walala wrote:> "Ben Pope" <spam@hotmail.com> wrote in message > news:bmuqba$qdnh9$1@ID-191149.news.uni-berlin.de... >> walala wrote: >>> Are there any guidelines on how to design good filters that applicable >>> to most(if not all) images? Discussion should be in two cases: a) with >>> source image and can get PSNR to compare results; b) with no source >>> image and only have the reconstructed images, so no PSNR can be >>> computed, how to design a "good" filter fit into this case? >> >> >> You need to first work out your metric... you say you use PSNR, but >> without a source image... so how can you possibly measure PSNR? >> > > Bob, thanks for your posting,Who is Bob?> That's the problem. The filter is supposed to work on all images. In this > "design" stage, I have a few source images to test PSNR... later on, the > filter is supposed to adapt to input and do filtering blindly... at that > time, no PSNR can be measured.Right.> So what I want to know is exactly how to design a filter that can > adaptively work on images to achieve satisfactory PSNR(although may not > be the best, but al least works for most cases) after the initial > training stage.Well if you're trying to clear up JPEG artifacts, then the best you can do is some kind of low pass filter. Wavelets can be used to great success as well. But thats whole new story :-) PSNR is not a great measure of perceived image quality anyway. We are more sensitive to brightness than colour, by a factor of about 4, yet PSNR does not model that. Additionally the edges that you see with JPEG artifacts is also not well described with PSNR, human vision is designed to pick out edges, we're very good at it, so although the error might be quite small in terms of PSNR, it looks ugly to us. Go back to my original reply... start defining your problem, are you trying to remove JPEG artifacts from images? Then the biggest issue I have with JPEG images are the discontinuities... so low pass filter it, which is what you have been doing. In terms of the specific best filter... you're fighting a losing battle without a huge training set. If you're images are primary photograph based, then just use Gaussian. Otherwise do a search for papers that have researched JPEG artifact removal - I suspect there are a considerable number of them out there with a wide ranging number of techniques. You can remove many types of noise from an image quite effectively with a simple blur (low-pass filter, guassian being very popular, along with unsharp mask), to increase PSNR by a good 10dB or so. Ben -- I'm not just a number. To many, I'm known as a String...
(image process) are there any guidelines on how to design good filters to enhance an image?
Started by ●October 18, 2003
Reply by ●October 19, 20032003-10-19
Reply by ●October 19, 20032003-10-19
walala wrote:> The input images are some jpeg images having block-artifacts. As you know > that JPEG images have block-artifacts under low bit rate. Someone else in my > group did some experiments to pre-filter the images to try to get rid of the > artifacts. But that also introduces other artifacts. So now it is a combined > artifact... and to be frankly, all processing techniques looked the same to > me on images, because they are nearly same, but in terms PSRN, they have big > difference... so I trial and error and found one with both good PSNR and > looking(but still ugly though)...Okay, now you are providing the type of information needed. So the task is to remove the blocking artifacts from jpegs. Here are some things you may want to consider: Jpegs have different levels of compression resulting in different levels of blocking artifacts. The compression techniques used in jpeg involve analysis of the frequency content of the image and the amount of blocking artifacts is somewhat related to the frequency content of the original input. Jpeg is designed for natural images like photographs. Artificial images like line drawings tend to have a lot more objectionable artifacts. And most important the compression in jpegs is done in part by quantizing the frequency levels of each block. This is done by formulae (derived by experiment) to retain the part of the signal that is the most perceptually important. This explains why you can't see the difference even though your calculations show a big difference in the amount of noise removed. Another thing is you are not limited to 3 x 3 filter. A common filter I've seen applied to this problem is [-1 0 5 8 5 0 -1]/16. This filter can be applied very efficiently with adds and shifts by iterating through the columns and rows of the image, resulting in what is effectively a 7X7 filter. I don't know if speed is an issue you are interested in. -jim> > But I cannot do trial-and-error search for every input, right? Because after > this "training" stage, I don't have source images available any more, right? > My fitler should work adaptively itself...-----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =-----
Reply by ●October 19, 20032003-10-19
jim" <"N0sp wrote:> And most important the compression in jpegs is done in part by quantizing > the frequency levels of each block....which is one of the reasons why a simple low pass filter increases PSNR. I guess predominantly however, it is because it reduces the high contrast edges, which is where the error is greatest. Since these edges tend to be the worst perceived artifact, low-pass filter helps drastically there. (Bearing in mind that PSNR is not entirely equivelent to perceived image quality) Ben -- I'm not just a number. To many, I'm known as a String...
Reply by ●October 20, 20032003-10-20
On Sun, 19 Oct 2003, walala wrote:> [...] I am doing "floating-point" search and trial & error ... so the > search space is huge. I just try to search for a few days...If you want to make this time much shorter, try evolutionary algorithms. We have written a small c++ library for that: http://www.iki.fi/o/opti/ok -olli
Reply by ●October 20, 20032003-10-20
> > Wiener filters come to mind... again, there are plenty filters and tricks > available for image processing. Whether you can do stuff completely > automatic or need a human operator in the loop, depends on theapplication.> > RuneRune, You said it... after doing searching a lot, I am tired of trial and error. This cannot let us write a report. I will try to apply a Wiener filter... just headache about how to get the Rxx of the input image...which is the covariance matrix of the input? Can you give me information on this? Thanks, -wa
Reply by ●October 20, 20032003-10-20
"walala" <mizhael@yahoo.com> wrote in message news:<bmvju8$dgl$1@mozo.cc.purdue.edu>...> > > > Wiener filters come to mind... again, there are plenty filters and tricks > > available for image processing. Whether you can do stuff completely > > automatic or need a human operator in the loop, depends on the > application. > > > > Rune > > Rune, > > You said it... after doing searching a lot, I am tired of trial and error. > This cannot let us write a report. I will try to apply a Wiener filter... > just headache about how to get the Rxx of the input image...which is the > covariance matrix of the input? Can you give me information on this?Sorry, can't help. I am not even sure Wiener filters work for image processing, but somehow I think I have seen the term used with images... perhaps you should ask at an image processing newsgroup? As for writing reports... I have the extremely controversial view that a report should reflect actual progress, not some imaginary path towards the desired goals. Some times the solving of a problem is just too difficult or require more resources than available in a project, and I believe the report should reflect that. That way there is a chance that planning a next project over the same theme has a chance to succeed. If used properly, such progress reports would actually be a great tool to show which managers actually know their job and which would be better off somewhere else. But beware! These counterproductive "non-team player" attitudes of mine forced me to resign from one job and got me into a long sick leave from the next. You may want to play your cards differently. Rune
Reply by ●October 20, 20032003-10-20
"Olli Niemitalo" <o@iki.fi> wrote in message news:Pine.GSO.4.58.0310200558460.289@paju.oulu.fi...> On Sun, 19 Oct 2003, walala wrote: > > > [...] I am doing "floating-point" search and trial & error ... so the > > search space is huge. I just try to search for a few days... > > If you want to make this time much shorter, try evolutionary algorithms. > We have written a small c++ library for that: > > http://www.iki.fi/o/opti/ok > > -olliOlli, Thanks a lot for the pointer... I have looked at the program but have a problem: Basically I am doing everything in Matlab, since I need the easy-manipulation of image data, and the easy-scripting, and the filtering operation, etc. And your program is in C++... I tried to compile Matlab script file into standalone executable and C++ source codes and then combine with yours... Before I can see any results, it just give me a lot of error during compilation. Also the C++ source code generated by Matlab is quite weird and not understandable. I wonder how to combine with yours. I suggest you to make a Matlab version or at least make a Matlab interface for your optimization code. Otherwise people may need go through a lot of trouble before they can try on your optimization code. Thanks a lot for your work! -Walalal
