# We do not draw polygons facing the other direction

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Unformatted text preview: vertices are mapped to pixels ○ Painter’s algorithm, BSP trees, portal culling ● Image space techniques: applied while the vertices are rasterized ○ Z-buffering * Back Face Culling. •We do not draw polygons facing the other direction •Test z component of surface normals. If negative – cull, since normal points away from viewer. •Or if N.V &gt; 0 we are viewing the back face so polygon is obscured. * Summary for Hidden Surface Removal ● Z-buffer is easy to implement on hardware and is a standard technique for hidden surface removal ● We need to combine it with an object-based method especially when there are too many polygons BSP trees, portal culling ● Need to do the front-to-back traversal to reduce the cost Overview Hidden Surface Removal ●Painter’s algorithm ●Z-buffer ●BSP tree ●Portal culling ●Back face culling ●Transparency ●Alpha blending ●Screen door transparency * Transparency Sometimes we want to render transparent objects We blend the colour of the objects along the same ray •Alpha blending •Screen door transparency * Alpha Blending Another variable called alpha is defined here This describes the opacity Alpha = 1.0 means fully opaque Alpha = 0.0 means fully transparent α=1 α = 0.5 α = 0.2 * Sorting by the depth First, you need to save the depth and colour of all the fragments that will be projected onto the same pixel in a list Then blend the colour from back towards the front * Colour Blending The colours of overlapping fragments are blended as follows: Co = α Cs + (1-α) Cd Cs : colour of the transparent object, Cd is the pixel colour before blending, Co is the new colour as a result of blending Co becomes Cd for the next round Sorting is expensive ● Need to use BSP Tree ○ Soring per-pixel is very expensive ● Any faster solution ? –Screen-door transparency Screen-door Transparency ● The object is solid bu...
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