Sampling Blank Alpha Slowing Down Render Times

[Reversetrio]

Hi. This is my first time posting to the forum.

I am working on a project in Cycles. I have thoroughly researched lowering my render times. I feel like my settings are in a good place for relatively quick, quality renders; but I still have one problem: rendering small objects against a background of alpha takes too long.

I am rendering a small object with alpha to composite later as a layer within a larger composition. At 1280x720, the renders of this small (13% the size of the total frame) object are taking about 2 minutes. Most of that time is spent sampling tiles that only contain blank space (at 750 samples).

I want to know if there is any sort of sample culling option for blank alpha tiles. If the engine could recognize that there is no geometry present within a tile after some baseline number of samples (say 10), then it could move on to the next tile and save precious render time and CPU cycles.

This feature, whether it exists or not, would drastically cut render times across my entire project. I could also consider rendering at a higher resolution.

Thank you!

Attachments

[organic]

Border render?

http://wiki.blender.org/index.php/Doc:2.6/Manual/3D_interaction/Navigating/Camera_View

[Love_3D]

Border render, that dosent work?, when you want to composit.

a kind of masking border thing, would be a great feature for cycles.

(automaticly of course when its alpha or transprant)

[Reversetrio]

Unfortunately, border render doesn’t suit my purposes. I need to be able to composite multiple images, which for my project means batch renders of multiple frames.

The masking idea is interesting because it would allow the artist to control what portions of the image should go un-sampled. I imagine a simple viewfinder placed in front of the camera, around the object, restricting what is able to be rendered. Actually sounds like border render again, but key-able.

It would be really great if there were a way to constrain samples in a render.