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Selecting HDRI For CGI Scenes

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HDRI comes from High Dynamic Range Imagery. These image files -mostly photographed- can be very useful in creating your 3d scene. We are going to emphasize the most contributing elements in selecting HDRI for CGI scenes.

The most common objective of HDRI is lighting up the scene with realistic global illumination. Little we use it as a background in the final render, especially if we have animations. HDRI lights our scene from all directions, also appearing as the reflected background if needed.

EV Element

EV stands for Exposure Value; to simply explain this, Lets say that we took a photo with an exposure level of 1/8 and at the same angle we took another photo with double the exposure 1/4. Merging those two photos we would get a final image with 1EV.

So higher EV comes from merging multiple images with variable exposure levels and static camera angle.

Say we took ten photos, each has the triple amount of exposure of the previous one. The final merged EV would be { 9 × 3 = 27 EVs } note that ten photos have nine gaps between them.

Roughly you will need around 12EVs for interior scenes, and around 22EVs for exterior scenes. As in the image above, the sun is perfectly seen when nothing around it is visible in the photo; yet it is too bright when the lake is captured. So you will need an HDRI with the complete dynamic range enough for you particular 3d shot, otherwise you will have incorrect lighting.

Resolution

If all you want from your HDRI is lighting. Resolution will not matter. Though if you are looking to get reflections and shadows out of it. Selecting HDRI with higher resolution will give a clearer higher quality shadows and reflections.

For technical info, you won’t need more than a 14k HDRI image for a 1920*1080 render shot. Yet these images consume a lot of RAM. These are 32-bit images. So each frame needs 32 bit of your memory. If you are using a 14000×7000 HDRI, it will use about 1GB of RAM. These bytes are precious if you are rendering with GPU. If you are CPU rendering, it won’t matter.

There are a lot of free websites that offer a large variety when selecting HDRI. One of the most common is https://hdrihaven.com/ , others like https://www.hdri-hub.com/ or https://hdrmaps.com/.


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