By default, Corona renders an image until you stop it. You can use render limits (3ds Max | C4D ) to specify the number of passes, rendering time, or noise level to terminate the rendering at. But what exactly happens when you use the pass limit? And how many passes are typically enough to get good image quality? This article answers those questions.
What exactly happens during one render pass?
The pass limit tooltip says: Each pass performs one antialiasing sample per pixel and a variable number of secondary samples set by the GI vs. AA balance parameter.
This means that when using the default render settings, during one pass, each pixel receives:
- 1 antialiasing sample;
- 16 GI samples (GI vs. AA balance = 16);
- 16*2=32 direct light samples (Light Samples Multiplier = 2).
This also means that when using the same render settings, each pass should take exactly the same amount of time to render. This time is proportional to the complexity of the rendered scene, the number of samples contributing to the rendering, and the total number of pixels (image resolution). Complex scenes require longer rendering times. Increasing the number of samples increases the time required to perform one pass but also increases the image quality per each single pass.
...so how many passes is enough?
The number of passes required for the final rendering is highly dependent on the kind of scene that is being rendered so there is no universal number that should always be reached. The total number of passes in Corona corresponds to the number of antialiasing samples cast per each pixel, so usually, to render detailed objects or textures, a minimum of 50-100 passes is required. In some cases, like complex GI or pronounced depth of field (3ds Max | C4D ), many more passes may be required (even 500 or more). The ratios between the number of passes, image quality, and rendering time are also dependent on such settings as:
- GI vs. AA balance
- Light Samples Multiplier
- Max Sample Intensity (3ds Max | C4D)
Generally:
- More passes = better image quality (in terms of anti-aliasing, GI, and direct light quality). This is always true.
- Higher GI vs. AA value = more time required to perform each pass, better per-pass GI and direct light quality - useful for scenes with complex lighting that don't require much antialiasing.
- Lower GI vs. AA value = less time required to perform each pass, worse per-pass GI and direct light quality - useful for scenes that require more antialiasing (DoF, motion blur, detailed geometry, fine textures) without complex lighting.
See also:
Adaptivity
Corona uses a feature called adaptivity in the rendering process and it is always enabled. Adaptivity slightly changes the way each subsequent pass affects image quality since the number of samples per pass is now dynamically adjusted to balance the noise level across the whole image. Generally, we can still assume that a fixed number of samples is cast per each pass, to understand how the process works, but in reality, it does vary slightly.
To learn more about adaptivity and how it affects image quality, see: what is adaptivity?
Examples
1. GI samples.
1.1. Image rendered at default settings. 4 passes. There is a lot of GI noise and geometry edges are not smooth.
Per-pixel:
1AA sample per pass * 4 passes = 4 AA samples total
16 GI samples per pass * 4 passes = 64 GI samples total
2 * 16 direct light samples * 4 passes = 128 direct light samples total
1.2. Image rendered at default settings. 16 passes. There is less GI noise and the geometry edges look better, but the image is still low-quality.
Per-pixel:
1AA sample per pass * 16 passes = 16 AA samples total
16 GI samples per pass * 16 passes = 256 GI samples total
2 * 16 direct light samples * 16 passes = 512 direct light samples total
1.3. Image rendered at default settings. 32 passes. There is less GI noise and geometry edges look good. This means that no further antialiasing is required for this scene (no more passes) but GI needs more samples to refine.
Per-pixel:
1AA sample per pass * 32 passes = 32 AA samples total
16 GI samples per pass * 32 passes = 512 GI samples total
2 * 16 direct light samples * 32 passes = 1024 direct light samples total
1.4. More GI samples may be added by increasing the number of passes. Image rendered at default settings. 200 passes. There is even less GI noise, but some more GI samples are still required.
Per-pixel:
1AA sample per pass * 200 passes = 200 AA samples total
16 GI samples per pass * 200 passes = 3200 GI samples total
2 * 16 direct light samples * 200 passes = 6400 direct light samples total
1.5. Let's leave the number of passes at 200 and increase the "GI vs AA balance" to 32. Now the image looks good both in terms of noise and antialiasing, but according to example 3., even 32 passes are enough for good antialiasing in this scene.
Per-pixel:
1AA sample per pass * 200 passes = 200 AA samples total
32 GI samples per pass * 200 passes = 6400 GI samples total
2 * 32 direct light samples * 200 passes = 12800 direct light samples total1.6. So let's try to render this scene with only 32 passes and 200 GI samples (swap passes number and "gi vs aa" value). The image looks virtually the same as the previous one in terms of quality because both 200 and 32 AA samples are enough to produce smooth edges and the same total number of GI and light samples was used in both cases.
Per-pixel:
1AA sample per pass * 32 passes = 32 AA samples total
200 GI samples per pass * 32 passes = 6400 GI samples total
2 * 200 direct light samples * 32 passes = 12800 direct light samples total2. GI Solvers.
2.1. Comparison between a scene rendered with the same settings, using Path Tracing + UHD cache, and Path Tracing as both solvers:- PT + UHD;
- default settings;
- 200 passes.
- PT+PT;
- default settings;
- 200 passes.
Conclusion: using the UHD cache as a secondary GI solver significantly reduces the number of required passes, thus reducing the render time.
3. MSI
3.1. Default settings, MSI = 20 (default value)
3.2. Default settings, MSI = 10
3.3. Default settings, MSI = 5
3.4. Default settings, MSI = 0,5
3.5. Default settings, MSI = 0,05
Conclusion: lowering the MSI value reduces noise in GI thus reducing the number of required passes at the cost of realism. It does not affect the noise visible in directly lit areas.
4. DoF & motion blur.
4.1. Scene with strong DoF effect - 4 passes:
4.2. Scene with strong DoF effect - 16 passes:
4.3. Scene with strong DoF effect - 64 passes:
4.4. Scene with strong DoF effect - 200 passes:
Conclusion: the number of passes (AA samples) decides about the quality of depth of field. This also applies to motion blur.
5. Fine geometry.
5.1. Fine geometry - 4 passes:
5.2. Fine geometry - 16 passes:
5.3. Fine geometry - 32 passes:
5.4. Fine geometry - 200 passes:
Conclusion: the number of passes (AA samples) decides about the quality of fine geometry, especially where dark and bright edges meet.
6. Fine textures.
6.1. Textured objects - 4 passes:
6.2. Textured objects - 16 passes:
6.3. Textured objects - 64 passes:
Conclusion: the number of passes (AA samples) decides about the quality of textured materials, especially if the texture is very fine.
Denoising.
Denoising can be used as a post-processing technique that helps reduce noise in rendered images resulting in smoother and cleaner final images with a lower numbers of passes.
To learn more about denoising - see: