The Sample Rate Render Element displays the work being done on the image as it is rendering, and will show the job done when the render completes.
A color is assigned to each pixel according to how many primary samples are being taken by the Image Sampler:
Fancy Bullets
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The Blue channel contains the areas which reached the desired image quality with few primary samples.
The Green channel shows areas which required more anti-aliasing (AA) samples to clean up than the Blue areas, but managed to achieve the Noise Threshold within the number of AA rays used.
The Red channel contains the parts of the image which didn't make it to the desired quality threshold, as they reached the maximum amount of AA rays before reaching the Noise Threshold.
If the Image Sampler type is Progressive, the White areas will represent the areas the sampler is currently working on, as they were found to not satisfy the Noise Threshold as of yet.
The way to ensure a render is correctly computed is to minimize the amount of red color in this render element. Raising secondary sampling through any means will reduce the primary sampling work in the corresponding areas, effectively moving red areas into green ones, and green ones into blue ones. Lowering secondary samples per pixel will do the opposite, potentially leading to more red areas. The suggested way to minimize the red color and optimize render times to increase the the Max AA Subdivs value (if the previous render was still noisy), or increase the Noise Threshold value (if the previous render was clean, but took too long to complete).
Note that some specific image features, like high-frequency detail often found in fur and hair, or high-contrast zones, will likely always try to reach the maximum sampling, so some red will appear there. This is perfectly normal and doesn't slow down the rest of the rendering while ensuring the problematic parts of the image will clean up as per the user's settings.
To use render elements, select from the Available render elements column and click the Add button to add the passes you wish to generate at render time. When you render the scene, the added render elements listed in the column on the right, will also render, which can be viewed from the VFB window's channel drop-down menu.
For more information on the parameters on this dialog, see the Render Elements tab page.
