This message appears when the system is running out of available RAM during rendering. It is triggered by Windows reporting low free memory. Once the memory runs out, the host software (3dsmax or C4D) might either crash or start paging memory onto the hard drive, resulting in extremely slow parsing and rendering (even when using fast SSD drives).
Note: the message refers to RAM, not disk space! Even with hundreds of gigabytes of free disk space, you can still run out of RAM.
Starting from Corona 3, the "System is running low on RAM" error message will display a breakdown of various rendering components and their RAM usage. This provides the user with information on what exactly should be optimized to lower the RAM usage. The same pop-up window will appear regardless if you're using 3dsmax or C4D.
Possible causes and solutions:
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Cause: Too high output resolution.
Solution: Lower the output resolution if possible.
Note: If you're using 3dsmax, disabling the VFB may also help (Render setup > System > System Settings > VFB Settings > Type: none); however, the memory saving may be negligible.
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Cause: Too many render elements.
Solution: Remove any unnecessary render elements (and lower your output resolution if possible).
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Cause: Denoising enabled for render elements.
Solution: Enable denoising only for those render elements where it is absolutely necessary.
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Cause: Too many high-resolution textures in the scene.
Solution: Reduce the number and resolution of bitmaps used in the scene.
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Cause: Displacement screen size value set too low.
Solution: Do not use very high displacement settings unless it's absolutely necessary. Usually, the screen size can be left at the default value. It should never be set to less than 1. Higher values mean lower quality and lower RAM usage.
Regardless if you're using 3dsmax or C4D, you can let the Autobump feature take care of fine detail.
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Cause: Using too detailed displacement maps.
Solution: Use less detailed displacement maps. Let bump take care of fine detail. Autobump does that automatically regardless if you're using 3dsmax or C4D.
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Cause: Too heavy scene geometry.
Solution: Optimize scene geometry if possible and reduce the number of objects. Use instancing whenever possible.
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Cause: The computer has insufficient RAM installed to render the specific scene.
Solution: install more RAM (at least 64 GB is recommended for most "standard" interior and exterior scenes; 32 GB is the bare minimum).
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Cause: Lots of memory-expensive applications running at the time of rendering.
Solution: Close any other applications running on your computer that could take a noticeable amount of memory (Such as Photoshop, compositing software, web browser with many tabs opened, etc.)
Additionally, you can enable the "Conserve memory (slow)" option in the Performance tab of the Render Setup window. This will slightly reduce RAM usage but will also slow down rendering by about 5%.
If you're using 3dsmax, you can activate it in Render Setup > Performance > Performance Settings rollout:
Location of the "Conserve memory (slow)" option in 3dsmax.
If you're using C4D, you can activate it in Render Settings > Corona > Performance Settings tab:
Location of the "Conserve memory (slow)" option in C4D.