Shadows and Texture Mapping

Transcription

1 Shadows and Texture Mapping Eric C. McCreath School of Computer Science The Australian National University ACT 0200 Australia

2 Overview 2 Shadows Textures Importance Co ordinates OpenGL Bump Maps MIP maps Cube Mapping

3 Shadows 3 There are many ways of producing shadows within a scene. Either we can augment approaches that don't naturally include shadows or we can move to rendering approach that directly incorporate shadows (ray tracing, radiosity, photon mapping). For real time rendering we wish to avoid having to for every fragment search the scene's data structure for objects between fragments and the light source. One simple way of including shadows is to pre calculate them, using an approach such as ray tracing, and then include them within your scene as fixed textures. This will work when light sources are not moving within the scene. It will clearly not include the shadows cast by moving objects in the scene.

4 Shadows Projecting the objects 4 One simple way of producing shadow effects is to draw the object in 'black' under a transformation that projects the object onto a plane. This only works with shadows on a flat planes. eg wall, or floor, etc. L O N S O L L=P P S. N =0 Solve for mu and then turn into a matrix for homogeneous coordinates. P

5 Shadows Textures 5 Another approach is create textures with the shadow and then add them to floors or walls. This can be done by: Move the viewing position to the light source, make the objects of the scene black, and drawing the scene into a texture. Map this texture to the flat surface you want the shadow to cast onto. Finally redraw the scene from the desired viewing position. Texture L

6 Shadows Shadow Mapping 6 Shadow mapping involves: Calculating a depth map based on the distance from the light's position (in a similar way to calculating the texture in the previous slide). Then as each fragment in the scene is rendered calculate the distance between that fragment and the light source. If this distance is larger than the depth one looks up in the depth map then the fragment is in shadow. Depth map L

7 Shadows Shadow Volumes 7 The shadow volume approach works by: Create a shadow volume. This is done by projecting rays out from the silhouette of the objects that make up the scene thus creating objects which are the shadows. The scene is rendered as if every thing is in shadow. A stencil is created using the shadow volume to determine which parts are in shadow and which are not. The scene is rendered again using the stencil as a mask, this time the full lighting model is used.

8 Textures 8 Textures can be defined by: an array of colours, or procedures which map texture coordinates to colours. Textures may be 1,2, or 3 dimensional. Texture co ordinates range form 0.0 to 1.0. Colour may be: single intensity, RGB, RGBA, etc Textures need to be mapped onto a: line, surface, or volume. Texture Space (s,t) Object Surface Space (u,v) Image Space (x,y) pixel

9 Textures in OpenGL 9 To render using textures you first need to load your texture. gl.glteximage2d(gl.gl_texture_2d, 0, GL.GL_RGB, width, height, 0, GL.GL_RGB, GL.GL_UNSIGNED_BYTE, ByteBuffer.wrap(textureData) ); Level Internal storage Byte array format Once this is done you enable textures: Raw bytes gl.glenable(gl.gl_texture_2d); Draw your surface setting the texture co ordinates at the vertices. Use: gl.gltexcoord2d(0.0, 0.0); Disable textures: gl.gldisable(gl.gl_texture_2d);

10 Some Texture Settings 10 The colour from the texture can either replace any lighting colour or it can be combined(gl_modulate) with the lighting: gl.gltexenvf( GL2.GL_TEXTURE_ENV, GL2.GL_TEXTURE_ENV_MODE, GL2.GL_REPLACE ); // GL_REPLACE, GL_MODULATE If the surface that the texture is rendered to is larger then texture's mapped size then either it can be repeated or clamped. gl.gltexparameteri( GL.GL_TEXTURE_2D, GL.GL_TEXTURE_WRAP_S, GL2.GL_REPEAT); gl.gltexparameteri( GL.GL_TEXTURE_2D, GL.GL_TEXTURE_WRAP_T, GL2.GL_CLAMP ); The texture will generally either need to be magnified and minimised. Thus either linear or nearest interpolation can be used. gl.gltexparameteri( GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MIN_FILTER, GL.GL_NEAREST ); gl.gltexparameteri( GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MAG_FILTER, GL.GL_LINEAR );

11 Bump Mapping 11 Many surfaces are bumpy. Textures can add a lot of detail to the surface, however, textures do not proved a good visual representation of a bumpy surface. This is especially noticeable as the view point or light sources move. So how do you add bumps without having to add lots more vertices in your tessellation? Simple, add a 'texture' which deviates the surface normal. Original surface normal Normalised surface normal N ' =N b v P u n b u n P v New surface normal Bump Map Surface

12 MIP Maps 12 MIP maps can be used to save on computation by pre calculating the texture at different scales. When the surface is rendered the appropriate scale is selected. for (int l=0;l<8;l++) gl.glteximage2d(gl.gl_texture_2d, l, GL.GL_RGB, 64/(1<<l), 64/(1<<l), 0, GL.GL_RGB, GL.GL_UNSIGNED_BYTE, ByteBuffer.wrap(mipmaps[l]) ); gl.gltexparameteri( GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MIN_FILTER, GL.GL_NEAREST_MIPMAP_NEAREST ); Level 0 Level 1 Level 2... Level 7

13 Cube Mapping 13 A cube map is a texture that lines the inside of a box. These texture boxes can be used as a 'skybox' to provide the distance background/sky of a scene. They can also be used form mapping the texture onto the surface of polygons. This can be used to create effects like: reflective surfaces, spectral highlights, bump maps, and some global illumination effects.