Technische Universität München Computer Graphics SS 2014 Graphics Effects Rüdiger Westermann Lehrstuhl für Computer Graphik und Visualisierung

Technische Universität München Computer Graphics Advanced graphics effects Effects which are not provided natively by the rasterization based graphics pipeline Scene reflections Shadows 2

Technische Universität München Computer Graphics Advanced graphics effects Reflections Note the difference between local illumination and scene reflection –Local illumination considers the reflection of direct light –Scene reflection considers the reflection of the scene Problem: which scene point reflects in an object point 3

Technische Universität München Computer Graphics Advanced graphics effects Scene reflections using rasterization based pipeline –Rasterization based pipeline projects scene onto the image plane –Camera position is projection center for all scene parts, ie. single view projection –To simulate scene reflections, the scene has to be rendered from multiple views, ie. from each object point that reflects towards the viewer –Requires many rendering passes, and is too costly in general 4

Technische Universität München Computer Graphics Advanced graphics effects Planar reflections –Reflect the scene about a planar mirror Basic algorithm: flip scene through the mirror and re- render 5

Technische Universität München Computer Graphics Advanced graphics effects Reflect of scene at the mirror plane requires finding the reflection matrix Re-render reflected scene but restrict drawing to the pixels through which the mirror is seen Mirror Reflected scene Wall 6

Technische Universität München Computer Graphics Advanced graphics effects The reflection matrix –p[3]: point on plane –n[3]: plane normal –d: n·p –Multiply reflection matrix with modeling matrix 7

Technische Universität München Computer Graphics Advanced graphics effects Restrict drawing of the flipped scene to the pixels through which the mirror is seen 8

Technische Universität München Computer Graphics Advanced graphics effects Restricted drawing via the stencil test –Stencil buffer: stores for each pixel a bit-mask –A stencil test can be performed after the depth test for each fragment Color buffer Depth buffer Stencil buffer 9

Technische Universität München Computer Graphics Advanced graphics effects Typical use of the stencil buffer/test stencil = 1 Initialize Stencil buffer Render pass 1Render pass 2 Test: ref = 0; stencil == ref Test: ref = 1; stencil == ref 10

Technische Universität München Computer Graphics Advanced graphics effects Mirror rendering algorithm A) Initialize modelview matrix, enable depth/stencil testing B) Render mirror to mask all covered pixels in stencil buffer mask = 1 11

Technische Universität München Computer Graphics Advanced graphics effects Mirror rendering algorithm C) Multiply modeling matrix with reflection matrix to flip the scene D) Render scene where pixels have been mask* *clip everything in front of the mirror plane using a clip-plane 12

Technische Universität München Computer Graphics Advanced graphics effects Mirror rendering algorithm E) Reset modeling matrix to initial state F) Render scene where pixels have not been mask + = 13

Technische Universität München Computer Graphics Advanced graphics effects Non planar reflections 14

Technische Universität München Computer Graphics Advanced graphics effects Environment mapping: –Simulate an infinitely small reflecting sphere in the center of the scene to be reflected –Compute (or measure) and store in a texture the incoming light at the sphere for all directions –At run-time, for a given reflection vector look-up the stored light from the texture and use as fragment color 15

Technische Universität München Computer Graphics Advanced graphics effects Cube maps –An alternative to environment maps which can be generated very efficiently using rendering functionality –Captures the incoming light through 6 faces of a cube centered at the environment center –Generated by rendering the scene 6 times from the cube center, with the respective cube face being the viewport 16

Technische Universität München Computer Graphics Advanced graphics effects See also

Technische Universität München Computer Graphics Advanced graphics effects Cube map is stored in 6 2D textures Cube map access: –For each fragment compute the reflection vector R –The largest component of R selects the cube face –By dividing the other components through the largest, the 2D texture coordinate in the selected face is given 18

Technische Universität München Computer Graphics Advanced graphics effects 19

Technische Universität München Computer Graphics Advanced graphics effects Cube map examples 20

Technische Universität München Computer Graphics Advanced graphics effects Effects which are not provided natively by the rasterization based graphics pipeline Scene reflections Shadows 21

Technische Universität München Computer Graphics Advanced graphics effects Why shadows? 22

Technische Universität München Computer Graphics Advanced graphics effects Shadows and visibility: Remember, the depth-test resolves visibility on a per- fragment basis –z-buffer stores minimum z-value of all fragments projecting into the same pixel –Testing during fragment processing Frustum (perspective) transformation preserves z-ordering Incremental z-value update (if fragment is closer to near plane than current one) –Visibility is with respect to the viewer 23

Technische Universität München Computer Graphics Advanced graphics effects z-buffer algorithm: colorbuffer  BackgroundColor z-buffer  1 foreach polygon foreach fragment x,y // generated by the rasterizer z = fragment x,y.depth if z < z-buffer[x,y] then// depth test z-buffer[x,y] = z colorbuffer[x,y] = fragment x,y.color 24

Technische Universität München Computer Graphics Advanced graphics effects Visibility: which objects can be seen from the view point? Shadows: which objects can be seen from the light source? © Bertrand Clerc Design 25

Technische Universität München Computer Graphics Advanced graphics effects Shadow mapping –Simulates shadows in two rendering passes (1)Renders the scene from the light source position to determine all fragments which are not in shade, ie. which are seen by the light (2)Renders the scene from the camera position and tests each generated fragment whether it was seen or not in the first pass  If it was seen in the first pass it receives light, otherwise it is in shade and its color is darkened  The test simply checks whether in the first pass there was a fragment closer to the light than the current fragment 26

Technische Universität München Computer Graphics Advanced graphics effects Shadow mapping – the 1st pass –Since we need the fragments from the first pass to perform the test in the second pass, these fragments have to be stored –What we store is the distances of these fragments to the light source, which is simply the content of the deph buffer after the first pass 27

Technische Universität München Computer Graphics Advanced graphics effects Shadow mapping – the 1st pass: the depth buffer after the first rendering pass is stored in a texture map – the shadow map –In DX this can be realized by a pixel shader that computes the fragments depth values and writes these values into a texture resource instead of the color buffer –First, set up the description of the texture, then create that texture, and finally use the texture to setup a render target view –The so-generated texture can then be made accessible to a pixel shader in the second rendering pass 28

Technische Universität München Computer Graphics Advanced graphics effects Shadow mapping – the 2nd pass renders from the camera –A pixel shader computes the fragments distances to the light source –The shader also projects the fragments towards the light source into the shadow map – the fragments coordinate after projection is used as texture coordinate into the shadow map Light Depth of fragment to light source 29

Technische Universität München Computer Graphics Advanced graphics effects Shadow mapping – the 2nd pass –Finally, the shader compares the fragments depth with the depth stored in the shadow map and decides whether it is in shade or not Light Depth in shadow map Depth of fragment to light source 30

Technische Universität München Computer Graphics Advanced graphics effects Shadow mapping –Requires one additional rendering pass per light source –Can only be used in combination with point light sources –Artefacts can occur due to the resolution of the shadow map 4096x x

Technische Universität München Computer Graphics Shadow volumes –Relies on a geometric representation of the spatial region that is shadowed by an object –Shadow polyhedra is generated by projecting the objects´ shilouette edges (as seen from the light source) onto the shadowed surface and connecting the respective points –Shadow algorithm now tests whether object points, ie. fragments, are inside or outside a shadow volume

Technische Universität München Computer Graphics Shadow volumes

Technische Universität München Computer Graphics Shadow volumes Shadow volume geometry Without shadows Shadow volume Final scene

Technische Universität München Computer Graphics Shadow volumes When is a point in shadow? Shadow volume algorithm [Crow 1977] –Intersect the rays from the eye with shadow volume boundaries –For each ray Start with counter = 0, and increment/decrement the counter at every shadow volume front/back face If ray hits an object and counter == 0, then the object point is not in shadow

Technische Universität München Computer Graphics Shadow volumes Algorithm [Heidmann 91] 1.Render the unlit scene and clear the stencil buffer 2.Loop over all light sources Clear stencil buffer, disable depth write and color write but leave depth test enabled Render front-facing polygons of the shadow volume and increment the stencil where they pass the depth test Render back-facing polygons of the shadow volume and decrement the stencil where they pass the depth test Render the lit scene where stencil value = 0 (with additive blending) Note: –Stencil value classifies fragments as shadowed or unshadowed Shadowed pixels end up with non-zero stencil

Technische Universität München Computer Graphics Shadowing object Light source Eye position zero Unshadowed object Shadow Volume Count = = 0 Illuminated (Behind Shadow Volumes)

Technische Universität München Computer Graphics Shadowing object Light source Eye position zero Shadowed object Shadow Volume Count = = 2 Shadowed (inside Shadow Volumes)

Technische Universität München Computer Graphics Shadowing object Light source Eye position zero Uns hadowed object Shadow Volume Count = 0 Illuminated (before Shadow Volumes)