Rendering high resolution images faster by first rendering the final gather map at a lower resolution.

Posted by MaxStation, 25 February 2011 12:00 am

When you render at high resolution you don't need to render the final gather map at the same resolution. By first rendering the final gather map at a lower resolution and then rendering the scene at the required resolution with the final gather map you've rendered earlier you can reduce the render time significantly and still get a good end result.

Once you're ready to render the scene first lower the output resolution (as an example 1024x1024 instead of 4096x4096).

Then set the final gather map file name: go to the indirect illumination tab of render setup dialog, scroll down to the Reuse (FG and GI Disk Caching) and click on the "..." before the filename field above the 'Generate Final Gather Map File Now' button.

Render Setup: mental ray Renderer - Indirect Illumination - Reuse (FG and GI Disk Caching)

And render the final gather map by clicking on the 'Generate Final Gather Map File Now' button. This should be something like 16 times faster than doing the final gather at full resolution.


Once that's done change the Final Gather Map to 'Read FG Points Only from Existing Map Files' so the subsequent render will use the file we've just rendered.

 Render Setup: mental ray Renderer - Indirect Illumination - Reuse (FG and GI Disk Caching)

Go back to the Common tab, set the render resolution to 4096x4096 and do the render. It will now skip computing the final gather mapand use the file we created earlier.


For more details on reusing final gather see the online help: Autodesk 3ds Max Help > Rendering > Renderers > mental ray Renderer > mental ray Renderer Interface > Indirect Illumination Panel > Reuse (FG and GI Caching) Rollout (mental ray Renderer).



Posted 25 February 2011 3:59 pm

That is really dependent on a situation. In some cases it might work, while in other cases it wont. Mainly if you render high resolution in order to show off some extremely detailed object, indirect shadow detail and quality loss will be quite obvious.

Brian Bradley

Posted 27 February 2011 4:24 pm

Its a usable trick for sure, BUT for all intents and purposes is Exactly the same as lowering the number of FG points in your FG solution!! - Bri


Posted 1 March 2011 2:45 pm

That's pretty much how my workflow works. I compute on Medium at 1/4 the intended resolution, and then I size up the resolution four times and render with FG set to read only. I know it's not perfect, but it works well for my current employer, and they are more than pleased with the results; in fact, most of the time they say I'm putting too much effort in - go figure!

Anyway it was really nice to hear you weigh in on this Jenni because I picked this technique up from you a couple of years ago - many thanks.

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