other formats that can store rgba such as tif or png, don't give you this option afaik.

These options don't seem quite right either - I end up with a bright ring around objects then- even after multiplying the alpha*RGB. I guess I'll just continue to use the "Remove black matte" option in photoshop -it seems to work OK but was looking for a render solution.

[Morbid Angel]

fusion is finicky that way...typically a good way to set mattes is to use bitmap node. And derrive alpha from luminance. That said, the alpha must be black on white, no other.

[mike_kennedy]

One trick we have used for Photoshop is to put the comp together in After Effects first since it handles this issue fine and then export a PSD file. AS far as other formats and Fusion I have no idea, we use PNG's 24 bit interlaced with Alpha and never have problems in After Effects. maybe trying one of the Render elements and using that to cut your alpha. I am guessing you might want to try another format like TGA.