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2 Vice President and Editorial Director, ECS: Marcia J. Horton Executive Editor: Tracy Johnson Assistant Acquisitions Editor, Global Edition: Aditee Agarwal Executive Marketing Manager: Tim Galligan Marketing Assistant: Jon Bryant Senior Managing Editor: Scott Disanno Production Project Manager: Greg Dulles Program Manager: Carole Snyder Project Editor, Global Edition: K.K. Neelakantan Senior Manufacturing Controller, Global Edition: Kay Holman Media Production Manager, Global Edition: Vikram Kumar Global HE Director of Vendor Sourcing and Procurement: Diane Hynes Director of Operations: Nick Sklitsis Operations Specialist: Maura Zaldivar-Garcia Cover Designer: Lumina Datamatics Manager, Rights and Permissions: Rachel Youdelman Associate Project Manager, Rights and Permissions: Timothy Nicholls Full-Service Project Management: Kalpana Arumugam, SPi Global MICROSOFT AND/OR ITS RESPECTIVE SUPPLIERS MAKE NO REPRESENTATIONS ABOUT THE SUITABILITY OF THE INFORMATION CONTAINED IN THE DOCUMENTS AND RELATED GRAPHICS PUBLISHED AS PART OF THE SERVICES FOR ANY PURPOSE. ALL SUCH DOCUMENTS AND RELATED GRAPHICS ARE PROVIDED AS IS WITHOUT WARRANTY OF ANY KIND. MICROSOFT AND/OR ITS RESPECTIVE SUPPLIERS HEREBY DISCLAIM ALL WARRANTIES AND CONDITIONS WITH REGARD TO THIS INFORMATION, INCLUDING ALL WARRANTIES AND CONDITIONS OF MERCHANTABILITY, WHETHER EXPRESS, IMPLIED OR STATUTORY, FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT SHALL MICROSOFT AND/OR ITS RESPECTIVE SUPPLIERS BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF INFORMATION AVAILABLE FROM THE SERVICES. THE DOCUMENTS AND RELATED GRAPHICS CONTAINED HEREIN COULD INCLUDE TECHNICAL INACCURACIES OR TYPOGRAPHICAL ERRORS. CHANGES ARE PERIODICALLY ADDED TO THE INFORMATION HEREIN. MICROSOFT AND/OR ITS RESPECTIVE SUPPLIERS MAY MAKE IMPROVEMENTS AND/OR CHANGES IN THE PRODUCT(S) AND/OR THE PROGRAM(S) DESCRIBED HEREIN AT ANY TIME. PARTIAL SCREEN SHOTS MAY BE VIEWED IN FULL WITHIN THE SOFTWARE VERSION SPECIFIED. Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world Visit us on the World Wide Web at: Pearson Education Limited 2016 The rights of Mark J. Guzdial and Barbara Ericson to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act Authorized adaptation from the United States edition, entitled Introduction to Computing and Programming in Python : A Multimedia Approach, Fourth Edition, ISBN , by Mark J. Guzdial and Barbara Ericson published by Pearson Education All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior written permission of the publisher or a license permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, Saffron House, 6 10 Kirby Street, London EC1N 8TS. All trademarks used herein are the property of their respective owners. The use of any trademark in this text does not vest in the author or publisher any trademark ownership rights in such trademarks, nor does the use of such trademarks imply any affiliation with or endorsement of this book by such owners. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN 10: ISBN 13: Typeset in 10.5/13 Times by SPi Global Printed and bound in Malaysia.

3 Section 5.4 Chromakey 157 FIGURE 5.15 Mark in front of a blue sheet. How It Works Here we take in just a source (with both foreground and background in it) and a new background bg. These must be the same size! Mark used the JES picture tool to come up with a rule for what would be blue for this program. He didn t want to look for equality or even a distance to the color (0, 0, 255) because he knew that very little of the blue would be exactly full-intensity blue. He found that pixels he thought of as blue tended to have smaller red and green values, and in fact, the blue values were greater than the sum of the red and the green. So that s what he looked for in this program. Wherever the blue was greater than the red and green, he swapped in the new background pixel s color instead. The function walks through all of the pixels using the variable px. For each one, the x and y positions are computed. If the blue of that pixel px is greater than the red plus the green, then we get the background pixel at the same x and y. We get the color from that pixel, and set it as the color in the source picture at the pixel px. The effect is really quite striking (Figure 5.16). Do note the folds in the lunar surface, though. You don t really want to do chromakey with a common color like red something that there s a lot of in your face. Mark tried it with the two pictures in Figure 5.17 one with the flash on and the other with the flash off. He changed the test to if getred(p) > (getgreen(p) + getblue(p)):. The one without the flash was terrible the student s face was jungle-ified. The one with the flash was better but the flash is still visible after the swap (Figure 5.18). It s clear why moviemakers and weather people use blue or green backgrounds for chromakey there s less overlap with common colors, like face colors. Getting chromakey to work just right takes some careful photography, e.g. good lighting and good choice of background color. But in an animation program, it is really easy to get all those aspects just right. Barbara generated the image in Figure 5.19

4 158 Chapter 5 Picture Techniques with Selection FIGURE 5.16 Mark on the moon. FIGURE 5.17 Student in front of a red background without the flash (left) and with flash on (right). using the programming language Alice. 1 It s the character Alice in front of a completely green background. (Alice has blue eyes and a blue dress, so green is a better choice for chromakey for her.) We wrote a slightly different chromakey function. This one replaces green pixels with a background color. Program 58: ChromakeyGreen: Replace All Green with the New Background def chromakeygreen(source,bg): for px in getpixels(source): x = getx(px) y = gety(px) if (getred(px) + getblue(px) < getgreen(px)): 1

5 Section 5.4 Chromakey 159 FIGURE 5.18 Using chromakey program with red background, flash off (left) and flash on (right). FIGURE 5.19 The character Alice on a completely green background. bgpx = getpixel(bg,x,y) bgcol = getcolor(bgpx) setcolor(px,bgcol) We ran this one twice. Once, with the image saved from Alice as JPEG and once as PNG. Consider the results in Figure Both clearly show Alice in the jungle, but look at the feet. Why does the PNG Alice have shoes on, while the JPEG Alice does not? JPEG is a lossy format. Some detail is lost in terms of color values when saving a picture as JPEG. PNG is a lossless format. PNG maintains color detail. Our suspicion is that the JPEG version of Alice blended the shoe color with the background color

6 160 Chapter 5 Picture Techniques with Selection FIGURE 5.20 Alice JPEG in the jungle (top) andpng(bottom). enough that the chromakey function got confused. The PNG version kept the black of the shoes distinct, so they were retained after the chromakey function. >>> alice = makepicture("alice.jpg") >>> chromakeygreen(alice,jungle) >>> explore(alice) >>> alice = makepicture("alice.png") >>> chromakeygreen(alice,jungle) >>> explore(alice) The devices and software that implement chromakey professionally use a somewhat different process than this. Our algorithm looks for the color to be replaced, then does the replacement. In professional chromakey, a mask is produced. A mask is of the same size as the original image, where pixels to be changed are white in the mask, and those

7 Section 5.5 Coloring in Ranges 161 that should not be changed are black. The mask is then used to decide which pixels are to be changed. An advantage to using a mask is that we separate the processes of (a) detecting which pixels are to be changed and (b) making the changes to the pixels. By separating them, we can improve on each, and thus improve the whole effect. 5.5 COLORING IN RANGES In this chapter, we have used a general technique of iterating across all pixels, computing the x- and y-coordinates, then using an if statement to decide if the pixel needed to be manipulated based on its position. We can use this general approach to do a wide variety of changes to a picture. We can put borders on a picture, or apply image techniques to only part of the picture Adding a Border For example, let s say that we wanted to put a blue and white border on the picture of Greek ruins. Blue and white are the colors on the Greek flag. We want to create the effect seen in Figure Program 59: Add Blue and White Borders def greekborder(pic): bottom = getheight(pic)-10 for px in getpixels(pic): y = gety(px) if y < 10: setcolor(px,blue) if y > bottom: setcolor(px,white) FIGURE 5.21 An image of Greek ruins (left) with a blue and white border added at top and bottom (right).