If you see the warning and want to maximize performance:
// Usage Example int main() RenderEngine engine;
Close all unnecessary applications before starting your render.
Are you a graphic designer, animator, or video editor who's encountered the warning message "Num samples per thread reduced to 32768 - rendering might be slower"? If so, you're not alone. This warning can be frustrating, especially when you're in the middle of a critical project with a tight deadline. In this article, we'll explore what this warning means, why it happens, and most importantly, how to address it to ensure optimal rendering performance. If you see the warning and want to
Why 32768? That number (2^15) is often a hardcoded safety limit imposed by the render engine or the underlying hardware/API (e.g., CUDA, OptiX, OpenCL). The warning appears when the renderer tries to allocate more than 32768 samples per thread, but something—memory constraints, driver limits, or device capabilities—forces it to that number to 32768.
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Some versions of CUDA, OpenCL, or Metal have internal limits on how many rays can be in flight per thread. NVIDIA’s CUDA runtime, for example, might cap the workgroup size. When the render engine requests a higher value, the driver clamps it down and returns a warning. This warning can be frustrating, especially when you're
While slower than a fully operational GPU render, switching V-Ray entirely to CPU production mode eliminates VRAM hardware limitations. It will leverage your system's RAM, which is far easier and cheaper to expand.
Understanding what chokes your VRAM is essential to preventing this warning:
Have you encountered this warning in your projects? Share your experience and any additional fixes in the comments below. For more rendering optimization guides, subscribe to our newsletter. That number (2^15) is often a hardcoded safety
Suppose you’re rendering a image (about 2 million pixels). You set 256 samples per pixel . Total samples = 2M × 256 = 524 million.
ApplicationWindow // ... setup code ...