At 9:14 PM -0400 4/28/05, Tom Barry wrote: >I think we've already once had something of this conversation on >AVS. I still tend to believe we see images in a blend of two (or >more) different modes. One would be some sort of edge and/or >shape perception which is probably not frequency based. But the >other is our perception of texture, and that one likely does rely >on frequency. It really seems to me our perception of realistic >sharpness depends upon both. It is at times like this that I take a bit of pride in the fact that the OpenDTV list keeps going, and going, and going. The discussions of the past few days have been first rate - I hope everyone is learning as much as I have. Tom is definitely on the right track with respect to the presence of specialized receptors in our foveal vision that are tuned to specific types of stimuli. The following is from the SPTE Task Force Report on Digital Imaging which I helped author in 1992. 3.2.2 Human Visual Processing Much of the research in visual science today is focused on the processing of data acquired by the image receptors. A variety of specialized analyzers in the eye process data from small localized regions and accumulate the results into channels which are processed by the brain to create an integrated view of the physicals environment. There is evidence that the brain directs the activity of the image receptors for processes such as establishing white balance and light sensitivity levels. Simple localized analyzers are used to enhance the data transmitted back to the brain. Some of these analyzers are sensitive to a particular edge orientation; there are sufficient analyzers at each location to represent a full set of edge orientations. Additional tuned analyzers cover portions of the range of human sensitivity for spatial frequency, spatial position, temporal frequency direction of motion; and binocular disparity. The data processed by these analyzers moves to the brain through two types of channels; a set of fast responding channels with relatively transient responses to stimuli, and a set of slower channels with relatively sustained responses to stimuli. Transient channels process the output of analyzers that are tuned for low spatial and high temporal frequency stimuli. Sustained channels process the output of analyzers that are tuned for high spatial and low temporal frequency stimuli. So yes, we do have a variety of image receptors that are tuned for the acquisition of various components of an image. One of the interesting findings of the research that I studied when writing the report is that we LEARN how to see different edge orientations. There was a study done with rats where they were raised in an environment that was devoid of certain edge orientations - I think there were vertical lines in the environment, but no horizontal lines. When these rats were later introduced into an environment with edges in all orientations, they could not see the edges that they had NOT learned to see; they would run into things because they could not see them. It is important to note that there is a huge difference between the perception of still images and moving images. With still images we have plenty of time to analyze the image and to perceive fine details. With moving images the amount of information can overwhelm the human visual system. We tend to filter out the most important information and used directed eye movements to acquire high resolution views of portions of a high resolution image. This is especially true for images that cover a large portion of our field of view, such as an large HDTV display - we cannot sample all of the information in the image, and are forced to track motion and acquire high resolution views of a portion of the image while ignoring most of the detail in other portions of the image. The only problem is that we cannot predict what a viewer will look at, so we need to have approximately the same level of detail everywhere. So while it is interesting to study the images that Tom and Jeroen have created for us, they only tell us part of the story. It takes several hundred milliseconds to acquire a high resolution view of any image, as the foveal receptors dart around to acquire a high resolution view. Thus in a motion imaging system it is likely that several temporal samples are contributing to the perception of sharpness. Detail from several frames can add to improve the perception of sharpness. One of the early compression systems for desktop computers simply threw away about 75% of the image samples. But it did this by moving the sampling points around in a four pixel region - over a four frame time period all of the sample points in the original image were presented. This actually worked quite well, enabling the perception of more image detail than simply down sampling to a frame 1/4 the size. On the other hand, objects that are moving need some motion blur - especially at lower frame rates like 24P - in order to fool the human visual system into seeing continuous motion. So a motion imaging system must deal with many issues as it attempts to fool the human visual system into seeing sharp, high resolution moving images. Regards Craig ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.