Beeldverwerking Prof. dr. ir. W. Philips Didactisch materiaal bij de cursus Academiejaar 2011-2012

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1 Beeldverwerking Prof. dr. ir. W. Philips Didactisch materiaal bij de cursus Academiejaar 2011-2012 philips@telin.UGent.be http://telin.UGent.be/~philips/beeldv/ Tel: 09/264.33.85 Fax: 09/264.42.95

2 © W. Philips, Universiteit Gent, 1999-2012versie: 26/10/2011 05a. 2 Copyright notice This powerpoint presentation was developed as an educational aid to the renewed course “Image processing” (Beeldverwerking), taught at the University of Gent, Belgium as of 1998. This presentation may be used, modified and copied free of charge for non-commercial purposes by individuals and non-for-profit organisations and distributed free of charge by individuals and non-for-profit organisations to individuals and non-for-profit organisations, either in electronic form on a physical storage medium such as a CD-rom, provided that the following conditions are observed: 1.If you use this presentation as a whole or in part either in original or modified form, you should include the copyright notice “© W. Philips, Universiteit Gent, 1998- 2002” in a font size of at least 10 point on each slide; 2.You should include this slide (with the copyright conditions) once in each document (by which is meant either a computer file or a reproduction derived from such a file); 3. If you modify the presentation, you should clearly state so in the presentation; 4.You may not charge a fee for presenting or distributing the presentation, except to cover your costs pertaining to distribution. In other words, you or your organisation should not intend to make or make a profit from the activity for which you use or distribute the presentation; 5. You may not distribute the presentations electronically through a network (e.g., an HTTP or FTP server) without express permission by the author. In case the presentation is modified these requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the presentation, and can be reasonably considered independent and separate works in themselves, then these requirements do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the presentation, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it. In particular note that condition 4 also applies to the modified work (i.e., you may not charge for it). “Using and distributing the presentation” means using it for any purpose, including but not limited to viewing it, presenting it to an audience in a lecture, distributing it to students or employees for self-teaching purposes,... Use, modification, copying and distribution for commercial purposes or by commercial organisations is not covered by this licence and is not permitted without the author’s consent. A fee may be charged for such use. Disclaimer: Note that no warrantee is offered, neither for the correctness of the contents of this presentation, nor to the safety of its use. Electronic documents such as this one are inherently unsafe because they may become infected by macro viruses. The programs used to view and modify this software are also inherently unsafe and may contain bugs that might corrupt the data or the operating system on your computer. If you use this presentation, I would appreciate being notified of this by email. I would also like to be informed of any errors or omissions that you discover. Finally, if you have developed similar presentations I would be grateful if you allow me to use these in my course lectures. Prof. dr. ir. W. PhilipsE-mail: philips@telin.UGent.be Department of Telecommunications and Information ProcessingFax: 32-9-264.42.95 University of GentTel: 32-9-264.33.85 St.-Pietersnieuwstraat 41, B9000 Gent, Belgium

3 Spatiale en temporele aspecten beeldopname en weergave Herhaling

4 © W. Philips, Universiteit Gent, 1999-2012versie: 26/10/2011 05a. 4 Een pixelsensor meet de beeldintensiteit in de omgeving van ( x k,y l ) Model voor een camera Camera: CCD (Charged- coupled device) pixelmatrix x, k y, l gewichtsfunctie, b.v. w ( x,y )=1 voor | x |<  en | y |<  en 0 daarbuiten Beeld 1Optisch systeem b i ( x,y ) Opmerking: h ( x,y ) b 0 ( x,y )  wiskundig model: lineair filter, gevolgd door “ideale” bemonstering

5 © W. Philips, Universiteit Gent, 1999-2012versie: 26/10/2011 05a. 5 Bemonsterde beelden… Voordeel: functie van x en y, zoals origineel beeld b ( r ) i.p.v. reeks getallen Definitie: “ideaal” bemonsterd beeld als distributie met eenvoudig verband met niet-bemonsterd beeld: Notaties (algemene bemonstering): Let op: b ( r )=( b i * h * w )( r ) is het beeld na in rekening brengen van lenskarakteristiek: impulsrespons h (r) pixelvorm: w (r)

6 © W. Philips, Universiteit Gent, 1999-2012versie: 26/10/2011 05a. 6 replica’s Bemonstering zonder aliasing in 2D Praktische beelden hebben een bandbeperkt spectrum (o.a. door verwaziging in de lens en uitmiddeling over camerapixels fxfx fyfy drager origineel spectrum (frequenties waar B ( f )≠0) Reciprook van rechthoekig rooster  x  y  x  y Indien de punten van het reciprook rooster voldoende ver uit elkaar liggen zal er geen aliasing optreden  punten van bemonsteringsrooster liggen voldoende dicht bij elkaar fxfx fyfy Reciprook van hexagonaal rooster

7 © W. Philips, Universiteit Gent, 1999-2012versie: 26/10/2011 05a. 7 Theoretische reconstructie… Besluit: reconstructie is mogelijk als replica’s niet overlappen, n.l. door B s ( f ) te vermenigvuldigen met goed gekozen R ( f ) (  lineair filter) fxfx fyfy drager origineel spectrum (frequenties waar B ( f ) ≠0) Reciprook rooster  x  y drager bemonsterd spectrum (frequenties waar B s ( f ) ≠0) fxfx fyfy R ( f ) =1 R ( f ) ≠0 R ( f )=0 Filterkarakteristiek R ( f ) R ( f ) = 1 waar B ( f ) ≠0 R ( f ) = 0 waar B ( f-Uk ) ≠0

8 © W. Philips, Universiteit Gent, 1999-2012versie: 26/10/2011 05a. 8  modelleren als lineair filter met impulsrespons h ( r ) en frequentiekarakteristiek H ( f ) Beeldreproductiesystemen bevatten soms lenzen (b.v. projector) Het oog filtert ook een deel van het frequentie-spectrum weg Praktische reconstructie Weergave met eindige pixels monster- waarden b k spectrum: cB s ( f ) W ( f ) w ( r ) =w ( x,y ) = 1 voor | x |<  en | y |<  en 0 daarbuiten Scherm- en/of oog-optica spectrum: cB s ( f ) W ( f ) H ( f ) Besluit: cW ( f ) H ( f ) moet dienen als benadering voor R ( f )  weinig keuze voor R ( f )  men moet overblijvende imperfecties aanvaarden grijswaarde= b k