TentamensKod: (Ifylles av student) Tentamensdatum: Tid: 10:00 12:00. Hjälpmedel: Inga hjälpmedel

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1 Computer Graphics Provmoment: Ladokkod: Tentamen ges för: Teoretisk tentamen 21DG2B Systemarkitektprogrammet 7,5 högskolepoäng TentamensKod: (Ifylles av student) Tentamensdatum: Tid: 10:00 12:00 Hjälpmedel: Inga hjälpmedel Totalt antal poäng på tentamen: 10 poäng För att få respektive betyg krävs: E >= 5, D >= 6, C >= 7, B >=8, A >= 9 Allmänna anvisningar: Skriv dina svar på separata blad Rättningstiden är i normalfall 15 arbetsdagar, till detta tillkommer upp till 5 arbetsdagar för administration, annars är det detta datum som gäller: Viktigt! Glöm inte att skriva namn på alla blad du lämnar in. Lycka till! Ansvarig lärare: Rikard König Telefonnummer:

2 1. The picture below describes the architecture of the Open-GL graphics pipeline. Briefly explain what happens in each step. (2p) 2. The Cohen-Sutherland Algorithm tries to eliminate as many cases as possible without computing intersections when clipping. Describe how the algorithm works and use line C-D in the picture bellow as an example. (1.5p) 3. The painter s algorithm is an algorithm used for visibility testing. Describe how the painter s algorithm works, and provide at least two examples that would cause the painter's algorithm to fail. Briefly explain how the examples you provided could be preprocessed so that they can be solved using the painter s algorithm. (2p) 4. The Z-buffer algorithm is another visibility testing algorithm; explain how it works. How does the Z-buffer algorithm solve the two difficult examples you provided above? 2

3 6. Consider the Blinn-Phong shading equation at a fragment with normal n and the material parameters cd, cs (colors) and m (shininess). The direction towards the directional light source is l and the direction to the viewer/camera is v. The lighting environment is described by the ambient light intensity Ia and the diffuse Id and specular Is light intensity of one directional light source. l n v The following HLSL effect file implements the Blinn-Phong shading equation: uniform const float4x4 World; uniform const float4x4 View; uniform const float4x4 Projection; uniform const float3 EyePosition; // Eye position in the world frame. // Material information from the application uniform const float4 DiffuseColor; uniform const float3 SpecularColor; uniform const int SpecularPower; uniform const texture DiffuseTexture; sampler DiffuseTextureSampler = sampler_state Texture = (DiffuseTexture); ; uniform const texture NormalMapTexture; sampler NormalMapTextureSampler = sampler_state Texture = (NormalMapTexture); ; // Lighting information from the application in the world frame uniform const float3 AmbientLightIntensity; uniform const float3 DirectionalLightDirection; uniform const float3 DirectionalLightDiffuseIntensity; uniform const float3 DirectionalLightSpecularIntensity; // Fog settings uniform const float FogEnabled; uniform const float FogStart; uniform const float FogEnd; uniform const float3 FogColor; struct VertexShaderInput float4 Position float3 Normal float3 Tangent float2 TexCoord ; : POSITION0; : NORMAL0; : TANGENT0; : TEXCOORD0; struct VertexShaderOutput float4 Position : POSITION0; float2 TexCoord : TEXCOORD0; float3 Normal : TEXCOORD1; float3 Tangent : TEXCOORD2; float3 Pos : TEXCOORD3; ; 3

4 VertexShaderOutput VertexShaderFunction(VertexShaderInput input) VertexShaderOutput output; float4 wp = mul(input.position, World); float4 vp = mul(wp, View); output.position = mul(vp, Projection); output.normal = mul(input.normal, World); output.tangent = mul(input.tangent, World); output.pos.xyz = wp.xyz; output.texcoord = input.texcoord; return output; float4 PixelShaderFunction(VertexShaderOutput input) : COLOR0 float4 diffusecolor = DiffuseColor * tex2d(diffusetexturesampler, input.texcoord); float4 ambient = 0.0; ambient.rgb = diffusecolor.rgb * AmbientLightIntensity; float3 Nb = normalize(input.normal); float3 Tb = normalize(input.tangent); float3 Bb = cross(nb, Tb); float3 Nt = 2.0*tex2D(NormalMapTextureSampler, input.texcoord) - 1.0; float3 N = normalize(nt.x*tb + Nt.y*Bb + Nt.z*Nb); float3 L = -DirectionalLightDirection; float3 E = normalize(eyeposition.xyz - input.pos.xyz); float3 H = normalize(l + E); float3 diffuseintensity = DirectionalLightDiffuseIntensity; float Kd = saturate(dot(l, N)); float4 diffuse = float4(kd * diffusecolor.rgb * diffuseintensity, diffusecolor.a); float3 specularintensity = DirectionalLightSpecularIntensity; float f = 1.0; if (Kd == 0.0) f = 0.0; float4 specular = float4(f * specularintensity * pow(saturate(dot(n, H)), SpecularPower) * SpecularColor, 0); float4 color = saturate(ambient + diffuse + specular); float fogfactor = FogEnabled * saturate((length(eyeposition - input.worldposition) FogStart) / (FogEnd - FogStart)); color.rgb = lerp(color.rgb, FogColor, fogfactor); return color; technique Technique1 pass Pass1 VertexShader = compile vs_3_0 VertexShaderFunction(); PixelShader = compile ps_3_0 PixelShaderFunction(); 4

5 Consider the HLSL effect above. Explain: (a) In which coordinate system/frame of reference it computes the directions (n, v, l) used by the Blinn-Phong shading equation and in which variables these are stored for the Blinn-Phong shading computation. (b) How the application communicates the material diffuse color, cd, for each fragment of a particular mesh to an instance of the effect, (i.e. list and explain all uniforms and/or vertex buffer channels used for this purpose). (c) In which coordinate system/frame of reference the supplied normal map is expected to be in and what color (by name or color values) a no-op normal map that just passes through the original normal would contain. 7. A 3d scene containing translucent surfaces is more prone to undesired rendering artifacts that scenes with only opaque surfaces. (a) Describe at least two types of rendering artifacts that are likely to occur if the objects of the scene (including semi-translucent surfaces) are rendered, as usual, in an arbitrary order but with alpha-blending on and why they occur. (b) Describe a solution (scene rendering process and/or shader modifications) that improves the situation. Does it solve all rendering artifacts concerning translucent surfaces? Which are solved and which (if any) remain unsolved? What are the additional costs compared to the standard plain approach described in (a)? 5