Hi Dave,
As Greg mentions, if you are not doing a photorealistic rendering, you want the fastest rendering from ArchiCAD possible to generate your epix file.
In particular, if you do NOT need shadows, reflections, etc., just use the internal engine with the fastest settings to get the material and depth channels with an ugly RGB channel to paint over.
If you need some of those things, then use LW, but use only the settings you require.
A trick for a high pixel result is this: the material and depth channels are the same no matter how you render - rendering settings only affect the RGB channel. So... generate the desired pixel size epix using the fastest rendering settings (internal at night = black for example).
Then, generate a normal LW render - at half the pixel dimensions (which is really 25% of the pixels) - which will be faster. Upsize it in Photoshop - doubling is rarely extremely noticeable, especially if you will be over-painting. Then in Piranesi use the Import RGB feature to bring in this upscaled RGB image to replace the lousy one. Do a Render to replace the 'restore RGB' channel and you're good to go. A bit of extra steps that only makes sense if the actual rendering process is required but is too slow.
(Also, as Greg mentions, if you require a LW render, dial down antialiasing and quality a notch as described by Dwight so that all cores can be used - at highest settings, rendering uses only one core.)
Cheers,
Karl
PS Was just in your neck of the woods this week in Petaluma - probably drove right by you on the 101 on the way to Muir Woods and Mt Tomapais on Monday. Beautiful.
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