Computer Graphics. Si Lu. Fall uter_graphics.htm 11/08/2016

Size: px

Start display at page:

Download "Computer Graphics. Si Lu. Fall uter_graphics.htm 11/08/2016"

Aldous Morrison
5 years ago
Views:

1 Cmputer Graphics Si Lu Fall uter_graphics.htm 11/08/2016

2 Last time Lighting and Shading 2

3 Tda Teture Mapping Hmewrk 4 available, due in class Nvember 20 Prject 2 Will publicize several times in the final week f classes when u can get ur prject graded Dem ur prgram t the instructr in persn Bring ur wn laptp r n a CS Windws Lab Machine Latest time t grade 5:00 pm, Frida, December 1, 2017 N late submissin! 3

4 Mapping Techniques Cnsider the prblem f rendering a glbe The gemetr is ver simple - a sphere But the clr changes rapidl, with sharp edges With the lcal shading mdel, s far, the nl place t specif clr is at the vertices T d a glbe, wuld need thusands f plgns fr a simple shape Same thing fr an range: simple shape but cmple nrmal vectrs Slutin: Mapping techniques use simple gemetr mdified b a detail map f sme tpe

5 Glbe and Orange 5

6 Teture Mapping + = 3D bject Teture Tetured bject Image surce: 6

7 Teture Mapping Teture mapping assciates the clr f a pint with the clr in an image: the teture Questin t address Which pint f the teture d we use fr a given pint n the surface? Establish a mapping frm surface pints t image pints Different mappings are cmmn fr different shapes We will, fr nw, just lk at triangles (plgns)

8 Eample Mappings

9 Basic Mapping The teture lives in a 2D space Parameterize pints in the teture with 2 crdinates: (s,t) These are just what we wuld call (,) if we were talking abut an image, but we wish t avid cnfusin with the wrld (,,z) Define the mapping frm (,,z) in wrld space t (s,t) in teture space T find the clr in the teture, take an (,,z) pint n the surface, map it int teture space, and use it t lk up the clr f the teture Samples in a teture are called teels, t distinguish them frm piels in the final image With plgns: Specif (s,t) crdinates at vertices Interplate (s,t) fr ther pints based n given vertices

10 Teture Interplatin Specif where the vertices in wrld space are mapped t in teture space (s,t) (,),(s,t) A teture crdinate is the lcatin in teture space that crrespnds t the verte t Linearl interplate the mapping fr ther pints in wrld space Straight lines in wrld space g t straight lines in teture space s Teture map teture crdinates Triangle in wrld space verte crdinates

11 Linear Interplatin (1) (s1) () (s) (2) (s2)? 11

12 Linear Interplatin (1) (s1) () (s) (2) (s2) 12

13 Interplating Crdinates ( 1, 1 ), (s 1, t 1 ) ( 2, 2 ), (s 2, t 2 ) ( 3, 3 ), (s 3, t 3 ) s s s R s s s L R L R L L L R L s s s 1 (, ), (s, t)

14 Barcentric Crdinates An alternate wa f describing pints in triangles These can be used t interplate teture crdinates Gives the same result as previus slide Methd in tetbk (Shirle) ),, Area( ),, Area( ),, Area( ),, Area(

15 Steps in Teture Mapping Plgns (triangles) are specified with teture crdinates at the vertices A mdeling step, but sme was t autmate it fr cmmn shapes When rasterizing, interplate the teture crdinates t get the teture crdinate at the current piel Previus slides Lk up the teture map using thse crdinates Just rund the teture crdinates t integers and inde the image Take the clr frm the map and put it in the piel Man was t put it int a piel (mre later)

16 Basic OpenGL Teturing Specif teture crdinates fr the plgn: Use gltecrd2f(s,t) befre each verte: Eg: gltecrd2f(0,0); glverte3f(,,z); Create a teture bject and fill it with teture data: glgentetures(num,&identifier) t get identifiers fr the bjects glbindteture(gl_texture_2d, identifier) t bind the teture Fllwing teture cmmands refer t the bund teture glteparameteri(gl_texture_2d,, ) t specif parameters fr use when appling the teture glteimage2d(gl_texture_2d,.) t specif the teture data (the image itself) MORE

17 Basic OpenGL Teturing (cnt) Enable teturing: glenable(gl_texture_2d) State hw the teture will be used: glteenvf( ) Teturing is dne after lighting Yu re read t g

18 Nast Details There are a large range f functins that cntrl the laut f teture data: Yu must state hw the data in ur image is arranged Eg: glpielstrei(gl_unpack_alignment, 1) tells OpenGL nt t skip btes at the end f a rw Yu must state hw u want the teture t be put in memr: hw man bits per piel, which channels, Tetures must be square with width/height a pwer f 2 Cmmn sizes are 3232, 6464, Smaller uses less memr, and there is a finite amunt f teture memr n graphics cards Sme etensins t OpenGL allw arbitrar tetures

19 Cntrlling Different Parameters The piels in the teture map ma be interpreted as man different things. Fr eample: As clrs in RGB r RGBA frmat As grascale intensit As alpha values nl The data can be applied t the plgn in man different was: Replace: Replace the plgn clr with the teture clr Mdulate: Multipl the plgn clr with the teture clr r intensit Similar t cmpsiting: Cmpsite teture with base clr using peratr

20 Eample: Diffuse shading and teture Sa u want t have an bject tetured and have the teture appear t be diffusel lit Prblem: Teture is applied after lighting, s hw d u adjust the teture s brightness? Slutin: Make the plgn white and light it nrmall Use glteenvi(gl_texture_2d, GL_TEXTURE_ENV_MODE, GL_MODULATE) Use GL_RGB fr internal frmat Then, teture clr is multiplied b surface clr, and alpha is taken frm fragment

21 Specular Clr Tpicall, teture mapping happens after lighting Mre useful in general Recall plastic surfaces and specularities: the highlight shuld be the clr f the light But if teturing happens after the lighting, the clr f the specularit will be mdified b the teture the wrng thing OpenGL lets u d the specular lighting after the teture Use gllightmdel ()

22 Teture Recap Triangle in 88 Teture Map, I(s,t) Triangle in wrld space t I(s,t) Interplated (s,t) we need a teture sample frm I(s,t) s We must recnstruct the teture image at the pint (s,t) Time t appl the ther f sampling and recnstructin

23 Tetures and Aliasing Tetures are subject t aliasing: A plgn piel maps int a teture image, essentiall sampling the teture at a pint The situatin is essentiall an image warp, with the warp defined b the mapping and prjectin Standard appraches: Pre-filtering: Filter the teture dwn befre appling it Useful when the teture has multiple teels per utput image piel Pst-filtering: Take multiple piels frm the teture and filter them befre appling t the plgn fragment Useful in all situatins

24 Mipmapping (Pre-filtering) If a tetured bject is far awa, ne screen piel (n an bject) ma map t man teture piels The prblem is: hw t cmbine them A mipmap is a lw reslutin versin f a teture Teture is filtered dwn as a pre-prcessing step: glubuild2dmipmaps( ) When the tetured bject is far awa, use the mipmap chsen s that ne image piel maps t at mst fur mipmap piels Full set f mipmaps requires at mst the strage f the riginal teture (in the limit) * *0.25*0.25+

25 Mipmaps Fr far bjects Fr middle bjects Fr near bjects

26 Mipmap Math Define a scale factr, =teels/piel A teel is a piel frm a teture is actuall the maimum frm and The scale factr ma var ver a plgn It can be derived frm the transfrmatin matrices Define =lg 2 tells u which mipmap level t use Level 0 is the riginal teture, level 1 is the net smallest teture, and s n If <0, then multiple piels map t ne teel: magnificatin

27 Pst-Filtering Yu tell OpenGL what srt f pst-filtering t d Magnificatin: When <0 the image piel is smaller than the teel: glteparameteri(gl_texture_2d, GL_TEXTURE_MAG_FILTER, tpe) Tpe is GL_LINEAR r GL_NEAREST Minificatin: When >0 the image piel is bigger than the teel: GL_TEX_MIN_FILTER Can chse t: Take nearest pint in base teture, GL_NEAREST Linearl interplate nearest 4 piels in base teture, GL_LINEAR Take the nearest mipmap and then take nearest r interplate in that mipmap, GL_NEAREST_MIPMAP_LINEAR Interplate between the tw nearest mipmaps using nearest r interplated pints frm each, GL_LINEAR_MIPMAP_LINEAR

28 Filtering Eample Level 2 Level 1 Level 0 s=0.12,t=0.1 =1.4 =0.49 NEAREST_MIPMAP_NEAREST: level 0, teel (0,0) LINEAR_MIPMAP_NEAREST: teel 0, teel (0,0) * level 1, teel (0,0) * 0.49 NEAREST_MIPMAP_LINEAR: level 0, cmbinatin f teels (0,0), (1,0), (1,1), (0,1) LINEAR_MIPMAP_LINEAR : Cmbinatin f level 0 and level 1, 4 teels frm each level, using 8 teels in all

29 Other Teture Stuff Teture must be in fast memr - it is accessed fr ever piel drawn If u eceed it, perfrmance will degrade hrribl Skilled artists can pack tetures fr different bjects int ne image Teture memr is tpicall limited, s a range f functins are available t manage it Smetimes u want t appl multiple tetures t the same pint: Multiteturing is nw in mst new hardware

30 Net Time Mesh and Mdelling 30

Topic 7: Transformations. General Transformations. Affine Transformations. Introduce standard transformations

Topic 7: Transformations. General Transformations. Affine Transformations. Introduce standard transformations Tpic 7: Transfrmatins CITS33 Graphics & Animatin E. Angel and D. Shreiner: Interactive Cmputer Graphics 6E Addisn-Wesle 22 Objectives Intrduce standard transfrmatins Rtatin Translatin Scaling Shear Derive