Reprocessing Historical Images - II, Restoring images from antiquated and/or poor quality sources |
Reprocessing Historical Images - II, Restoring images from antiquated and/or poor quality sources |
Aug 29 2008, 03:34 PM
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Interplanetary Dumpster Diver Group: Admin Posts: 4404 Joined: 17-February 04 From: Powell, TN Member No.: 33 |
I figure that it is time for another thread like this. I still find it astonishing to see the versions of a lot of images that are reproduced over and over again. For example, this is the last mosaic of Triton taken before the close encounter began. The version on the Planetary Photojournal is on the left, my version on the right. Clearly, this image was produced as part of the "instant science" campaign. They did a superb job getting images to the public in a speedy manner, but they are extremely rough, since the team was busy running the spacecraft. However, it is this version that keeps being recycled. Worse, the version on the photojournal is clearly scanned from a printed copy, causing further degradation.
Here is a similar comparison, this time using Proteus (still 1989N-1 on the Planetary Photojournal!). This discussion started in the thread about Viking crescents but was getting off topic, so I thought I would start a new thread here. -------------------- |
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Feb 3 2009, 06:13 PM
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Member Group: Members Posts: 378 Joined: 21-April 05 From: Portugal Member No.: 347 |
Hi all,
I wrote a small program in Actionscript 3 (Flash) that performs linear interpolation to 8 bits on images with low bit levels originally. It's very basic and only works in one axis, but i can get around that simply rotating the original image 90º and averaging the results with the non rotated interpolation. I tried it on the Voyager 2 Proteus image discussed in earlier posts. I must say I like the results. The raw image was histogram stretched and cleaned of noise before interpolation. The interpolated image was then processed in Photoshop. What I got is the image bellow. Any ideas regarding other good images to test the program, besides the Viking sunset/sunrise images that I've already thought about? -------------------- _______________________
www.astrosurf.com/nunes |
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Feb 4 2009, 02:46 AM
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#3
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Interplanetary Dumpster Diver Group: Admin Posts: 4404 Joined: 17-February 04 From: Powell, TN Member No.: 33 |
That looks like a good result, but could you explain your use of "interpolated." Usually it is used to remove missing lines and pixels, but I am assuming this is used to reduce the "stair-stepping" effect. Am I correct?
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Feb 4 2009, 11:13 AM
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#4
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Member Group: Members Posts: 378 Joined: 21-April 05 From: Portugal Member No.: 347 |
"I am assuming this is used to reduce the "stair-stepping" effect."
Yes, that's it. I'll try to explain better: Let's imagine that he have an image with pixel values, along a row, something like this: 100 110 110 110 120 120 110 100 What the program does is reading each pixel value and counting for how many pixels is that value the same as the previous pixel. It's just like saying, in common language: there are 3 pixels in a row with a value of 110, and then the next pixel's value is 120. Then the program divides the total pixel value difference (in this example 120-110=10 levels) by the amount of pixel the value stayed the same (3 pixels). These 3 pixels with the same value are then replaced by a gradient (110 113 116). So the resulting image data would be something like: 100 110 113 116 120 120 110 100 Hope this makes things a little clearer. I've tried it on the viking images, and it does give a smooth sky, but noise creates artifacts. So i need more time to optimize the code and try to ignore level changes due to isolated pixels. -------------------- _______________________
www.astrosurf.com/nunes |
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Feb 4 2009, 11:48 AM
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Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
So i need more time to optimize the code and try to ignore level changes due to isolated pixels. So it's something similar to deblocking algorithms for MPEG video decoders. It's somewhat of a black art - you ignore too many "isolated" noise pixels and all of a sudden you realize you're starting to destroy valid details too... EDIT: That sort of thing could be helpful with problems described in this thread, namely color banding after massively enlarging a Titan image. If done with 16 bit color, dithering down to 8 bits could remove the banding effects. -------------------- |
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