White Paper Grayscale resolution: How much is enough? What’s inside? • Look-Up tables explained • What defines image quality • What defines Paul Matthijs DICOM precision General Manager Medical Imaging Systems • Facts and figures Barco for existing LUT [email protected] topologies BarcoView Th. Sevenslaan 106 B-8500 Kortrijk, Belgium www.barcomedi cal.com Barco white paper: Grayscale resolution - Company Confidential Is 8 to 12 bits better than 10 to 10 bits? Is the highest number of bits a guarantee for best image quality? This white paper starts with the basic concepts defining resolution and compares the image quality performance of each topology. Copyright © 2003 BARCO n.v., Kortrijk, Belgium All rights reserved. No part of this publication may be reproduced in any form or by any means without written permission from Barco. Paul Matthijs Final Version 26/11/2003, Page 2 www.barcomedi cal.com Barco white paper: Grayscale resolution - Company Confidential Table of contents 1. TERMINOLOGY: WHAT’S IN A NAME?........................................................5 1.1. Look–up tables explained ......................................................................5 1.2. LUT characteristics translated to image quality parameters ........................7 1.3. The DICOM Grayscale Standard Display Function .................................... 10 1.4. What defines image quality? ................................................................ 12 2. DISPLAY GRAYSCALE RESOLUTION AND DICOM PRECISION ...................... 13 2.1. How LUT topologies define DICOM precision........................................... 13 2.2. Defining DICOM precision: the theoretical approach ................................ 14 2.3. DICOM precision for various LUT topologies ........................................... 16 2.4. Summary about DICOM precision ......................................................... 19 3. CONTRAST RESOLUTION DEFINING IMAGE QUALITY ................................ 22 3.1. Contrast resolution explained............................................................... 22 3.2. Contrast resolution for various LUT topologies ........................................ 24 3.3. Summary about contrast resolution ...................................................... 27 4. MACH BANDING .................................................................................. 29 5. CONCLUSIONS .................................................................................... 32 Acknowledgement I hereby thank Tom Tuytschaever for his support and advanced Excel skills that were key to the completion of this white paper. Paul Matthijs Final Version 26/11/2003, Page 3 www.barcomedi cal.com Barco white paper: Grayscale resolution - Company Confidential List of acronyms Acronym Definition 8 to 8 bit LUT LUT having 8 bits wide address (= LUT input) and 8 bits wide data (= LUT output) CRT Cathode Ray Tube DDL Digital Driving Level GSDF DICOM Grayscale Standard Display Function JND Just Noticeable Difference: the luminance difference that the average human observer can just perceive at a specified luminance level and viewing conditions LCD Liquid Crystal Displays LUT Look-Up Table Paul Matthijs Final Version 26/11/2003, Page 4 www.barcomedi cal.com Barco white paper: Grayscale resolution - Company Confidential 1. TERMINOLOGY: WHAT’S IN A NAME? We all heard it before: Look-up tables, 8 to 10 bits, 1024 simultaneous shades of gray. In the jargon and figures describing grayscale resolution, it is easy to get lost. One may assume that the higher the numbers are, the better the image quality will be. But is this really so? The answer to this question is more subtle. In fact, the highest numbers do not always yield the best image quality. The purpose of this white paper is to make technically as well as non-technically trained people understand what the figures and specifications mean and what to watch for when specifying display systems for medical imaging applications. 1.1 Look-up tables explained A good understanding of the concept and technical capabilities of Look-up tables1 (LUT) is essential for an in depth discussion of grayscale resolution. Look-up tables come in a large variety and are a key component defining what the final grayscale or color capabilities of a display system will be. A LUT is a piece of memory inserted between the video memory, located on the graphic board or display controller, and the display. As every memory, the LUT has address lines defining which memory location we want to address. The address lines is called the input to the LUT and is connected to the video memory. Thus, the pixel content stored in the video memory is defining where to point in the memory block of the LUT. The output of the LUT is formed by the contents of the memory cell itself, in general terms called data. The output of a LUT is connected to the display input. In analog systems, the digital output of the LUT is first converted to an analog value before feeding it to an analog display. In case of digital displays - LCD2 displays are typically driven using digital signals – the digital LUT output is fed directly to the display. The situation of a digital display is illustrated in Fig. 1 below. 1 LUT: Look-Up Table 2 LCD: Liquid Crystal Displays Paul Matthijs Final Version 26/11/2003, Page 5 www.barcomedi cal.com Barco white paper: Grayscale resolution - Company Confidential LUT (8 to 8 bit example) DIGITAL DISPLAY 0 b7........b2.b1.b0 1 b7........b2.b1.b0 2 b7........b2.b1.b0 Pixel 3 b7........b2.b1.b0 Pixel Input Value Output Value 4 b7........b2.b1.b0 VIDEO MEMORY 8 bits wide 8 bits wide From video to memory display 255 b7........b2.b1.b0 GRAPHICS ADDRESS DATA PROCESSOR Display Controller located in workstation Host System PCI BUS Fig. 1 Situating the LUT in the display system In this particular case, the video memory is 8 bits wide, meaning that every video memory location contains pixel values of 8 bits wide. The data output of the video memory is connected to the address lines of the LUT, that in its turn, has 8 bits wide memory locations. The data output of the LUT is then connected to the digital display. This scheme is reproduced for every color Red (R), Green (G) and Blue (B) in case of a color display system. In case of a grayscale display system, the scheme is only reproduced once describing the intensity values of the image. The LUT in the case of Fig. 1 is said to be an 8 to 8 bit LUT3 since both input and output are 8 bits wide. Other options for LUT input and output widths will be discussed further in this paper. It should be clear that the LUT scheme does nothing else than a transformation of the pixel values into other pixel values, as defined by the content of the LUT. This transformation capability of a LUT is mostly used to define an arbitrary conversion from pixel values as stored in the video memory – sometimes called p-values - into display values to be fed to the display. This conversion may be required to compensate for the physical behavior of the display converting digital values into luminance values. This is illustrated in Fig. 2 below. The LUT content can be changed on the fly, without interrupting the image to the display. Thanks to that possibility, LUTs are also intensively used to change the image representation dynamically, e.g. enhance contrast, or apply window and leveling functions in medical applications. Yet another application for LUTS is situated in color display applications to change the color palette available to the application. 3 8 to 8 bit LUT: LUT having 8 bits wide address (= LUT input) and 8 bits wide data (= LUT output) Paul Matthijs Final Version 26/11/2003, Page 6 www.barcomedi cal.com Barco white paper: Grayscale resolution - Company Confidential LUT Output 255 Display Response Curve LUT Response Curve LUT Input 0 255 Fig. 2 LUT response compensating display transfer curve to obtain linear relationship 1.2 LUT Characteristics translated to image quality parameters The physical characteristics have a dramatic impact on the color and grayscale capabilities of the display system. The input as well as the output resolution of LUTs are each related to different image properties. We refer to Fig. 1 above and Fig. 3 below. Paul Matthijs Final Version 26/11/2003, Page 7 www.barcomedi cal.com Barco white paper: Grayscale resolution - Company Confidential 8/10 bit LUT 10/10 bit LUT 0 b9........b2.b1.b0 0 b9........b2.b1.b0 1 b9........b2.b1.b0 1 b9........b2.b1.b0 2 b9........b2.b1.b0 2 b9........b2.b1.b0 Pixel Pixel Pixel 3 b9........b2.b1.b0 Pixel 3 b9........b2.b1.b0 Input Value Output Value Input Value Output Value From 4 b9........b2.b1.b0 From 4 b9........b2.b1.b0 to video video to 8 bits wide 10 bits wide display 10 bits wide 10 bits wide me mor y memor y display 256 discrete values 1024 discrete values available, available, each defined with each defined with 10 bits precision 10 bits precision 255 b9........b2.b1.b0 1023 b9........b2.b1.b0 1 2 1 2 3 3 4 4 256 5 1024 5 6 6 a selection of 256 discrete values a selection of (=all) 1024 discrete values 0 12345 1023 0 1 2 3 4 5 1023 "10 bits precision" "10 bits precision" Fig. 3 LUT physical aspects and their impact on image parameters The LUT in Fig. 1 above was referred to as an 8 to 8 bit LUT. This means that both the input as well as the output resolution of the LUT were 8 bits wide, yielding 256 possibilities at each side. Other set-ups are possible. As is illustrated in the Fig. 3, it is possible to have the LUT output be of different width than the input. To the left of the figure, an 8 to 10 bit LUT is illustrated. The input values fed from the video memory to the LUT input are 8 bits wide. This means that at any given time, there are not more than 256 discrete and simultaneous possibilities to describe a pixel value in the image.