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#11
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Compression in JPEG files in digital cameras
ray wrote:
On Sat, 18 Aug 2007 11:55:45 -0700, aniramca wrote: I need some help in explaining the JPEG compression feature in digital cameras. My camera (which likely is similar to most others) has the feature to compress the photo JPEG files in the storage card. It also has the choice to have different pixel sizes (example: 3000x2250, 2000x1500, 1024x768, etc). What is the difference of the above two features? If you store a 3000x2250 pixel data in compressed mode, does it loose its quality? Can it be re-instated to full uncompressed size without loosing photo quality?. When I compressed the data, it will fit more pictures in a single storage card. But, is it the same if I choose 2000x1500 pixel and no compression instead? Thanks for info. It will uncompress to the full resolution - however it will not be an exact replica of the original image. JPEG is a 'lossy' compression algorithm - meaning that some detail is lost in the act of compressing the data. There are 'lossless' compression schemes as well, but the level of compression with them is generally considerably less. From a practical standpoint, you can do a lot of compression on a JPEG image before you see noticeable loss of information. Before I got a camera capable of saving RAW images, I always chose the highest resolution and the smallest amount of compressin on JPEG images - now I simply save nearly everything in RAW - even though it does take a lot of space. Memory cards are very cheap now - IMHO it is better to get some extra cards and save everything using the best method possible. I currently have 2-1gb cards and 1-2gb card for my 5mp camera - they run around $15 and $30 respectively. Currently several places in my local area sell 2GB SD cards for under $20. 4GB cards go for about $40. Very little excuse to opt for more compression, or lower resolution, just to save space. Note that if you have a need for fast shooting, writing very large files (RAW/TIFF), may slow down your shot rate, considerably. |
#12
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Compression in JPEG files in digital cameras
On Aug 18, 1:55 pm, wrote:
I need some help in explaining the JPEG compression feature in digital cameras. My camera (which likely is similar to most others) has the feature to compress the photo JPEG files in the storage card. It also has the choice to have different pixel sizes (example: 3000x2250, 2000x1500, 1024x768, etc). What is the difference of the above two features? If you store a 3000x2250 pixel data in compressed mode, does it loose its quality? Can it be re-instated to full uncompressed size without loosing photo quality?. When I compressed the data, it will fit more pictures in a single storage card. But, is it the same if I choose 2000x1500 pixel and no compression instead? Thanks for info. This is a complicated question, and not suited to quick answers- I'd get a decent book on digital photography. Actually, the Dummies Guide series has a reasonable book on the subject. Briefly, there are two different ways of reducing the size of the file (memory size) of an image. One method is to "downsample", or average pixels to create a new array of numbers with fewer pixels. All the pixels in the image chip are used, but the downsampling or averaging math reduces the "resolution" or sharpness of the image. Data compression schemes such as JPEG do not downsample in the same way. Depending on the subject a jpeg will retain all or most of the resolution. However, subtleties of color disappear, gradients of color or brightness get flattened, and little regions of the wrong color begin cropping up. Jpeg is a lossy compression scheme, meaning this loss of image quality cannot be reversed after the image file is compressed. There ARE other compression schemes that ARE reversible. With the cheapness of very large memory cards these days, and also large memory banks for computers, there is little reason to downsample in the camera, or to use excessive values of compression (many cameras allow you to select how much compression to use). The most common advice these days is to shoot full resolution (the 3000 x 2250 mentioned), and select the least amount of jpeg compression. This is frequently called picture quality, as in super high quality, high quality, medium, or some such combination of words. |
#13
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Compression in JPEG files in digital cameras
In rec.photo.digital harrogate3 wrote:
JPEG is a lossy form of saving the picture - The Bayer sensor is already lossy, having only one of RGGB at any of four pixel locations, so one could make the argument that JPEG imposes no further loss. Google on 'explanation of jpg' and it will give you the detail or read http://photo.net/jpeg/learn/ |
#14
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Compression in JPEG files in digital cameras
Bill Tuthill wrote:
In rec.photo.digital harrogate3 wrote: JPEG is a lossy form of saving the picture - The Bayer sensor is already lossy, having only one of RGGB at any of four pixel locations, so one could make the argument that JPEG imposes no further loss. You could argue that until you are blue in the face, and it still wouldn't be anything near correct though! :-) -- Floyd L. Davidson http://www.apaflo.com/floyd_davidson Ukpeagvik (Barrow, Alaska) |
#15
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Compression in JPEG files in digital cameras
Bill Tuthill wrote:
In rec.photo.digital harrogate3 wrote: JPEG is a lossy form of saving the picture - The Bayer sensor is already lossy, having only one of RGGB at any of four pixel locations, so one could make the argument that JPEG imposes no further loss. However, JPEG imposes an additional loss, because of the approximations it makes. You can control the degree of approximation by the quality setting on cameras, or by the compression or quality settings in your software. David |
#16
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Compression in JPEG files in digital cameras
In rec.photo.digital Floyd L. Davidson wrote:
JPEG is a lossy form of saving the picture - The Bayer sensor is already lossy, having only one of RGGB at any of four pixel locations, so one could make the argument that JPEG imposes no further loss. You could argue that until you are blue in the face, and it still wouldn't be anything near correct though! :-) I'm not going to argue it that long, but I would say that nobody has quantified the loss that comes from the Bayer sensor versus the loss that comes from JPEG. The real problem with JPEG is that it is not edit-safe, rather than loss of information at creation time. |
#17
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Compression in JPEG files in digital cameras
In rec.photo.digital Bill Tuthill wrote:
I'm not going to argue it that long, but I would say that nobody has quantified the loss that comes from the Bayer sensor versus the loss that comes from JPEG. The real problem with JPEG is that it is not edit-safe, rather than loss of information at creation time. Is it not obvious to you that the "loss" from using a bayer sensor (which isn't really a loss in the same sense) and the loss due to JPEG compression are going to be cummulative in nature. Thus, the JPEG losses will only add to any losses due to using a bayer sensor. -- Thomas T. Veldhouse We have more to fear from the bungling of the incompetent than from the machinations of the wicked. |
#18
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Compression in JPEG files in digital cameras
added these comments in the current discussion du jour ...
I need some help in explaining the JPEG compression feature in digital cameras. My camera (which likely is similar to most others) has the feature to compress the photo JPEG files in the storage card. It also has the choice to have different pixel sizes (example: 3000x2250, 2000x1500, 1024x768, etc). What is the difference of the above two features? If you store a 3000x2250 pixel data in compressed mode, does it loose its quality? Can it be re-instated to full uncompressed size without loosing photo quality?. When I compressed the data, it will fit more pictures in a single storage card. But, is it the same if I choose 2000x1500 pixel and no compression instead? Thanks for info. Resolution is measured by horizontal and vertical pixels for an image, the area of which constitutes mega pixels. JPEG compression is how much or little the uncompressed data is made smaller. Since JPEG is a so-called "lossy" format, it literally throws away pixels in order to drastically shrink the file size. The idea of the algorithm is to choose pixels to disgard that have a mathmatical probability of not being noticed by the human eye. However, when compression begins to get even moderately high, defects begin to appear, commonly called "JPEG artifacts", artifact meaning "what is left behind". This can easily be seen by blobs, streaks, blurry areas, minor destruction of fine detail, and sometimes posterization. "Better" digital cameras will give you choices as to how much to compress for a given mega pixel image. Lots of words used but "basic", "normal", and "fine" are common. I don't think any cameras tell you the actually number they use nor the Chroma subsampling they use, but under most circumstances, one can quickly show themselves that "basic" is pretty awful, "normal" MAY produce artifacts 5, 10, 15% of the time, and "fine" rarely does. However, many - not all - of the less expensive P & S cameras only give you marketing BS like "good", "better" and "best", but what they're really doing is maintaining the same JPEG compression but upping the MP. The reason that so many lower cost but high MP cameras do that is that they also want to advertise how many pictures you can fit on even a small memory card. I'm not sure about the rest of your question to exampand more on my answer. Some cameras allow TIFF, which is lossless, and even better cameras - certainly DSLRs - can also save in RAW. Without starting another religious war, if you can get by with JPEG and it fulfills what you want and need the camera to do for you, you'll be just fine. It is universally readable, lots of free or almost free editing apps as wells as commercial apps, and you can save money on memory if that is a consideration. But, since I know of NO camera buyer who isn't interested in the best possible quality, I would look for cameras that offer a choice of compression so that you can run some tests for yourself and make up your own mind. -- HP, aka Jerry |
#19
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Compression in JPEG files in digital cameras
Bill Tuthill added these comments in the current discussion du jour
.... In rec.photo.digital harrogate3 wrote: JPEG is a lossy form of saving the picture - The Bayer sensor is already lossy, having only one of RGGB at any of four pixel locations, so one could make the argument that JPEG imposes no further loss. Google on 'explanation of jpg' and it will give you the detail or read http://photo.net/jpeg/learn/ Bill, I'm not nearly enough of a mathematicion to understand this, but it is WAY over the OP's head. Might it be better to describe the issue in more qualitative than theoretical ways he can understand? -- HP, aka Jerry |
#20
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Compression in JPEG files in digital cameras
David J Taylor added these comments in the current discussion du
jour ... JPEG is a lossy form of saving the picture - The Bayer sensor is already lossy, having only one of RGGB at any of four pixel locations, so one could make the argument that JPEG imposes no further loss. However, JPEG imposes an additional loss, because of the approximations it makes. You can control the degree of approximation by the quality setting on cameras, or by the compression or quality settings in your software. Here we go again! David, you and I get along OK. You're level headed and not any kind of elitist, while at the same time, you have an excellent foundation in the theory of all of this. I think the OP is a pretty rank novice and has probably gone unconscious by now in this thread. You have such a great way to express your ideas in words that people of all technical levels can understand that I bet you can do far better than my feeble attempt at helping the OP. -- HP, aka Jerry |
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