In V-Ray GPU, an image sampler refers to an algorithm for calculating a pixel's color based on the colors within and around it.

Each pixel in a rendering can have only one color. To get the color of a pixel, V-Ray calculates it based on the object's material, direct light striking the object, and indirect lighting in the scene. But within a single pixel there might be multiple colors, which may come from multiple objects whose edge intersect at the same pixel, or even difference in brightness on the same object due to changes in object shape or falloff and/or shadowing of light sources.

To determine the right color for such a pixel, V-Ray looks at (or samples) colors from different parts of the pixel itself as well as the pixels around it. This process is called image sampling.

UI Path: ||Render Setup window|| > Settings tab > Image sampler rollout (Renderer set to V-Ray GPU)

^{Image courtesy of Tuna Unalan}

Multiple colors within a single pixel. What color should the pixel be? 

Image filter – Enables sub-pixel filtering. When this option is disabled, an internal 1x1 pixel box filter is used. Using image filters may slightly increase rendering times.

Size – Specifies the size of the image filter.

Type – Specifies the filter type: Area, Blend, VRayLancsozFilter and VRayBoxFilter¹. For more information, see The Anti-aliasing Filters example. 

Adaptivity clamp – Specifies an intensity limit for the adaptive bucket and progressive samplers to avoid excessive sampling of overexposed areas. Lower values mean a lower limit and potentially noisy overexposed areas. Higher values produce more samples in overexposed areas.

Render mask – Enables the use of a render mask to determine which pixels of the image are calculated. The rest of the pixels are left intact.

None – A render mask is not used.
Texture – Render mask is a texture. Black values in the map define pixels which are not rendered. Pixels with any other values are rendered. The texture should use the screen mapping type. 
Selected – Only pixels that belong to the currently selected object(s) are rendered. V-Ray still calculates the entire image, but only renders parts that belong to the selected object(s). This mode is useful if you want to isolate or re-render just some objects from the scene. 
Include/Exclude list – Same as Selected, but objects are selected by list.
Layers – Renders only objects that belong to the currently selected layer. 
Object IDs – Renders only objects with specified Object IDs. Separate Object IDs with a comma (,).
Clear previous render – Removes the previous frame for the VFB. 

For more information on how to use the Render mask feature, see the Render Mask tutorial.