There are a lot of stuff going on.
1. Render quality - Most of the renders cut from an image usually are dragged down to LQ because of the below reasons. Sounds silly but plays a part.
2. The finer details of the cut - The hair, the shoes, the tuck-ins and cloth areas are to be preserved as they are in the original image.
3. Tricky areas - The inside parts of a render such as the Helmets, shoe laces, and all the gaps that are disconnected from the outer empty areas.
4. Matte finish around the edges - If you observe the outer areas of the renders come along with the surrounding part of the image the render has been cut from, the contracting of that area from the render is a clean matte.
5. The clean cut look. - Says it all. The overall render must look clean that anything else at the bottom, last half of the process.
This is the basis of judging and whereupon the competitors have to take care of.
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