清华大学：《计算机图形学基础》课程教学资源（授课教案）阴影 Shadow

Shadow ( Shadow (阴影) • Shadow – Shadows are important elements in creating realistic imag pg es and in providing the user with visual cues(暗示) about object placement. – Shadow generation is a fundamental problem Shadow generation is a fundamental problem in Computer Graphics. Today, we will present the most important real-time shadow algorithms

Shadow • Why is shadow important? – Let’s see an example

Shadow • Why is shadow important? – Do you know where are the balls located? ( ) Without shadows) – It is hard to determine the actual position

Shadow • Why is shadow important? – We know where the balls located are ( ) With shadows)

Shadow • Why is shadow important? – Different shadows implies different balls location

Shadow • Why is shadow important? – From this example, we could conclude that: • Shadows gp j ive an important visual cue of object position. • Same images with different shadows implies Same images with different shadows implies different object positions. – S hd i i So, shadow is important

Shadow • What is Shadow? – Definition and Terminations • Consider a light source L illuminating a scene: • receivers are objects of the scene that are py y otentially illuminated by L. • A point P of the scene is considered to be in the umbra(本影) if it can not see any part of L. if it can not see any part of L. • If P can see a part of the light source, it is in the penumbra(半影)

Shadow • What is Shadow? – Definition and Terminations • Shadow is the union of the umbra is the union of the umbra (本影) and the penumbra (半影), is the region of space f hi h l i f h li h for which at least one point of the light source is occluded. • Objects that hide a point from the light source are call de occld ( u ers 遮挡物)

Shadow • What is Shadow? D fi iti Light Source – Definition and Terminations g Occluder b penumbra Reciever Umbra penum ra penumbra

Shadow • Types of Shadows – attached shadows attached shadows: occurring when the normal of : occurring when the normal of the receiver is facing away from the light source; – cast hd s a ows: occurri h h d f ll ing when a shadow falls on an object whose normal is facing toward the light source. – Self-shadows: are a specific case of cast shadows th t h th h d f bj t i that occur when the shadow of an object is projected onto itself, i.e. the occluder and the receiver are the same receiver are the same