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These parameters affect the calculation phase of the light cache; they do not affect the final rendering.

Subdivs – Determines how many paths are traced from the camera. The actual number of paths is the square of the subdivisions (the default 1000 subdivs mean that 1 000 000 paths will be traced from the camera).   For more information, see the Subdivisions Parameter example below.

Sample Size – Determines the spacing of the samples in the light cache. Smaller numbers mean that the samples will be closer to each other, the light cache will preserve sharp details in lighting, but it will be noisier and will take more memory. Larger numbers will smooth out the light cache but will loose detail. This value can be either in world units or relative to the image size. For more information, see the  Sample Size Parameter example below.

Screen Space – The units are fractions of the final image (a value of 1.0 means the samples will be as large as the whole image). Samples that are closer to the camera will be smaller, and samples that are far away will be larger. Note that the units do not depend on the image resolution. This value is best suited for stills or animations where the light cache needs to be computed at each frame. 
World Units – The sizes are fixed in world units everywhere. This can affect the quality of the samples - samples that are close to the camera will be sampled more often and will appear smoother, while samples that are far away will be noisier. This value might work better for fly-through animations, since it will force constant sample density everywhere.

Retrace – When enabled, this option improves the precision of the global illumination in cases where the light cache will produce too large an error. This is especially obvious with the Use light cache for glossy rays option, or near corners where light leaks might be possible because of the light cache interpolation. For glossy reflections and refractions, V-Ray dynamically decides whether to use the light cache or not based on the surface glossiness and the distance from it so that the errors due to the light cache are minimized. Note that this options may increase the render time. For more information, see the Retrace Parameter example below.

Use Camera Path – When enabled, V-Ray calculates the GI samples for the entire camera path instead of just the current view. This option is available only if there is an animation set up in the project.

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Disk Caching rollout is available only in advanced mode of the Global Illumination rollout.

Mode – Controls the mode of the irradiance map:

Single Frame – When enabled, the last GI map calculated stays in memory after the rendering has finished
From file – When enabled, V-Ray does not compute the photon map but loads it from a file. Hit the Browse button on the right to specify the file name.

Keep File – When enabled V-Ray keeps the photon map in memory after the scene rendering has finished. When disabled, the map is deleted and the memory it took is freed. Enabling this option can be especially useful if you want to compute the photon map for a particular scene only once and then re-use it for further rendering.

Auto-Save File – When enabled, V-Ray automatically saves the caustics photon map to the provided file when rendering is completed. Specifies the file location where the caustics photon map is saved after rendering.

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