The need for shadowsHard ShadowsSoft Shadows

Outline

1 The need for shadows

2 Hard ShadowsShadow mapsShadow volumesHardware support: stencil bufferFake shadows

3 Soft Shadows

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

The need for shadows

Shadows come “for free” in raytracing and radiosity methods,but in projective methods theyrequire some extra effort.

However, these efforts arewell-spent: shadows addrealism and depth, as well asgive a better understanding ofthe images.

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Hard and soft shadows

Point light sources give hardshadows, whereas area lightsgive soft shadows.

For soft shadows, the regionthat receives no light at all iscalled the umbra, and theregion that receives partiallight is called the penumbra.

The penumbra gives additionalcues with respect to the size ofthe light source and thedistance from the occluder tothe receiver.

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow maps

A popular hardware supportedshadow rendering algorithmuses shadow maps.

The scene is first renderedfrom the light sourcepoint-of-view to fill a depthbuffer, stored with the light.

light source

view point

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow maps

Next, the scene is renderedfrom the camera point-of-view.The z-coordinate renderedpolyon for a pixel is convertedwith a transformation matrixto a z-coordinate in the lightsource coordinate system.

If the light source z-buffer hasa smaller z-value stored, thenthe part of the polygonprojected to the current pixellies in shadow.

light source

view point

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow maps

Advantages:

hardware-supported(z-buffer and matrix ops),so very fast.

Disadvantages:

Suited for spot lights, notreally for omni-directionallights.

Precision problems due touse of floating points anddiscrete sampling.

light source

view point

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow volumes

An alternative method forshadow computation usesshadow volumes.

For a light source and anoccluder, the 3D shadowvolume is determined by thesilhouette edges of theoccluder w.r.t the light source.

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Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow volumes

The faces of the shadowvolumes are either front facingor back facing w.r.t thecamera. This is easilydetermined with the normalsof the faces.

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Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow volumes

The idea: when a pixel isrendered, we follow the rayfrom the viewpoint to theobject for the pixel.

Whenever the ray crosses ashadow volume face, weincrement or decrement acounter, depending on whetherthe face is front-facing orback-facing.

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Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow volumes

Objects for which the counteris zero are lit; objects forwhich the counter is positiveare in shadow. 1

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Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Hardware support: stencil buffer

Obviously, we don’t want to doray tracing to determineshadows in real-time renderingmethods.This is where the stencil buffercomes in.

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Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Hardware support: stencil buffer

The stencil buffer is comparable to the z-buffer (i.e., there is aone-to-one correspondence of pixels in the frame buffer and entriesin the stencil buffer), but here every entry is a counter.

It supports

resetting, incrementing and decrementing the counters.

idem, but conditionally, depending on a test against thez-buffer.

conditional drawing in the frame buffer, i.e., only draw a pixelif the corresponding counter is zero/non-zero.

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Hardware support: stencil buffer

Computing shadows using thestencil buffer:

Draw the scene withambient lighting only(i.e., everything is drawnas if it is in shadow).

Compute the shadowvolumes

“draw” the shadowvolume faces in the stencilbuffer.

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Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Hardware support: stencil buffer

Computing shadows using thestencil buffer (cont’d):

Drawing a pixel in the stencilbuffer means:

Test the shadow face“pixel” agains thez-buffer. If its z value issmaller than that of thedrawn (real) object, then:

If it is a front-facingface, increment thecounter.If it is a back-facingface, decrement thecounter.

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Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Hardware support: stencil buffer

Computing shadows using thestencil buffer (cont’d):

Once all shadow volume facehave been drawn, the stencilbuffer contains zeros for pixelsthat are not in shadow.

Next, draw the entire sceneagain (only the real objects),this time including lightingcalculations, but tell thegraphics card to ignore pixelsthat have a non-zero stencilbuffer entry.

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Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow volumes: problems

Problems with the shadowvolume approach:

The view point may lie inone or more shadowvolumes.

The near clipping planemay slice open shadowvolumes.

light source

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow volumes: problems

The described problems can be solved by capping the shadowvolumes: the near clipping plane (partially) becomes one of thefaces of each volume. Drawback: precision problems close to nearclipping plane.

The described algorithm uses a depth-pass approach: only shadowvolume faces in front of objects are drawn in the stencil buffer.

Alternative: depth-fail approach: only draw/count shadow volumefaces behind object. Problems with near clipping plane are tradedfor problems with far clipping plane.

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Shadow volumes: depth-pass vs. depth-fail

Depth-pass approach:

Depth-fail approach:

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Fake shadows

A fast and easy (but not extremely realistic) way of creatingshadows: fake shadows. Only few (important) occluders areselected, and shadows are projected onto supporting planes of onlyfew (important) surfaces (e.g.: table has shadows, but chairsaround it haven’t).

Draw the scene, including lighting.

Project occluders onto designated planes. Improvement: don’trender projections black, but blend with original colors.

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Fake shadows

Problems with fake shadows:

projected shadows have same depth as shadow receivers.

Shadows may stick out beyond shadow receivers (since weproject on planes instead of objects).

Multiple occluders may give rise to double blending.

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Shadow mapsShadow volumesHardware support: stencil bufferFake shadows

Fake shadows

Solution to the problems with fake shadows: use stencil buffer.

Reset stencil buffer counters.

Draw scene. When shadow receivers are drawn, incrementcorresponding stencil buffer counters.

Project shadow polygons, but only draw/blend for pixel thathave a non-zero stencil value. Reset stencil entry afterdrawing, to avoid double blending.

Graphics, 2nd period 2007/08 Lecture 13: Shadows

The need for shadowsHard ShadowsSoft Shadows

Soft Shadows

Soft shadows are expensive. Usual approach: approximate bysampling area lights, treating every sample as a point light source.Compute individual shadows, and average.

Drawback: many samples are needed to get smooth penumbras,and this hurts performance.

Graphics, 2nd period 2007/08 Lecture 13: Shadows