GI on big images, tips how to reduce rendering times
by Jason Saunders
With version 5 there is now an option in the render setting to set the quality to High, Medium or Low. For Hires images I find low is much quicker with little trade off to quality.
Don't forget to set the post GI effect to image not box and turn off safety area in the Post Process tab. You will get a significant speed up in rendering times as you are reducing the calculation considerably. Changing objects properties/Wire/Shaded quality to 1 instead of 2 if quality permits, reduces render times too.
This gets tricky however if you have a complex scene and want to render over 2000(approx) pixel images. You will be likely to reach the Windows 32bit 2 gig file size limit with the realsoft Raw channel data file and the post process will fail. The obvious way around this is to render and post per box with safety area on (do not use render backup). This makes your renders four time longer on average and often un-useable in a production environment.
The other work around for lager than 2000 res images is to use the box render option in the render window, with post per image in say 1800 wide sections. Make sure the box sections you render overlap relevant to the blur iteration count set in your GI blur settings. This way the separately rendered section will all stitch together without seams.
I generally split my geometry into different GI shader sections relevant to their use in the image / scene. For example, I put glass objects in a different level with their own Gi shader set with higher ray count levels two and three if need be. Same for reflective surfaces when necessary. I often set the glass level to be shadow invis and also reflection invis (except when looking through) to speed up render times.
Glass as we all know is a tricky one at the best of times. Some of my best results are using the crudest methods. Simply setting the objects channels for transparency, reflection, optical thickness and colour are all that is required to get good glass or reflective objects. Don't be afraid to use negative or multiple values when experimenting with these channels.
Oh and try ramping up the GI glass shader reflection colour value to say 1.5 1.5 1.5. This ramps up reflections of areas not receiving direct light. Can look muddy, but play around with it and ray count values along with different Post/ GI blur and Gi Specular iteration values.