Why are shadows so hard to render in video games?

Because in a video game, you cannot just shine a light at an object and get the shadows “for free”, automatically (like in a real world). Instead, you have to make really complicated calculations to find out which other objects will get obscured by the shadows, and how. read more

The depth buffer (shadow map is saved as one) is not continuous leaking to issues like this. Some pixels are"deep" enough there is a shadow and others are not leading to shadow acne. When rendering shadows you can render the back facing (surface normal points away from the camera) polygon to the shadow map. read more

Oh you mean “computationally or artistically difficult”. Sorry, but I couldn't resist the joke, especially given the recent incident with watch dogs 2. They did really prove how hard it is to render genitals in a video game - it neither looked very good, nor was it tolerated very well. For a real answer, most others will say hair. read more