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15.1 Si23_03 SI23 Introduction to Computer Graphics Lecture 15 – Visible Surfaces and Shadows

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15.2 Si23_03 The Story So Far for 3D Graphics n We now understand: – how to model objects as a set of polygonal facets and create a 3D world (Lectures 10 & 11) – how to view these worlds with a camera model, projecting the facets to 2D (Lectures 11&12) – how to calculate reflection off a surface (Lecture 13) – how to shade a single projected facet using the reflection calculation (Lecture 14) n Next step: rendering the visible facets in a scene

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15.3 Si23_03 First a Word On Phong Shading and OpenGL n OpenGL supports two types of shading: glShadeModel(rendering Type) GL_FLAT: flat shading GL_SMOOTH: Gouraud shading n Why not Phong? n Phong needs normals passed through the pipeline to the screen space n OpenGL shades vertices in viewing co-ordinates and then discards normals from the pipeline.. So Phong shading impossible

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15.4 Si23_03 Rendering Polygons n We are now ready to consider rendering a set of polygon facets visible n For efficiency, we only want to render those that are visible to the camera

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15.5 Si23_03 Back Face Culling polyhedron back-facing n If the facets belong to a solid object (a polyhedron) we do not need to render back-facing polygons Here only three facets need to be drawn - those that face towards the camera

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15.6 Si23_03 Back Face Culling n A polygon faces away from the viewer if the angle between the surface normal (N) and the viewing direction (V) is less than 90 degrees V.N > 0 camera V N

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15.7 Si23_03 Back Face Culling n It is efficient to carry this out in the viewing co-ordinate system – camera on z-axis pointing in negative z- direction – so V = (0,0,-1) n Thus the V.N>0 test becomes a test only on z-component of normal vector N z < 0 – ie test if z-component of normal points in negative z-direction

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15.8 Si23_03 Back Face Culling n Back face culling is an extremely important efficiency gain in rendering and is typically the first step in visibility processing glEnable(GL_CULL_FACE) glCullFace(GL_BACK) n We are left with a set of front facing polygons...

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15.9 Si23_03 The Next Problem visible n Some facets will be obscured by others - we only want to draw (ie shade) the visible polygons

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15.10 Si23_03 Solution - Z Buffer Algorithm n Suppose polygons have been passed through the projection transformation, with the z-co-ordinate retained (ie the depth information) - suppose z normalized to range 0 to 1 z x y view plane window For each pixel (x,y), we want to draw the polygon nearest the camera, ie largest z 0 1 camera

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15.11 Si23_03 Z Buffer Algorithm n We require two buffers: – frame buffer – frame buffer to hold colour of each pixel (in terms of RGB)... typically 24 bits plus 8 bits for transparency (A or ) – z-buffer – z-buffer to hold depth information for each pixel... typically 16, 24 or 32 bits n Initialize: – frame buffer to the background colour of the scene colour (x,y) = (I RED, I GREEN, I BLUE ) background – z-buffer to zero (back clipping plane) depth (x,y) = 0

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15.12 Si23_03 Z Buffer Algorithm n As each polygon is scan converted and shaded using Gouraud or Phong shading: – calculate depth z for each pixel (x,y) in polygon – if z > depth(x,y), then set: depth (x,y) = z; colour (x,y) = (I RED, I GREEN, I BLUE ) gouraud/phong n Opportunity to exploit coherence here… how? n After all polygons processed, depth buffer contains depth of visible surfaces, and frame buffer the colour of these surfaces

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15.13 Si23_03 Z Buffer - Strengths and Weaknesses n A major advantage of the z-buffer algorithm is its simplicity n Weaknesses are: – Amount of memory required (now of decreasing importance) – Some calculations are wasted – why? – Limited precision for depth calculations in complex scenes can be a problem: z- fighting (why is this exacerbated by perspective transformation?) n In OpenGL: glEnable(GL_DEPTH_TEST) glClear(GL_DEPTH_BUFFER_BIT)

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15.14 Si23_03.. And this years Oscar winner is n The z-buffer was invented by Ed Catmull in 1973 while a student… n … received Oscar in 2001 for Toy Story 2 working for Pixar n … but if only he had patented the z- buffer idea!

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15.15 Si23_03 Transparency n Polygons in practice may be opaque or semi-transparent – in OpenGL =1 represents opaque n Simple rendering: – render opaque polygons first, generating colour (x,y) – for each semi-transparent polygon (with opacity ) render into another buffer as polygon (x,y) – and combine using: ( 1 - ) * colour (x,y) + * polygon (x,y)

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15.16 Si23_03 Better Transparency n Better results by storing for each pixel the depth and transparency of each surface n Surfaces can then be composited back to front in order to give more accurate images

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15.17 Si23_03 Shadows n Z buffers also give us a nice way of doing shadows n The z buffer is a way of determining what is visible to the camera n For shadows, we need a way of determining what is visible to the light source

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15.18 Si23_03 Shadow Z Buffer n We require a second z-buffer, called a shadow z-buffer n Two step algorithm: – scene is rendered from the light source as viewpoint, with depth information stored in the shadow z-buffer (no need to calculate intensities) – scene is rendered from the camera position, using Gouraud or Phong shading with a z-buffer algorithm... but we need to adjust colour if point is in shadow

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15.19 Si23_03 Shadow Z Buffer n To determine if point is in shadow: – take its position (x O, y O, z O ) in the camera view, and transform it into the corresponding position (x O, y O, z O ) in the light source view – look up the z value, say z L, in the shadow z-buffer at the position (x O, y O ) – if z L is closer to the light than z O, this means some object is nearer the light and therefore the point is in shadow... in this case only the ambient reflection would be shown at that point

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