[BradT]

Many of the scenes I create contain a lot of empty pixels. I've found that rendering to a base SL of 7, then extra sampling based on Alpha to SL15 can reduce my render time tremendously. In the scene I'm currently working on, it takes 10 minutes to get to SL15 this way, versus nearly 2 hours rendering straight to SL15.

I'm curious why this isn't just a permanent built-in optimization. I have yet to find a scenario where it hurts to do it this way. Having it built-in would also allow me to still use the custom-alpha-based extra sampling if I need to, without a submitting a separate pass to the render queue.

Maybe this should be a wish-list item?

[Mihai]

Could you post an example? I'm not sure what you mean by "empty pixels" in this case, you mean simply empty space in your scene/black? It's a bit strange in that case that the difference should be so huge, maybe a better measure would be noise levels between the two.

[choo-chee]

I use render region a lot, already posted about it in the wish list section. marking a region (many times it's actually 30-50% of the frame) on render to be used as a photo montage of a 5000X3333 pixels shortens time significantly.

[F. Tella]

Sounds good to me.

I made a fast test with a sphere in the sun with quite a lot of empty space around. Even in this simple scenario the differences are quite remarkable.

Full render.png

Full with alpha.png

Extra sampling.png

I'll suggest this to the developers.

Best,
Fernando

[BradT]

I'm not sure what you mean by "empty pixels" in this case, you mean simply empty space in your scene/black?

Yes. That's what I mean. For example, a satellite in space. Maxwell is really good at rendering hard-surface models, so I'd use maxwell for the satellite, rendered against black/transparent. I'd then composite that over the Earth and stars from another renderer or matte painting in the background.

I'd post an example scene, but Fernando's simple scene shows the problem just as well. As the models and materials get more complex, the time advantage increases.

Also, thanks Fernando!

[Mihai]

It's weird, in my opinion there shouldn't be any difference in render time in these cases between using just an alpha channel vs using extra sampling and its alpha. Yeah, good opportunity to speed things up!

[Hillmeister]

I've tested it with one of my scenes where I have a ship as Maxwell Reference which I choose as my Custom Alpha Object. It's really speeding up things!

Would be very welcome to have this automated somehow. Offcourse one could use region render, but the Alpha is cutting the image perfectly!

Cheers