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Thread ID: 16553	2002-03-11 11:00:00	.jpg images	Guest (0)	Press F1

Post ID	Timestamp	Content	User
38757	2002-03-11 11:00:00	Hi folks. When I save a .jpg image, that is already on my pc (eg from a Kodak still, or 'grabbed' video frame etc), via MS Photo Editor the file size is reduced to about 10% of the presaved version (eg 721KB becomes 74KB). Can someone please explain in simple terms what happens to the image? Is there a loss of quality? They look the same to me. How does it work? Thanks in advance. Peter.	Guest (0)
38758	2002-03-11 11:19:00	Pictures from devices usually have irrelevant information included in the jpeg like the device that took that picture and the version drivers you used for it. I guess the graphic editor removes this type of information. Depending on the quality setting you can lose quality for a smaller sized jpeg. Some graphic editors may not tell you that quality maybe effected but Photoshop does and the quality is still good.	Guest (0)
38759	2002-03-11 12:05:00	I'm not sure quite what you're asking Peter, but if you want to know what happens to a jpeg image during the save process, then read on. If not, just ignore this. The program creating the jpeg is analysing the information in the picture and determining the amount of redundant data that the picture contains. Wherever it finds that data, it replaces it with a token describing the redundant information. For a simple example, suppose the image contained pixels of a certain colour we'll call 'red', and there are 150 pixels of this colour. An uncompressed file might store this information by writing 'red' 150 times. The jpeg program instead writes one token that says '150 red', rather than 150 references to 'red'. So the information stored requires a significantly smaller file size than if it were not compressed. Also, the jpeg format allows the user to be more, or less, aggressive in how much data is compressed. For instance, using our red analogy once more, on a less aggressive compression, 'red' means exactly that, whereas on a more aggressive compression, the value of red could be expanded to colours a little each side of red; including, say 'orangey red' on one side and 'purpley red' on the other. That allows the number of pixels of our picture represented by our 'red' token to be increased considerably without increasing the size of the token, resulting in a much greater compression. And I'm sure you'd agree, much lower colour accuracy. (By its very nature, though, a jpeg file will *always* be compressed). This same 'token' approach applies to position and other file details, not just colour. When you save to a jpeg, you determine how much loss of accuracy is in the final file by selecting a value, normally from 1-12, with the small number representing the smallest file size and the lowest quality, and vice versa. Hope that wasn't as clear as mud, and it's actually what you wanted to know. :-)	Guest (0)
38760	2002-03-11 20:41:00	Hi Kame & Andy Thanks for your explanations. I didn't say very clearly that the original image was a *.jpg then saved as a *+.jpg without actually changing the picture. So if I continualy save, then save, then save etc the file-size will continue to reduce in value and I am left with the same picture? I need to email the best quality image with a reasonable size file. Is .jpg better than .bmp or .tif for this purpose? Peter.	Guest (0)
38761	2002-03-11 20:44:00	Hi Kame & Andy Thanks for your explanations. I didn't say very clearly that the original image was a *.jpg then saved as a *+.jpg without actually changing the picture. So, if I continualy save, then save, then save etc the file-size will continue to reduce in value and I am left with the same picture? I need to email the best quality image with a reasonable size file. Is .jpg better than .bmp or .tif for this purpose? Peter.	Guest (0)
38762	2002-03-11 21:31:00	In simple terms each time you save a jpg file image information is trimed out, supposidly the image information that your eye can't detect. Each time you save as jpg more information is lost making the file smaller, but only up to a point and you will begin to notice the degrading image. In the better image editors there is a tool called jpeg artifact removal to counter this. I would reccomend saving .PNG which compresses in the same way as ZIP files are and looses no image detail.	Guest (0)
38763	2002-03-11 22:33:00	If it's only going to be viewed on a monitor, and not printed out jpeg is fine. If you want to email an image, that is going to be printed, then eps is a good way to go, as long as you have the necessary program to open it. eg Photoshop, Pagemaker, etc.	Guest (0)
38764	2002-03-12 00:33:00	Something I forgot to mention. If you decide to switch to PNG yo can open and use PNG file with any image editor except windows paint. You can even use MS photo editor that comes with office	Guest (0)
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