Lecture 6 3D Rendering Pipeline III Prof Hsien Hsin Sean Lee School of Electrical and Computer Engineering Georgia Institute of Technology Rasterization Shading a Triangle RGB 255 0 0 RGB 0 0 255 RGB 0 255 0 Paint each pixel s color by calculating light intensity on your display Typically object based Gouraud Shading Intensity Interpolation of vertices 2 Gouraud Shading RGB 255 0 0 RGB 127 127 0 RGB 127 0 127 Scan line RGB 127 64 64 RGB 0 0 255 RGB 0 255 0 Scan conversion algorithm 3 Comparison of Shading Methods Source Michal Necasek Flat shading Gouraud shading Phong shading Phong shading requires generating per pixel normals to compute light intensity for each pixel not efficient for games Can be done on GPGPU today using Cg or HLSL Gouraud shading is supported by Graphics hardware 4 Double Buffering Display refreshes at 60 75 Hz Rendering could be faster than the refresh period Too fast leads to Frames not shown Too slow leads to New and old frame mixed Flickering Front Buffer Back Buffer surface1 surface2 swap surface1 surface2 Solution Double or multiple buffering 5 Z Buffer z1 z2 z3 Also called depth buffer Draw the pixel which is nearest to the viewer Number of the entries corresponding to the screen resolution e g 1024x768 should have a 768k entry Z buffer Granularity matters 8 bit never used 16 bit z value could generate artifacts 6 Z Buffer Bandwidth Could Be an Issue Perform Z test before drawing a pixel How much bandwidth needed for RZ WZ RC WC TR per pixel Overdrawn rate is about 3 out of 4 One every 4 pixels drawn is for clearing the Z buffer Some cards perform Z compression to alleviate bandwidth issues 7 Aliasing Jagged line or staircase Can be improved by increasing resolution i e more pixels 8 Anti Aliasing by Multisampling Example Supersampling 1 2 1 2 4 2 3x3 Virtual Pixels 1 2 1 Bartlett window 255 159 159 255 255 255 255 255 255 255 0 0 255 255 255 255 0 0 255 0 0 255 255 255 255 255 255 255 255 255 Actual Screen Pixels Example GPU samples multiple locations for a pixel Several different methods e g grid as shown random GeForce s quincunx Downside Blurry image Increased memory e g z buffer storage for subpixel information 9 Anti Aliasing Example Ideal No MSAA With MSAA 10 Visualizing Anti Aliasing Example No MSAA With MSAA 11 Texture Mapping Rendering tiny triangles is slow Players won t even look at some certain details Sky clouds walls terrain wood patterns etc Simple way to add details and enhance realism Use 2D images to map polygons Images are composed of 2D texels Can be used to substitute or blend the lit color of a texture mapped surface 12 Texture Mapping Introduce one more component to geometry Position coordinates Normal vector Color Texture coordinates Texture info u v coordinates for each vertex Typically range from 0 to 1 0 0 from upper left corner 13 Texture Mapping 0 0 v7 1 0 v0 0 1 v1 1 1 Texture hunk jpg v1 x v2 x v0 x v3 x v8 v1 y v2 y v0 y v3 y v1 z v2 z v0 z v3 z u 1 1 0 0 v 0 1 0 1 v3 v2 14 Texture Mapping 0 0 Texture 1 1 0 0 0 Texture 2 v7 v0 0 1 1 1 v8 v1 1 1 v3 v2 15 Texture Mapping 0 0 1 0 v2 v3 v0 1 1 0 1 v1 x v2 x v0 x v3 x v1 v1 y v2 y v0 y v3 y v1 z v2 z v0 z v3 z u v 0 5 5 0 0 0 3 3 16 Texture Mapping v1 x v2 x v0 x v3 x v1 y v2 y v0 y v3 y v1 z v2 z v0 z v3 z u v 0 6 6 0 0 0 6 6 17 Magnification Texels Pixels on screen Texel and pixel mapping is rarely 1 to 1 Mapped triangle is very close to the camera One texel maps to multiple pixels 18 Nearest Point Sampling for Magnification Texels Pixels on screen Choose the texel nearest the pixel s center 19 Bi linear Filtering for Magnification R 247 G 237 B 141 R 253 G 230 B 145 Texels R 255 G 204 B 102 R 214 G 193 B 110 R 102 G 102 B 51 Pixels on screen Average for the 2x2 texels surrounding a given pixel 20 Minification Color Texels Pixels on screen Texel and pixel mapping is rarely 1 to 1 Multiple texels map to one pixel 21 Nearest Point Sampling for Minification Pixels on screen Choose the texel nearest the pixel s center 22 Bi linear Filtering for Minification R 234 G 189 B 0 R 0 G 0 B 0 R 252 G 219 B 96 R 155 G 132 B 30 R 135 G 119 B 23 Pixels on screen Average for the 2x2 texels surrounding a given pixel 23 Mip mapping Multiple versions are provided for the same texture Different versions have different levels of details E g 7 LOD maps 256x256 128x128 64x64 32x32 16x16 8x8 4x4 Choose the closet maps to render a surface Maps can be automatically generated by 3D API Accelerate texture mapping for far away polygons More space to store texture maps 24 Mip mapping API or Hardware can Generate lower resolution mip maps automatically Choose the right one for the viewer Good performance for far triangles Good LOD for close by objects Tri linearly interpolate 25 Tri linear Filtering using Mip maps R 147 G 114 B 117 R 155 G 132 B 30 R 58 G 0 B 0 R 106 G 80 B 74 R 229 G 208 B 119 R 178 G 179 B 90 R 66 G 0 B 0 R 233 G 227 B 143 Lower Res Mip Map Higher Res Mip Map R 94 G 49 B 48 R 199 G 187 B 96 R 147 G 118 B 72 Screen Pixel Interpolate between mipmaps 26 Anisotropic Filtering Not isotropic Preserving details for oblique viewing angles non uniform surface Bilinear filtering Trilinear filtering AF calculates the shape of the surface before mapping The number of pixels sampled depends on the distance and view angles relative to the screen Very expensive 16x Anisotropic filtering 64x Anisotropic filtering Source nvidia 27 Color Blending and Alpha Blending Transparency effect e g Water glasses etc Source color blended with destination color Several blending methods Additive C SrcPixel 1 1 1 1 DstPixel 1 1 1 1 SrcPixel DstPixel Subtractive C SrcPixel 1 1 1 1 DstPixel 1 1 1 1 SrcPixel DstPixel Multiplicative C DstPixel SrcPixel Using Alpha value in the color Alpha Blending C SrcPixel DstPixel 1 1 1 1 And many more in the API 28 Alpha Blending Inverse Source form No transparency Src 0 2 triangle Dest 0 8 square Src 0 5 triangle Dest 0 5 square Src 0 8 triangle Dest 0 2 square 29 Another Example Without …
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