Texture Mapping and Special Effects

Transcription

1 Texture Mapping and Special Effects February 23 rd 26 th 2007 MAE , Virtual Reality Applications and Research Instructor: Govindarajan Srimathveeravalli

2 HW#5 Due March 2 nd Implement the complete Triangle and Vector classes Add functionality to draw triangles meshes By complete faces By index values As triangle strips (where and when possible) Will these functions be part of the triangle class or some separate drawing class? Load objects from STL files to test your code. Texture these objects automatically Make additional objects (spheres, cylinders etc. ) using parametric functions Draw these objects using display lists. Connect from 2D to 3D Modify your 2D code to write out point information Point information to be used to Create polygons which are to be extruded, swept or revolved What about convexity? How would you check? Axis information for extrusion and sweeping Extrusions and sweeps with necks and variations in object size. Extra Credit? 3D sweeps and extrusions 2

3 Texture mapping Done in the rasterizer phase of the graphics pipeline Texture increase scene realism Texture space -> Polygon coords -> Pixel values Provide better 3-D spatial cues (perspective) They reduce the number of polygons in the scene increased frame rate Application Geometry Rasterizer Compute Object space Location Use projector Function for Parameter space Corresponder Function to Find pixel Apply value Transform function Modify Illumination equation 3

4 Textured polygons Shaded Lit polygons 4

5 How to create textures: Create your own in code! Models are available online in texture libraries of cars, people, construction materials, etc. Custom textures from scanned photographs or Using an interactive paint program to create bitmaps Textures courtesy mayang.com 5

6 Texture mapping [0,1] Screen space Object surface [0,1] Texture space 256x256 texture 6

7 Image Texture: It glues an image to a polygon. Size of texture is restricted by graphics accelerators to 2 m 2 n or 2 m 2 m square. The lower limit for OpenGL is texels. If the size of the polygon is much larger than the size of the texture then the OpenGL performs magnification If there are much fewer pixels than texels minification Both techniques use bilinear interpolation to assign colors to pixels. 7

8 Texture minification: Texture Polygon Uses various filters to approximate the color of the pixel: nearest neighbor (to texel closest to the pixel center is selected, bilinear interpolation, etc.) 8

9 VR Geometric Modeling Tree, higher resolution model 45,992 polygons. Tree represented as a texture 1 polygon, 1246x1280 pixels ( 9

10 The GL Part Enable usage of textures first glenable( GL_TEXTURE_2D ) gltexenvf( GL_TEXTURE_ENV, GL MODE, GL_REPLACE MODULATE DECAL) Load and initialize the texture glpixelstore( GL_UNPACK_ALIGNMENT, VAL) Name the texture, glgentextures( 1, name(int) ) glbindtexture(gl_texture_2d, name ) gltexparameter(gl_texture_2d, parameter, value) Set up the drawing glteximage2d( Lots of parameters ) Define coordinates gltexcoord2f() OpenGL requires your image be of order 2 n Overcome (kind of) using gluscaleimage() glcopyteximage2d() may also be of interest 10

11 Texturing Curved Surfaces 11

12 Multi-texturing: texturing: Several texels can be overlaid on one pixel; A texture blending cascade is made up of a series of texture stages Interpolated vertex values Texture value 0 Stage 0 Stage 1 Stage 2 Texture value 1 Texture value 2 Polygon/ Image buffer Images courtesy, Steven Jones 12

13 The GL Part for MultiTexturing Needs to use GLSL and Glext.h GL Shading Language. Needs initialization and use of following two parameters PFNGLMULTITEXCOORD2FARBPROC glmultitexcoord2farb, glactivetexturearb glactivetexturear = (PFNGLACTIVETEXTUREARBPROC)wglGetProcAddress("glActiveTextureARB"); glactivetexturearb(gl_texture0_arb) Typicalljy upto 32 textures can be used at a time for multitexturing. Rest of the procedure is common Binding the texture glmultitexcoord2farb(gl_texture0_arb, 1.0f, 0.0f); Can be used for both Multi-texturing Lightmapping 13

14 Multi-texturing texturing for lighting: Several texels can be overlaid on one pixel; One application in more realistic lighting; Polygonal lighting is real-time but requires lots of polygons (triangles) for realistic appearance Vertex lighting of low polygon count surface lights are diffuse tessellated. Vertex lighting of high polygon count surface lights have realistic appearance. High computation load 14

15 Multi-texturing texturing (texture blending): Realistic-looking lighting can be done with 2-D textures called light maps Light maps are independent of lighting 15

16 Normal texture Allow more complex textures Reflectivity texture Background texture Transparency texture Bump maps 16

17 Bump mapping per-pixel pixel shading Normal texture Multi- texture Bump texture 17

18 Bump Mapping Courtesy Paul s Projects For any lighting model you need the vectors in the image to calculate pixel color information Texture and tangent space (s,t) Need to apply normals per pixel Similar to applying a texture! Represent vectors as follows, with condition that all values lie within [0 1] r = (x+1)/2; g = (y+1)/2; b = (z+1)/2; 18

19 Bump Mapping Courtesy Paul s Projects Look at a typical bump map generated using previous routine Cube maps A 3D representation of textures Can be used to store normal information Vector passed in is returned normalized Why all this trouble? The vector diagram from prev slide, has to be w.r.t. surface! 19

20 GLSL Courtesy Light House 3D Vertex opns Transforms, lighting, textures etc. Primitive assy Gives out fragments (contrast with pixels, that they hold normal, texture and other assoc info) Graphics cards offer new functionality Vertex transformations Vertex position transformation Normal transformations Per Vertex Lighting Texturing Fragment operations Similar operations on fragment level 20

21 Blending Anti Aliasing Images courtesy Inventor Mentor Keyword GL_BLEND glblendfunc( source, dest ) Also, stipple and stencils glhint( target, value ) 21

22 glenable( GL_FOG ) Again achieved through carefully blending object pixel color with fog color Color Quality of fog Other parameters Fog hint Fog, Smoke etc. 22