Vision Systems

Image Acquisition
The first stage of any vision system is the image acquisition stage.

After the image has been obtained, various methods of processing can be applied to the image to perform the many different vision tasks required today.

However, if the image has not been acquired satisfactorily then the intended tasks may not be achievable, even with the aid of some form of image enhancement

2D Image Input

The basic two-dimensional image is a monochrome (greyscale) image which has been digitised.

Describe image as a two-dimensional light intensity function f(x,y) where x and y are spatial coordinates and the value of f at any point (x, y) is proportional to the brightness or grey value of the image at that point.

A digitised image is one where

• spatial and greyscale values have been made discrete.
• intensity measured across a regularly spaced grid in x and y directions
• intensities sampled to 8 bits (256 values)

For computational purposes, we may think of a digital image as a two-dimensional array where x and y index an image point. Each element in the array is called a pixel (picture element)

2D Input Devices
TV Camera or Vidicon Tube

A first choice for a two-dimensional image input device may be a television camera -- output is a video signal:

• Image focused onto a photoconductive target.
• Target scanned line by line horizontally by an electron beam
• Electric current produces as the beam passes over target.
• Current proportional to the intensity of light at each point.
• Tap current to give a video signal.

This form of device has several disadvantages.

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PNG - The Definitive Guide

By Greg Roelofs

Part I, Using PNG
Part I is intended for designers, web site owners, casual image creators, and web surfers--anyone who wants a quick start on using PNG images in a variety of applications. Such users may need only a brief overview of PNG features, but they want to know what applications support the format and to what extent, how to invoke PNG-specific features within the applications, and how to work around certain bugs or incompatibilities in the applications. Of course, a book like this cannot possibly stay current, particularly not when it comes to software, but every effort has been made to ensure that the information is accurate as of the day this is written (mid-April 1999).

• Chapter 1, "An Introduction to PNG", covers some basic concepts of computer images and file formats, explains how PNG fits in and where using it is most appropriate (and most inappropriate!), and ends with an in-depth look at an image-editing application with particularly good PNG support.
• Chapter 2, "Applications: WWW Browsers and Servers", looks at PNG support in web browsers and servers and shows how to use the HTML OBJECT tag and server-side content negotiation to serve PNG images to browsers capable of viewing them.
• Chapter 3, "Applications: Image Viewers", lists more than 75 applications capable of viewing PNG images, with support for a dozen operating systems. Viewers that are additionally capable of converting to or from other image formats are so noted.
• Chapter 4, "Applications: Image Editors", looks at PNG support in five of the most popular image editors, showing how to invoke such features as gamma correction and alpha transparency, and indicating some of the problems unwary users may encounter.
• Chapter 5, "Applications: Image Converters", covers five conversion applications in detail, including one specifically designed to optimize PNG images and another designed to test PNG images for conformance to the specification. In addition, the chapter lists another 16 dedicated image converters beyond those in Chapter 3, "Applications: Image Viewers".
• Chapter 6, "Applications: VRML Browsers and Other 3D Apps", looks at PNG as a required texture format of the VRML 97 specification and investigates the level of conformance of seven browsers. It also lists a dozen PNG-supporting applications designed for the editing or rendering of 3D scenes.

Part II, The Design of PNG
Part II looks at the PNG format from an historical and technical perspective, detailing its structure and the rationale behind its design. Part II is intended for more technical readers who want to understand PNG to its core.

• Chapter 7, "History of the Portable Network Graphics Format", looks at the events leading up to the creation of PNG, some of the design decisions that went into the format, how it has fared in the subsequent years, and what to expect for the future.
• Chapter 8, "PNG Basics", covers the basic ``chunk'' structure of PNG files and compares PNG's level of support for various fundamental image types against that of other image formats.
• Chapter 9, "Compression and Filtering", delves into the heart of PNG's compression engine, provides the results of some real-world compression tests, and offers a number of tips for improving compression to both users and programmers of the format.
• Chapter 10, "Gamma Correction and Precision Color", discusses one of the least understood but most important features of PNG, its support for platform-independent image display. That is, in order for an image to appear the same way on different computer systems or even different print media, it is necessary for both the user and the program to understand and support gamma and color correction.
• Chapter 11, "PNG Options and Extensions", details the optional features supported by PNG, including text annotations, timestamps, background colors, and other ancillary information.
• Chapter 12, "Multiple-Image Network Graphics", is a brief look at PNG's multi-image cousin, MNG, which supports animations, slide shows, and even highly efficient storage of some types of single images.

Part III, Programming with PNG
Part III covers three working, libpng-based demo programs in detail, and lists a number of other toolkits that offer PNG support for various programming languages and platforms. It is intended for programmers who wish to add PNG support to their applications.

• Chapter 13, "Reading PNG Images", is a detailed tutorial on how to write a basic PNG-reading display program in C using the official PNG reference library. The application is divided into a generic PNG back end and platform-specific front ends, of which two are provided (for 32-bit Windows and the X Window System).
• Chapter 14, "Reading PNG Images Progressively", inverts the logic of the previous chapter's demo program, simulating the design of a web browser's display-as-you-go PNG code. Progressive display of interlaced, transparent PNG images over a background image is supported.
• Chapter 15, "Writing PNG Images", shows how to create a basic PNG-writing program. The supplied code compiles into a simple command-line program under both Windows and Unix, and it includes support for interlacing, gamma correction, alpha transparency, and text annotations.
• Chapter 16, "Other Libraries and Concluding Remarks", lists a number of alternative libraries and toolkits, both free and commercial, including ones for C, C++, JavaTM, Pascal, tcl/tk, Python, and Visual Basic. The chapter ends with a look back at what parts of the PNG design process worked and what didn't, and also a look forward at what lies ahead.
  The References section lists technical references and resources for further information, both printed and electronic.
• The Glossary defines a number of acronyms and technical terms used throughout the book.

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Compute!'s First Book of Atari Graphics

The Basics Of Atari Graphics

Tom R. Halfhill

If you are new to the Atari and have acquired a bit of familiarity with BASIC, but have not yet taken the plunge into graphics, this article will introduce you to the fundamentals.

For some reason, many people are intimidated by the programming steps required to create computer graphics. Probably this is because creating computer graphics is not as easy as it looks. The typical buyer of a personal computer is dazzled in the store by all the fantastic arcade games and impressive graphics demos with which the sales people are armed. It all looks so simple. Then the buyer eagerly unpacks the computer at home and quickly discovers that even crude pictures cannot be created without screenfuls of cryptic programming that seemingly have more in common with Sanskrit than English.

But there is hope. It's not really that hard--honest. Nobody is promising that you'll be able to duplicate Star Raidersor PacManany time soon, but the basics of computer graphics are quite easy to grasp for anyone who has some knowledge of BASIC programming. You don't need to be a math wizard, either. The most valuable attributes are a willingness to learn and to experiment. And, of course, to be creative.

Choosing A Graphics Mode
Atari graphics are particularly challenging to learn, mainly because the Atari computers have extremely versatile graphics. Luckily, Atari made it easier for us by including many special keywords in Atari BASIC that are dedicated to graphics. The first step, then, is to learn those keywords. And by the way, if you don't already have your Atari BASIC Reference Manualhandy, take a second to grab it. This book and the Manualshould help to explain each other.

The most basic of the keywords is the GRAPHICS command. This tells the computer which graphics mode you want, which in turn determines how the screen will look. The format is GRAPHICS (aexp), where (aexp) is any arithmetic expression that results in a positive integer (in other words, not a negative number or a fraction). For example, GRAPHICS 6 is a valid command which tells the computer you want graphics mode six. GRAPHICS 3 + 3 or GRAPHICS 3*2 would do the same thing.

Depending upon how old your Atari is, the GRAPHICS command gives you access to either nine or twelve different graphics modes. The reason for the difference is that earlier Ataris (generally, those shipped before late 1981) came with a TV controller chip called the CTIA. Later Ataris have a GTIA chip instead. The chips are fully compatible--programs written on CTIA Ataris will run on GTIA machines and vice versa--but the GTIA adds three new graphics modes. Users with CTIA chips can have their computers upgraded if they wish. (See "Atari Video Graphics And The New GTIA" in Chapter 6.)

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