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Bài giảng Nhập môn thị giác máy tính - Chapter 7: Seam carving - Lê Thanh Hải

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Chapter 7 – Seam Carving  
7-1  
Chapter 7  
Seam Carving  
Michael Rubinstein, MIT  
Shai Avidan, Mitsubishi Electric Research Labs  
Ariel Shamir, The Interdisciplinary Center & MERL  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-2  
Contents  
Image resizing  
• Seam Carving algorithm  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-3  
Display Devices  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-4  
Content Retargeting  
iPhone  
PC  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-5  
Page Layout  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-6  
Simple Media Retargeting Operators  
LettSecrabloinxging  
Departmen
Chapter 7 – Seam Carving  
7-7  
Content-aware Retargeting Operators  
“Important”  
content  
Content-  
aware  
Content-  
oblivious  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-8  
Content-aware Retargeting  
Scale  
Crop  
Content-aware  
“less-Important”  
content  
Input  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-9  
Image Retargeting  
• Problem statement:  
• Input Image I with size n × m, and new size n’ × m’  
• Output Image ꢀ푰 of size n’ × m’ which will be “good  
representative” of the original image I  
• To date, no agreed definition, or measure, as to what a  
good representative is in this context!  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-10  
Image/Video Retargeting  
• In large, we would expect:  
1. Adhere to the geometric constraints (display/aspect ratio)  
2. Preserve the important content and structures  
3. Limit artifacts  
4. Perhaps a new representation that will support different  
sizes?  
• Very ill-posed!  
o How do we define important? Is there a universal ground  
truth?  
o Would different viewers think the same about a retargeted  
image?  
o What about artistic impression in the original content?  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-11  
Importance (Saliency) Measures  
• A function S: p [0,1]  
Wang et al. 2008  
• More sophisticated: attention models, eye tracking (gazing  
studies), face detectors, …  
Judd et al. ICCV09 Learning to predict where people look  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-12  
General Retargeting Framework  
1. Define an energy function E(I)  
(interest, importance, saliency)  
2. Use some operator(s)  
to change the image I  
Setlur et al.  
[2005]  
Recompose  
Santella  
et al.  
Crop  
Warp  
[2005]  
Gal et al.  
[2006]  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-13  
Previous Retargeting Approaches  
• Optimal Cropping Window  
• For videos: “Pan and scan”  
Still done manually in the movie industry  
Liu and Gleicher, Video Retargeting: Automating Pan and Scan (2006)  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-14  
Cropping  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-15  
Seam Carving  
• Assume m × n m × n’, n’ < n  
• Basic Idea: remove unimportant pixels from the image  
o Unimportant = pixels with less “energy”  
• Intuition for gradient-based energy:  
o Preserve strong contours  
o Human vision more sensitive to edges – so try remove content  
from smoother areas  
o Simple, enough for producing some nice results  
o See their paper for more measures they have used  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-16  
Pixel Removal  
Least-energy pixels  
(per row)  
Optimal  
Least-energy columns  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-17  
A Seam  
• A connected path of pixels from top to bottom (or left to right).  
Exactly one in each row  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-18  
Seams in Images  
. Efros & Freeman [2001] – Texture synthesis  
. Kwatra et al. [2003] – Image and video synthesis  
. Agarwala et al. [2004] – Digital Photomontage  
Mostly used for composition of  
. Perez et al.
. Jia et al. [20
two (or more) images or patches…  
. Rother et al. 
. Wang and Cohen [2006] – simultaneous matting and  
compositing  
Agarwala et al., Interactive Digital Photomontage  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-19  
Finding the Seam?  
Department of Mechatronics  
Chapter 7 – Seam Carving  
7-20  
The Optimal Seam  
s* = argmin E(s)  
∂
∂
E(I) =| I | + | I |  
∂x ∂y  
S
Department of Mechatronics  
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