# cg13_2013 - Computer Graphics Lecture 13 Hidden Surface...

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± &omputer *raphics /ecture ²³ +idden 6urface 5emoval ´ 7ransparency 7aku .omura

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± 2verview /NnnOS =ZWPKMO <ORT[KQ ł :KNSYOW⁖X KQqTWNYrR ł D±LZPPOW ł 8=: YWOO ł :TWYKQ MZQQNSq ł 8KMp PKMO MZQQNSq ł >WKSXUKWOSM^ ł 7QUrK LQOSnNSq ł =MWOOS nTTW YWKSXUKWOSM^
± :hy +idden 6urface 5emoval" ● \$ correct rendering requires correct visibility calculations ● :hen multiple opaque polygons cover the same screen space² only the closest one is visible ³remove the other hidden surfaces´ 3ainter’s algorithm² =µbuffer wrong visibility correct visibility

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± 2verview /NnnOS =ZWPKMO <ORT[KQ ǎ VJRWwNUłv JuPXURwQ± ǎ C²KxOONU ł 8=: YWOO ł :TWYKQ MZQQNSq ł 8KMp PKMO MZQQNSq ł >WKSXUKWOSM^ ł 7QUrK LQOSnNSq ł =MWOOS nTTW YWKSXUKWOSM^
± 3ainters algorithm ● 'raw surfaces in back to front order – nearer polygons “paint” over farther ones² ● 1eed to decide the order to draw – far objects first

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± 3ainters algorithm ● .ey issue is order determination² ● 'oesn’t always work – see image at right²
± 3ainters algorithm ● \$nother situation it does not work ● ,n both cases² we need to segment the triangles and make them sortable

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± =²buffer ● \$n image²based method applied during the rasterization stage ● \$ standard approach implemented in most graphics libraries ● (asy to be implemented on hardware ● %y :olfgang 6traßer in ³" ´
± =²buffer %asic =²buffer idea# )or every input polygon ● )or every pixel in the polygon interior³ calculate its corresponding z value ´by interpolationµ ● &ompare the depth value with the closest value from a different polygon ´largest zµ so far ● 3aint the pixel with the color of the polygon if it is closer

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± =
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± :hy is =²buffering so popular " \$dvantage •6imple to implement in hardware³ –0emory for z²buffer is now not expensive •'iversity of primitives – not just polygons³ •8nlimited scene complexity •1o need to sort the objects •1o need to calculate object²object intersections³ 'isadvantage •:aste time drawing hidden objects •=²precision errors ´aliasing problemsµ

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=±buffer aliasing
± =²buffer performance •0emory overhead# 2³´µ •7ime to resolve visibility to screen precision# 2³nµ –n# number of polygons –1eed to be combined with other culling methods to reduce complexity

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± 5endering &omplex 6cenes AO NTS Y \KSY YT \KXYO MTRUZYKYnTSKQ WOXTZWMOX WOSNOWnSq UWnRnYn[OX \rnMr NTS Y MTSYWnLZYO YT YrO PnSKQ nRKqO +WK\nSq UTQ^qTSKQ PKMOX TS XMWOOS MTSXZROX 9:? M^MQOX O\$q\$ 3QQZRnSKYnTS
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cg13_2013 - Computer Graphics Lecture 13 Hidden Surface...

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