Saturday, January 26, 2008

Grainy Quality to mr Renders

Q + A With Jamie Cardoso: Grainy Quality to mr Renders

Question: I bought your book Realistic Arch. Vis. And have gone through the first interior room tutorial. I’ve been trying to learn Mental Ray for a year now, but one thing I can’t figure out is why the renders of your files have this strange grainy quality to them. I’ve never produced a render with this using my own files, and I can’t figure out what it is about your settings which could be causing this. Can you please advise?

Jamie: This is to do with the fact that the render is at a small output size and also when the light's radius values are very high. This artifact happens with every high end rendering engine. Moreover, it frequently happens in enclosed scenes where lights (more than one) are bouncing across far too many objects.

There are 2 ways of by passing this:

1-To keep it at a small render size and get rid of the graininess, one should increase the U & V mr light samples (under area light parameters) to a point where there are no more grains (the value of 10 should be enough). This process will require some test renders, and it will increase the render times dramatically. By the way, this subject is mentioned in the book, at the end of the interior daylight tutorial.

2-The most common option amongst Hollywood film companies and 3D visualizers, is to render at higher resolution (IE 2500 pixels and above), and later decrease its original output size (IE 800 pixels) in photoshop etc, for web or e-mail purposes. Higher resolution renders are normally used for movies, publishing or marketing purposes. By the way, the big size render tutorial of the book cover image is also covered in the book.

When going through this tutorial, as mentioned in the book: save first the fg file by checking the read/write file function and clicking render at draft sampling settings (IE box filter; samples per pixel minim. 1/4 maxim. 4), the render size should not be higher than 500 pixels . Once rendered, the fg should be saved by then. The next step is to keep the read/write file function checked and also check the read only (fg freeze) this time around to ensure that the fg process will be bypassed. Finally set the final render output size to 2500 pixels or higher followed by also increasing the Sampling settings (IE gauss or mitchell filter type, samples per pixel min. 1 max. 16 or 64).

In both options, the rendering times will increase considerably however; the 2nd option is more reliable and professional.

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