14 - Additional Topics

Transcription

1 CSc 165 Computer Game Architecture Game-Loop Approaches Tightly-Coupled Fixed frame rate Variable-timestep update() Gameloop-managed update timing Fixed logic rate Renderer frame interpolation Multi-threading 2 Review: Game Structure Tightly-Coupled Approach Start initsystem() initgame() mainloop() shutdown() while (!gameover) handleinput() logicupdate() render() One iteration = 1 Frame Difficulties : Different machine speeds produce different game flow effects Changes to game logic (e.g. AI) can slow down presentation (frame rate) Problem source: The two steps have different inherent frequencies o logicupdate() frequency should be fixed; o render() frequency should optimize hardware use Exit Tightly-coupled game loop Conclusion: Need to somehow decouple the steps 3 4 Fixed frame rate Constant frame and update rate (one update per frame) Variable-timestep updating update() is responsible for accounting for time variation desiredframeduration = (1/desiredFPS)* ; //nsec handleinput() update() render() compute sleeptime sleep() while (!gameover) { framestarttime = System.nanoTime(); handleinput(); update(); render(); frameendtime = System.nanoTime(); frameduration = frameendtime framestarttime; while (frameduration < desiredframeduration) { sleeptimemsec = (desiredframeduration frameduration) / ; sleep (sleeptimemsec); frameduration = System.nanoTime() framestarttime; yield(); //in case we never sleep Compute Elapsed time handleinput() update(time) render() lastupdatetime = System.nanoTime(); while (!gameover) { timenow = System.nanoTime(); elapsedtime = timenow lastupdatetime; lastupdatetime = timenow; handleinput(); update(elapsedtime); render(); 5 6

2 Time Since Last Update Too Long? Yes handleinput() update() render() Gameloop-Managed Timed update Game-loop accounts for time variation No 7 desiredupdateperiod = (1/desiredUPS)* ; lastupdatetime = System.nanoTime(); while (!gameover) { curtime = System.nanoTime(); timesincelastupdate = curtime lastupdatetime; if (timesincelastupdate > desiredupdateperiod) { handleinput(); update(); lastupdatetime = System.nanoTime(); render(); More updates needed & allowed? Yes handleinput() update() render() Fixed logic rate Multiple logic updates between renders as necessary to maintain a fixed logic rate (i.e., to catch up ) No updateperiod = (1/desiredUPS)* ; starttime = System.nanoTime(); while (!gameover) { currenttime = System.nanoTime(); timesincelastupdate = currenttime-starttime; loopcount = 0; while (updateratetoolow() && loopcount<max ) { handleinput(); update(); starttime += updateperiod; timesincelastupdate = currenttime-starttime; loopcount ++ ; render(); boolean updateratetoolow() { if (timesincelastupdate > updateperiod) return true ; else return false ; 8 Interpolated frames desiredupdateperiod = (1/desiredUPS)* ; starttime = System.nanoTime(); while (!gameover) { currenttime = System.nanoTime(); timesincelastupdate = currenttime-starttime; o render() assists in accounting for time variation (e.g. by smoothing animations) More updates? Yes No Multi-threaded Approach Separate threads for Update() and Render() Control handleinput() Yes //...loop here to do multiple updates as before... update() Compute update fraction //check if update loop hit MAX if ( updateratetoolow()) { //yes, loop hit MAX; advance start time to // within period of current time starttime = currenttime desiredupdateperiod; percentwithinupdateperiod = min ( 1.0, ((currenttime starttime)/desiredupdateperiod)); Logic Update loop: - Runs at controlled (platform-independent) speed - Performs world state update, AI calculations, etc. Logic Update Start Threads World Data Render Presentation loop: - Runs as fast as possible - Draws visible world - Makes maximum use of hardware capabilities render(fraction) render(percentwithinupdateperiod); 9 10 Game Engine Timing Support AbstractGame BaseGame... # mainloop() Lighting Real world lights have a frequency spectrum o White light: all (visible) frequencies o Colored light: restricted frequency distribution Simplified model: VariableFrameRateGame # final mainloop() + final setmaxframerate(int) + final getframespersec():float FixedLogicRateGame # final mainloop() + final setdesiredupdaterate(int) + final getupdatespersec():float Light characteristics o Ambient, Diffuse, Specular reflection characteristics o Red, Green, Blue intensities Light type UserGame o Positional, Directional, update() render()

3 The ADS lighting model The ADS lighting model Ambient reflection simulates a low-level illumination that equally affects everything in the scene. Diffuse reflection brightens objects to various degree depending on the light s angle of incidence. Specular reflection conveys the shininess of an object by strategically placing a highlight of appropriate size on the object s surface where light is reflected most directly towards our eyes. specular highlights Using an ADS lighting model requires specifying contributions due to lighting on a pixel s RGBA output value. Factors include: The type of light source, and its ambient, diffuse, and specular characteristics The object s material s ambient, diffuse, and specular characteristics The object s material s specified shininess The angle at which the light hits the object The angle from which the scene is being viewed ambient diffuse Point source o Location, intensity Light Types Positional (or point ) light May include an optional attenuation factor. For example: 1 attenuationfactor = k c +k l d+k q d 2 Directional ( distant ) Spot o Direction, intensity o Location, direction, intensity, coneangle, falloffrate 15 θ Ф Spot light D = spotlight direction V = direction to vertex θ = cutoff angle φ = light off-axis angle intensityfactor = cos exp (φ) Combines elements of positional and directional lights. Iconic since Pixar s animated short Luxo, Jr. in D V φ θ RAGE Light Classes Materials <<interface>> SceneObject <<interface>> Entity <<interface>> ManualObject <<interface>> Light name : String Type : light.type ambient : [r g b a] diffuse : [r g b a] specular : [r g b a] Range : float falloffexponent : float etc. <<interface>> Tessellation <<interface>> SkeletalEntity Models the reflectance characteristics of surfaces. Usually modeled in ADS with four components: Ambient, Diffuse, and Specular Shininess (to determine size of specular highlights) GenericLight 17

4 some common materials OBJ example # File: cube.obj # This file uses OBJ format to define a cube which has a # different material applied to each of its faces. # Define 8 cube vertices v v v v v v v v # Specify the file (library) containing materials mtllib cube.mtl Barradeu, N., #continued OBJ example (cont.) OBJ Material Files (.mtl) # file cube.obj (cont.) # Define a group (g) and material for each face (f) g front usemtl red f g back usemtl blue f g right usemtl green f g top usemtl yellow f g left usemtl magenta f g bottom usemtl cyan f #end of cube.obj 21 #File: cube.mtl #Defines the named materials used in rendering the Cube newmtl red Kd // diffuse reflection coefficients newmtl green Kd newmtl blue Kd newmtl yellow Kd newmtl cyan Kd newmtl magenta Kd More Complex Materials newmtl flatwhite Ka Kd illum 1 newmtl shinyred Ka Kd Ks illum 2 Ns newmtl clearblue Ka Kd illum 1 Tr // material name // coeffs of amb refl // coeffs of diff refl // flat (no Ks) // coeffs of spec refl // specular material // shininess // transparency ADS lighting computations I observed = I ambient + I diffuse + I specular Ambient computation is the simplest: I ambient = Light ambient Material ambient Note that each item has R, G, and B components. So the computations actually are as follows: red I ambient green I ambient blue I ambient red red = Light ambient Material ambient green green = Light ambient Material ambient blue blue = Light ambient Material ambient 23

5 Diffuse computation depends on the angle of incidence between the light and the surface: light source L pixel θ N (Lambert s cosine law [1760] ) Specular computation depends on the angle of reflection of the light on the surface, and the viewing angle of the eye. light source I diffuse = Light diffuse Material diffuse cos(θ) Rightmost term determined simply using dot product: I diffuse = Light diffuse Material diffuse (N L) Only include this term if the surface is exposed to the light: I diffuse = Light diffuse Material diffuse max ( N L, 0) L pixel θ N θ ϕ R V Shininess modeled with a falloff function. Expresses how quickly the specular contribution reduces to zero as the angle ϕ grows. cos(ϕ) cos 2 (ϕ) cos 3 (ϕ) combining light and textures Color = texturecolor * ( ambientlight + diffuselight ) + specularlight or Color = texturecolor * ( ambientlight + diffuselight + specularlight ) or Color = (amblight * ambmaterial) + (difflight * diffmaterial) + speclight fragcolor = 0.5 * texturecolor * lightcolor cos 50 (ϕ) I spec = Light spec Material spec max(0, R V n ) Normal Mapping Example normal maps: The (X,Y,Z) values of the normal can be stored as (R,G,B) values in an image file called a normal map. Converting normal ( ) to RGB (0...1) is easy: R = (N X + 1)/2 G = (N Y + 1)/2 B = (N Z + 1)/2 The normal map can then be stored in a texture! Values in normal maps are usually stored as offsets from vertical relative to the plane tangent to the surface, with the Z (or B) coordinate set to 1.0. This is why normal maps appear bluish. The surface tangent plane is defined by mutually-perpendicular tangent and bi-tangent vectors (both perpendicular to the normal).

6 Moon example: Normal map lighting effects on moon: Sphere with moon texture map: Sphere textured with corresponding normal map: Note the changes in specular highlights at the indicated locations. Combining texture map and lighting with normal map -- effect on moon: Height Mapping Using a texture image to perturb the vertex locations. Use a grayscale image, where: White == high Black == low Small variation in height Large variation in height

7 Example: height map Edge-on view Height map image (height interpretation) standard texture Result when applied to a 100x100 rectangular grid height map Texture-Mapped Fonts standard texture Create a texture map image containing font characters: texture map each desired character to a single polygon Build output image strings from texture-mapped polygons 1,1 normal map Result when applied to a sphere 0,0 40 (1,1) Texture-Mapped Fonts (cont.) Y (100,100) t (0,0) s Texture Space Texture Space x4, y4 x3, y3 s4, t4 s3, t3 H x1, y1 x2, y2 s1, t1 s2, t2 e l l o (100,100 ) Drawbacks: Requires an external application to create the font texture Scaling can affect font appearance Difficult to implement proportionally-spaced fonts Uses up a texture unit World Window (0,0) QUADS see the Wikibook Modern OpenGL Tutorial Text Rendering

8 Basic approach: Billboards o Replace a complex 3D model with a 2D image o Maintain image orientation toward camera Works well when 3D model World background o Model is sufficiently far away o Model tends to have axial symmetry 2D image approximation Transparent background Basic steps: o Create flat object with 2D texture-mapped image o Translate object to tree location in world o Rotate object so that it points to the camera θ Y U X N Camera X = {1,0,0 N = Camera Loc Y = V = {0,1,0 U = V N Θ = X U Atmospheric Effects - Fog A useful effect: blend pixel color with another color (e.g. gray) based on distance from eye. Fog is not just for simulating fog, but also for enhancing the sense of 3D depth for the viewer. Fog (simple approach) Pixel color based (mostly) on object Fog color (contribution varies with distance from eye) There are simple models, as well as sophisticated models that include light scattering effects. Pixel color based (mostly) on fog color a more comprehensive model: Extinction rate of decay from image color to black Inscattering rate of change from image color to fog color f e = e -zx f i = e -zs z = distance from eye to object, x = extinction coefficient, s = inscattering coefficient 47 48

9 Atmospheric Effects Environment Mapping OpenGL cubemap = a single texture object with six 2D faces Texture coords (s,t,r) = vector from the cube center +Y +Z +Y Texel Environment Mapping Useful technique for rendering mirror objects Create texture cube map describing the environment Shade object points by following reflection of eye vector (to surface normal) into the map Environment +X -X +X -Z +Z Vector (s,t,r) Shiny Object Environment Map -Y using OpenGL cubemaps makes environment mapping easier Eye Texel used for shade Point to be shaded D Textures 2D Texture Images are painted on and sometimes they look like it Desired: make it look like an object is made out of some material 51 Image source: ACM SIGGRAPH Education Slide Set, R. Wolfe, DePaul Univ. 52 3D Texture Example: Wood-grain Wood grain is caused by tree rings wood-grain example (cont.) Computing cylindrical location for (x,y,z) object point Y Top View Concentric growth ring cylinders Top View R (x,y,z) z x X Object to be carved from wood Object point (x,y,z) Needed: a function f(x,y,z) mapping an object location into the wood space i.e. obtaining the wood color Z 2 2 R x z tan z 1 x H y Z Y X 53 54

10 wood-grain sample results Variations in ring density Perlin Noise Ken Perlin (NYU), 1985 o Classic Noise (1985) Gradient noise using cubic Hermite interpolation o Improved Noise (2001) Gradient noise using quintic interpolation and precomputed gradients o Simplex Noise (2005) Hardware implementation Image source: ACM SIGGRAPH Education Slide Set, R. Wolfe, DePaul Univ. Variations in Ring Width Computer Graphics V19 No.3, July 1989 AMPAS Oscar, Randomness is important in many 3D textures: o Natural materials (wood, marble, granite ) o Man-made materials (stucco, asphalt, cement ) o Natural phenomena (clouds, fire, smoke, wind effects ) o Model imperfections (rust, dirt, smudges, dents ) o Pattern and motion imperfections (bumps, wobbles, jitters ) 57 Image source: ACM SIGGRAPH Education Slide Set, R. Wolfe, DePaul Univ. 58 Example Jade Atmospheric Effects - Clouds Clouds are complex: scatter and reflect light in diverse ways drift and morph over time There are simple models, as well as sophisticated models that include light scattering effects. Simple models often use 3D textures and Perlin noise

11 Atmospheric Effects - Clouds Mixing 3D noise at different levels of precision: Atmospheric Effects - Clouds 3D texture allows efficient morphing by slowly changing the Z parameter when accessing texture map: Immediate-Mode Graphics The PROGRAMMABLE Graphics Pipeline CPU transfers RAM data over main bus o Time consuming o Bus Contention glbegin(gl_triangles); glcolor3f (1.0, 0.0, 0.0); glvertex3f(0.0, 0.0, 0.0); glcolor3f (0.0, 1.0, 0.0); glvertex3f(1.0, 0.0, 0.0); glcolor3f (0.0, 0.0, 1.0); glvertex3f(1.0, 1.0, 0.0); glend(); CPU GPU RAM Video Memory State Variables Application Program Shader Source Code Vertex Processor Vertex Program Compiler Linker Assembly Projection Clipping/Culling Persp. Divide MapToViewport Graphics card Rasterizer Fragment Processor Fragment Program Fragment Testing (HSR, etc.) Frame Buffer Programmable Shaders: Vertex shader Tessellation Control Tessellation Evaluation Tessellator (grid) Hardware ( Shader ) Programming High-level languages: o HLSL ( High-Level Shading Language ) Proprietary (Microsoft) Powerful Specific to DirectX o Cg ( C for graphics ) Proprietary (nvidia) Supports both DirectX and OpenGL APIs (more complex) Fragment Shader Geometry Shader o GLSL ( OpenGL Shading Language ) Open standard Compiles to all common vendor chips Can run on top of DirectX, or directly on hardware 65 66

12 Vertex array pointer Texture Coord pointer Vertex Arrays 67 x y z x y z x y z x y z x y z... Vertex Texture Coordinates s t s t s t s t s t... Steps to use Vertex Arrays: Vertex Geometry Array Enable use of each array type (geometry, tex, etc.) Point to each array to be used Load each array with data (geometry, tex, etc.) Specify what to draw, and from where, in the arrays Index (pointer) array Vertex array Color array Indexed Vertex Arrays Face 0 Face 1... x y z x y z x y z x y z x y z x y z... s t s t s t s t s t s t Vertex Buffer Objects (VBOs) Map an application buffer directly to highspeed video memory (such as DRAM) Buffers can include vertex arrays Methods like gldrawarrays() operate on video memory data see CSc