In Corona, when using using pt+pt or pt+hd, you have two main parameters responsible for image quality: "path tracing samples" and "passes". Generally, we can say that total number of samples used to render each pixel is PTS*passes. We know that each pass consists of sending a primary ray and then calculating secondary rays. That's why rendering an image with such settings: PTS: 100, passes: 1 will be faster than rendering an image like this: PTS: 1, passes: 100. Probably there won't be much difference in GI noise because number of used samples will be the same (100) but there will be significant difference in quality of geometry edges, thin lines, textures, generally antialiasing. So passes work basically like supersampling in conventional renderers. Each pixel is sampled with and averaged, this works like "subdividing" a pixel X times and averaging the output. So here is my question: does it make sense to render a final image with more than 16-32 passes? I know it depends on what kind of scene you have and what resolution you are rendering at. But generally, isn't rendering with 100 or more passes an overkill? The less passes you use, the faster the rendering will finish with the same GI quality, right? We want as little passes as possible. So here is my workflow:
1. I saved the HD cache solution to have the same results all the time. Then I determined (more or less, this is just a test ;) ) how many samples are required to render the GI noiseless - I set passes to 1 and increased PTS until the noise was acceptable. This time I decided it was 2048, but it would really require even more.
2048 PTS x 1 PASS - time: 21:31:![](http://s24.postimg.org/5lxi0xep1/2048x1.jpg)
2. Ok, so total 2048 samples are required to render nice cleanGI. Now - how much pixel subdivision is required for noisy edges to look nice? I did some test while still maintaining 2048 total samples. I increased passes (2, 4, 8, etc) and divided PTS by them (2048/2=1024, 2048/4=512, 2048/8=256 etc) so the GI quality was the same and antialiasing better. I found out that 16 passes are enough for very thin elements to look ok:
128 PTS x 16 passes - time: 24:40:![](http://s24.postimg.org/ga18zroo5/128x16.jpg)
So in this case this is the optimal value. Increasing passes and decreasing PTS wouldn't increase quality, only time.
You can see that rendering the same image with 16PTS x 128 passes produces a very similar result but takes more time to render:
16 PTS x 128 passes - time: 28:50:![](http://s24.postimg.org/7t6c2l9d1/16x128.jpg)
Hope it makes sense and maybe clarifies some Corona settings to new users.
Of course I encourage everybody to argue with me, shout at me and even disagree. ;)