Dithering, also called Halftoning or Color Reduction, is the process of rendering an image on a display device with fewer colors than are in the image. Digital Image Processing: Some Special Techniques Dithering presents about it.
Digital Image Processing Some Special Techniques Dithering 21/11/15 Duong Anh Duc - Digital Image Processing Dithering Dithering, also called Halftoning or Color Reduction, is the process of rendering an image on a display device with fewer colors than are in the image (Mateus Pins and Hermann Hild) The number of different colors in an image or on a device is used called its Color Resolution 21/11/15 Duong Anh Duc - Digital Image Processing Dithering If the display device has a higher spatial resolution than the image that you are trying to reproduce, it can show a very good image even if its color resolution is less This is what we will call 'dithering' and is the subject of this work Dithering is a one-way operation Once an image has been dithered, although it may look like a good reproduction of the original, information is permanently lost Many image processing functions fail on dithered images 21/11/15 Duong Anh Duc - Digital Image Processing Dithering 21/11/15 Duong Anh Duc - Digital Image Processing Dithering Grey-scale and colour simulation Dithering on a screen or printer is analogous to the half-toning techniques used in the print industry A CRT can be considered to be a complex colour “dithering” device with variable colour intensity 21/11/15 Duong Anh Duc - Digital Image Processing Dithering Grey-scale and colour simulation We need to display colour and grey-scale images on output devices that have a lower information-carrying capacity Cheap printers are bi-level or CMYK - clearly we need to add colours/intensities to approximate an image 21/11/15 Duong Anh Duc - Digital Image Processing Dithering Methods (Digital Halftoning) Threshold dithering ordered dither stochastic dither dot diffusion 21/11/15 Error diffusion dithering Floyd-Steinberg Burkes Stucki Sierra Jarvis, Judice and Ninke Stevenson and Arce … Duong Anh Duc - Digital Image Processing Dithering in Printing Industry Newspapers black ink on light paper, rasterization of theimages enables also grey levels, equal pointdensity everywhere, variable size Color printing every primary color is rasterized separately,different printing angles ensure unbiased results 21/11/15 Duong Anh Duc - Digital Image Processing Simple shading techniques A series of examples Original picture, half-toning simulation by a non-PostScript laser printer The original image has an 8bit grey scale palette The laser printer has only got a 1-bit palette (ie bi-level, black and white) and must simulate the original shading 21/11/15 Duong Anh Duc - Digital Image Processing Simple shading techniques An example Bayer - Ordered Dithering This method uses a set of regular arrays of values, leading to a regular (and visually poor) output This method creates abrupt changes between areas, changes that not exist on the original Such artefacts are not desirable 21/11/15 Duong Anh Duc - Digital Image Processing 10 Colour Systems The RGB colour cube A system with three independent variables can be represented by a three-dimensional position The RGB colour cube represents all of the colours that an RGB monitor can create, in a non-normalized form 21/11/15 Yellow Green Cyan White (Greys) Black Blue Duong Anh Duc - Digital Image Processing Red Magenta 42 Colour Systems The RGB colour cube A system with three independent variables can be represented byCyan a three-dimensional position The RGB colour cube represents all of the Blue colours that an RGB monitor can create, in a non-normalized form 21/11/15 Green Duong Anh Duc - Digital Image Processing Yellow White Red Magenta 43 Colour Systems The HSV model Hue, Saturation, Value is a more intuitive model “Value” is brightness, constant value hexagons lie parallel to the top surface Grey shades run up the vertical axis, black at the bottom and white at the top 21/11/15 Green 120 Yellow 60 Cyan 180 V=1 Red Magenta 300 Blue 240 V=0 Duong Anh Duc - Digital Image Processing 44 Colour Systems The HLS model The Hue, Lightness, Saturation L=1 model was developed by Tektronix HLS is similar to HSV but with a Green 120 double cone Yellow 180 This and other models are Red 240 combinations of the CIE, RGB and HSV models Translations are always possible 21/11/15 Duong Anh Duc - Digital Image Processing Cyan 60 Blue Magenta 300 L=0 45 References http://www.efg2.com/Lab/Library/ImageProcessin - dither.txt – everything you ever wanted to know about dithering! Computer Graphics, (C version) by D Hearn and P Baker: Section of Chapter 15, Halftone Patterns and Dithering Techniques Chapter 15, Colour Models and Colour Applications 21/11/15 Duong Anh Duc - Digital Image Processing 46 Digital Image Processing Some Special Techniques Thinning (Lọc xương) 21/11/15 Duong Anh Duc - Digital Image Processing 47 Thinning Các pixel ảnh chia làm loại: Pixel Pixel thuộc đối tượng Ở ta quan tâm đến loại sau Các pixel thuộc đối tượng lại chia làm loại: Điểm biên Điểm bên Thinning trình biến đổi để ảnh điểm biên 21/11/15 Duong Anh Duc - Digital Image Processing 48 Thinning Thinning trình loại bỏ pixel phụ (dư thừa) để làm đối tượng trở nên đơn giản hơn, gồm thành phần mảnh, khơng có diện tích Thinning giống với phép co: xóa liên tiếp pixel dư thừa khung xương đối tượng 21/11/15 Duong Anh Duc - Digital Image Processing 49 Thinning Thinning phải thỏa tính chất sau: Đối tượng kết phải mảnh, có độ rộng pixel Các pixel tạo nên khung xương phải định vị gần tâm mặt cắt đối tượng Đảm bảo tính liên thơng giống đối tượng ban đầu 21/11/15 Duong Anh Duc - Digital Image Processing 50 Thuật toán Zhang – Suen Tư tưởng bản: Việc giải có xóa hay khơng xóa pixel phụ thuộc vào pixel lân cận với Bốn quy tắc để định xóa hay khơng xóa pixel: 21/11/15 Duong Anh Duc - Digital Image Processing 51 Quy tắc Pixel p xóa < N8(p) < với N8(p) số lân cận p Điều kiện N8(p) > đảm bảo điểm đầu mút đối tượng khơng bị xóa, khơng bị bào mòn Điều kiện N8(p)