Seam carving

Original image to be made narrower

File:Broadway tower edit scale.png

Scaling is undesirable because the castle is squished.

File:Broadway tower edit cropped.png

Cropping is undesirable because part of the castle is removed.

File:Broadway tower edit Seam Carving.png

Seam carving

Seam carving (or liquid rescaling) is an algorithm for content-aware image resizing, developed by Shai Avidan, of Mitsubishi Electric Research Laboratories (MERL), and Ariel Shamir, of the Interdisciplinary Center and MERL. It functions by establishing a number of seams (paths of least importance) in an image and automatically removes seams to reduce image size or inserts seams to extend it. Seam carving also allows manually defining areas in which pixels may not be modified, and features the ability to remove whole objects from photographs.

The purpose of the algorithm is image retargeting, which is the problem of displaying images without distortion on media of various sizes (cell phones, projection screens) using document standards, like HTML, that already support dynamic changes in page layout and text but not images.

Image Retargeting was invented by Vidya Setlur, Saeko Takage, Ramesh Raskar, Michael Gleicher and Bruce Gooch in 2005. The work by Setlur et al. won the 10-year impact award in 2015.

Seams

Seams can be either vertical or horizontal. A vertical seam is a path of pixels connected from top to bottom in an image with one pixel in each row. A horizontal seam is similar with the exception of the connection being from left to right. The importance/energy function values a pixel by measuring its contrast with its neighbor pixels.

Computing Seams

Computing the seam consists of finding the path of minimum energy cost from one end of the image to another. This can be done via Dijkstra's algorithm, dynamic programming, or graph cuts.

Dynamic Programming

Dynamic programming is a programming method that stores the results of sub-calculations in order to simplify calculating a more complex result. Dynamic programming is used in seam carving for computing seams.

If attempting to compute a vertical seam (path) of lowest energy, for each pixel in a row we compute the energy of the current pixel plus the energy of one of the three possible pixels above it.

This is better described by this image:

File:DynamicProgrammingLeastEnergyPathA.png

Each square represents a pixel, with the top-left value in red representing the energy value of that said pixel. The value in black represents the cumulative sum of energies leading up to and including that pixel.

The first row has no rows above it, so the sum (black) is just the energy value of the current pixel (red).

The second row, if we look at the second pixel for example, we see its energy value is 2 (red). If we look above it, it has a choice of either 1, 4, or 3 (black). Since 1 is the minimum number of the three values, we ignore the other two and set the sum of the pixel to its energy value which is 2 (red) plus 1 (black).

After the above operation is carried out for every pixel in the second row, we go to the third row:

File:DynamicProgrammingLeastEnergyPathB.png

We repeat the process in row two in row three to end up with the final cumultive sums for the seams/paths. The lowest value or values are the seams with the lowest energy, which would be in this example the seams with '5' in the last row.

To trace the seam/path, work from the last row and follow the green arrows:

File:DynamicProgrammingLeastEnergyPathC.png

Algorithm

1) We start with an image such as:

File:BroadwayTowerSeamCarvingA.png

2) We then calculate the weight/density/energy of each pixel. This can be done by various algorithms: gradient magnitude, entropy, visual saliency, eye-gaze movement. In this example gradient magnitude gives 'satisfactory results.'

File:BroadwayTowerSeamCarvingB.png

3) After we have the energy of the image, we generate a list of seams. Seams are ranked by energy, with low energy seams being of least importance to the content of the image. We can choose to calculate seams via the dynamic programming approach.

Seams shown with the energy function:

File:BroadwayTowerSeamCarvingC.png

Seams shown with the original image:

File:BroadwayTowerSeamCarvingD.png

4) We then remove the seams from the image, reducing the size of the image as a result:

File:BroadwayTowerSeamCarvingE.png

The recalculated energy function of the image would be:

File:BroadwayTowerSeamCarvingF.png

Issues

1. The algorithm may need user-provided information to reduce errors. This can consist of painting the regions which are to be preserved. With human faces it is possible to use face detection.
2. Sometimes the algorithm, by removing a low energy seam, may end up inadvertently creating a seam of higher energy. The solution to this is to simulate a removal of a seam, and then check the energy delta to see if the energy increases. If it does, prefer other seams instead.

Implementations

Adobe Systems acquired a non-exclusive license to seam carving technology from MERL, and implemented it as a feature in Photoshop CS4, where it is called Content Aware Scaling. As the license is non-exclusive, other popular computer graphics applications, among which are GIMP, digiKam, ImageMagick, as well as some stand-alone programs, among which are iResizer, also have implementations of this technique, some of which are released as free and open source software.

Improvements and extensions

• Better energy function and application to video.
• Multi-operator: Combine with cropping and scaling.
• Much faster removal of multiple seams
• On-demand, server-side seam carving

A 2010 review of eight image retargeting methods found that seam carving produced output that was ranked among the worst of the tested algorithms. It was, however, a part of one of the highest-ranking algorithms: the multi-operator extension mentioned above (combined with cropping and scaling).

See also

• Inpainting
• Texture synthesis

External links

• Seam Carving demonstration videos:
  • on YouTube
  • on Ariel Shamir's pages on the Interdisciplinary Center web site (higher resolution)
• Explanation of seam carving (Liquid rescaling) at the ImageMagick website