From CS294-10 Visualization Sp11

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Lecture on Feb 2, 2011




  • Perception in visualization. Healey. (html)
  • Graphical perception. Cleveland & McGill. (jstor)(Google Scholar)
  • Chapter 3: Layering and Separation, In Envisioning Information. Tufte.

Optional Readings

  • Gestalt and composition. In Course #13, SIGGRAPH 2002. Durand. (1-up pdf) (6-up pdf)
  • The psychophysics of sensory function. Stevens. (pdf)
  • Crowdsourcing Graphical Perception: Using Mechanical Turk to Assess Visualization Design. Heer & Bostock. ACM CHI 2010. (html) - Feb 09, 2011 01:56:43 pm

Regarding Figure 29 in the Cleveland & McGill paper, I would argue that this graph and a more traditional heat or patch map serve two different purposes. Although in this map it is easier to compare the murder rates from one state to another it does not take advantage of the preattentive processing described in the Healey paper. A traditional heat map gives a good overview of quantitative variables but it allows for only minimal comparison of similar values. This type of map gives a poor overview of quantitative variables and requires postattentive processing for finding anomolies, but allows for much more nuanced comparison of individual states' values even when the difference between them is minimal.

Michael Cohen - Feb 09, 2011 05:03:53 pm

I was frustrated by the "perceptually-motivated" visualizations featured in the Healey article, particularly the electoral map at the top and the weather history map (Fig. 17). I think this frustration spilled over into our discussion of distinguishing orientation and length in class and led me to question whether using both together would be a good idea in practice. I think Healey's visualizations are impressive in that they demonstrate that you can cram so many dimensions of data into a single visualization and have it all be fundamentally perceptible. However, in practice I think both visualizations are difficult to read and draw useful conclusions from. In the electoral map, the depth of each state corresponds roughly to its importance in the election, but since depths obscure other depths they are very difficult to compare except between geographically adjacent states. The depth dimension also causes some states to obscure others (e.g., California & Oregon). In the weather map, the use of size and density of strokes to encode some information means that the Southeast has significantly lower data density than the North on dimensions encoded by color and orientation. It's also difficult to disentangle density and size visually. I think the weather map, in particular, could have been better handled with small multiples.

To me, these visualizations illustrate that more information in a single display isn't always better, even if your perceptual theory argues that all of the information is available. The interference between dimensions is an crucial factor.

Saung Li - Feb 09, 2011 06:49:58 pm

I like how this topic of perception combines the fields of psychology and visualization. I did not know there are fields such as "psychophysics." The idea of preattentive processing, in which visual properties are detected quickly via a low-level system, can help direct us in our design of graphics. We are good at detecting differences in color and shape; after playing around with the Java applet it seems like differences in color are easiest to spot, though different shapes still work well. The change blindness phenomena shows us how little information we actually retain from looking at an image. I found it extremely hard to find the differences in the GIFs-- I could only find one if I focused on one small part of the image, and doing this can be quite painful to the eye. These ideas suggest that we should not clutter too much information into a graphic unless we make use of preattentive cues effectively, so that readers can pick up the most relevant information quickly.

Boheekim - Feb 09, 2011 10:12:51 pm

I just wanted to add a couple thoughts to today's class. I wanted go back to the trade off we face when we decide to make accurate visualizations or visualizations that are accurately perceived. Perhaps if we are only to absorb the data rather than analyze it, maybe it would be better to work towards making visualizations that are accurately perceived. Also on pie charts: I know we discussed why pie charts are often a poor data visualization. Do we use them because they are so familiar, because they are more aesthetically pleasing or for some other reason?

Lastly, the Healy paper talked a bit about directing the eye of the audience to the areas that are important. I think it's important to consider what mediums we are publishing the data visualizations in then, taking into account eye tracking when it is web for example.

Natalie Jones - Feb 10, 2011 01:51:39 am

I find the work on perception to be fascinating, and particularly the findings about misperceptions as they pertain to visualizations. It also makes me think about accuracy in visualizations in general. It seems important before starting a visualization to decide what accuracy means for that particular dataset and audience, and how adamantly it might be necessary to stay faithful to the data in order to get the intended point across. In some cases, it might be much more important than in other cases to be orthodox about scaling the data "correctly," and I could imagine that in some cases it might actually make sense to choose design over data, or at least meet in the middle somewhere. I also feel like I see a lot of visualizations that don't even seem to try hard at all to make certain aspects of the visual representation correspond to data, and I wonder if there are visualizations I've seen that appear attractive and interesting, but that may not have communicated as much as I thought they did, or as much as they could have. It's certainly something I'll be more on the lookout for now.

Karl He - Feb 10, 2011 02:40:31 am

I was particularly amused by the images on change blindness. It took me at least five minutes to realize that the airplane engine was missing in the second frame of the animation.

This phenomenon makes clear that human perception is extremely important for visualization. It doesn't matter how accurate your visualization is if it is interpreted incorrectly. Techniques such as apparent-size drawings I am hesitant to utilize since it involves distorting the data. However, it is a valid technique as long as nothing is being exactly measured. Apparent sizes of bars on a bar chart would likely be a bad idea, since people may try to measure exact sizes, but for the example given of dots on a map representing newspapers delivered, I feel it is valid since the disc sizes aren't meant to exactly correlate to the numbers anyway.

While it should be used with caution, it is a good idea to distort data for human perception to get the correct message across, especially if it is difficult to interpret without aid.

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