A few scanning tips

What does JPG Quality Losses mean?   What are JPG artifacts?

The next images (a small crop from a snapshot on a cruise ship) are saved with JPG compression varying from 20 to 100 JPG Quality, and also one PNG (lossless compression). TIF would look the same as PNG (both are lossless), but browsers typically don't show TIF files. The JPG files were saved with Irfanview 4.33 menu File - Save AS - JPG. IMO, Faststone 4.6 gives same results and sizes, no differences I can notice - guessing that they may possibly share the same code library. However, the numbers are not absolute or unique, and since there are several parameters involved with JPG compression, different programs typically use different option parameters. The details are not just a single number, so the same numerical value is not necessarily the same exact effect in different programs. Irfanview JPG Quality runs 1..100, but Adobe Photoshop has two scales, 0..12 in Save As (more conservative, less compression), and another 0..100 in Save For Web, which is different itself. Generally if using JPG for your good photo images, do start at a high value, like 9 or 90.

To know and understand JPG losses, we of course must learn to recognize JPG artifacts. There are a couple of types of JPG artifacts. One is 8x8 pixel blocks seen in smooth featureless areas (pixels made to be the same one color in the block, which compresses better). Another is harsh artifacts around sharp edges. Due to these artifacts, some pixels now have different color values. Viewing at 3x actual size helps to learn to see JPG artifacts. They are real, regardless if you are aware or not.

This image is a small 300x300 pixel 100% crop (actual size) from a 4288x2848 12-megapixel RAW image (Nikon D300). All are of course identical copies of the same one pristine image (from RAW, it had never seen JPG before), just with different text and JPG compression added. A PNG image is added for comparison, because PNG uses lossless compression, no JPG artifacts (and browsers can show PNG).

This is how the images came out of the JPG files (but enlarged now, by interpolation, to recognize it easier). All were the same one image going into the files. The differences (between what we put in, and what we get out), is called "losses", due to JPG artifacts caused by lossy compression. Lossy compression means the process is allowed to change the data a little (the RGB data is the color of each pixel) to make compression more effective, simply ignoring difficult details - and the file can become quite small that way. There are of course diminishing differences with higher Quality, but the PNG just looks sort of "pure" (lossless). The added text is all the same font and the same identical operation for all, but PNG just comes out more pure. In contrast, lossless compression cannot be as effective making a tiny file, because it must honor higher standards, and absolutely insure no pixel color value is altered in any way - what comes out of the file is an exact copy of the data that went into the file.

The examples above show what is meant by JPG losses. It is about losses of quality - meaning, the pixels that come out of the file are simply not as good to look at as what went into the file. The image still has the same count of pixels, and the size is still three bytes per pixel when opened again, etc, but now some pixels have altered colors, called JPG artifacts... like dirty pixels, which is quality losses. Remember, artifacts continue to accumulate, getting worse, every time you edit and Save the file as JPG again. Each Save is lossy compression again. And this loss is not repairable, so be aware, don't do this repeated times. There are better plans.

Make no mistake - JPG 100 is NOT 100%, and it is NOT an unaffected image. It is always still JPG, and JPG 100 is NOT a perfect copy. Above is the pixel difference in the original JPG 100 image and the PNG image, as shown by the Adobe Photoshop CS5 Calculations tool, when comparing only the blue channels (red channels look essentially the same as this, but the green channels are pretty dark in this one). This is enlarged some, but is shown at default brightness, the actual difference, no fudged adjustments. Seems to me the differences are in the anti-aliasing on the edges of objects (almost like a line drawing). These were all the same image with the same pixels, until compression, so my notion is that JPG apparently didn't consider saving all of these infrequent colors as important. Faststone has a menu that counts colors in images, and reports the PNG image has 35,046 colors, and the JPG 100 image has 23,368 colors (only 2/3 as many now.) There is more color count in the lower JPG values, which are the artifacts probably. Photoshop JPG does the same reduction of colors, in about the same degree. The JPG 100 file, when saved again now as PNG, is nearly 5% smaller than the original PNG file, because its modified data is easier to compress now. JPG 100 is a relative number, it does NOT mean 100%, and it is NOT lossless (but yes, 100 is still pretty good).

If you start your comparisons from an original JPG image, you may not see this much difference remaining (you have no base of reference then), but remember, the file here started out as RAW, it had never seen JPG before. This may be minor visually, but there is no way we can argue that they are still identical images. We might argue "good enough", and Quality 90 probably is good enough (mostly numerical differences, we have to look harder to see visual artifacts). However, if this is the master copy of our prize image, we may not be willing to consider any argument at all about quality. But JPG Quality 100 is pretty good, and the file sure is small. I would have no concern about sending the JPG 90 or 100 images out to be printed (printers gloss over small details too), but not to keep as my archived master copy. The actual worry is about creating more differences next time it is saved as JPG. JPG artifacts accumulate at every Save as JPG (compressed again at every Save).

The only point here is that even JPG Quality 100 is definitely always still JPG, and it is never going to be lossless compression. In contrast, lossless means every pixel's data value (color) comes back out of the file exactly the same as what when in - no artifacts. That's always a good thing, if quality is a major concern. If you have read this far, I assume you are concerned about image quality.

Speaking on a 1..10 scale (Adobe), JPG Quality 4 might sometimes be "good enough" for some unimportant web page images, especially back in dial-up days. We could debate it, but Quality 10 may Not be good enough to archive your prized best pictures, at least not for mine. At bare minimum, for your good stuff, 1) you should always use a high value of JPG Quality, if not 10, at least 9. And 2) you should minimize any occurrence of additional Saves as JPG (each Save compresses again, and losses accumulate). When emailing pictures of the kids to Grandma, sure, resample a small copy for video, and JPG 8 is good enough for email. That is an expendable copy, but archives should be the good stuff.

The degree of compression varies with the saving program, and also with image scene detail. Sort a large folder of camera images by file size, of many varied JPG images all of same original camera size and size settings, and these will vary maybe over a 2:1 size range (depending on scene content). Large blank areas, like sky and walls, compress much smaller than extremely detailed areas, like a tree full of leaves. That said (different scenes vary in file size, and programs vary too), and so speaking very generally, the average JPG file size ought to be at least 10% of the data size to be "decent". Use at least JPG Quality 9 for your good stuff. I would consider High Quality as being 20% of data size (24-bit data size is three bytes per pixel). If you ever saved it ONCE at lower quality, lower is the best you've got now. The larger you can make the JPG file, the better quality the image will be (and the largest JPG is still tiny, compared to the size of the data). And BTW, another factor, excessive USM sharpening is another factor that can also cause false edges, and sharpening aggravates the JPG artifacts around sharp edges. If you want the smallest file with less artifacts, blur it the slightest amount before saving it.

JPG Quality 10 is pretty good indeed, but it is still JPG. JPG is used where small file size is more important than absolute image quality, like web pages or email, or small memory cards. Conversely, if absolute image quality is important, why consider JPG at all? JPG Quality 8 to 10 may be "good enough" for most "viewing" uses, EXCEPT there is always the distinction that JPG is not good for editing, involving saving repeated times - which accumulates and compounds the JPG artifacts each time saved as JPG. Your images are either important or not, and maybe absolute quality is not your biggest concern, but if we take liberties, experience knows the time will always come when it will matter. We cannot undo JPG damage.

There are always ifs and buts, difficult to quantify. There are lossless methods to rotate or flip JPG images, rearranging and rotating the 8x8 pixel blocks without uncompressing and recompressing (Irfanview plugins offers this), so there are no additional losses in those special lossless cases. Applications like Photoshop take heroic pains to try not to recompress image areas with no change (when possible, still has to be the same Quality level too). But bottom line, saving JPG again is a little like Russian Roulette - every time may not get you, but there certainly are risks, and when it does bite us, it always seems at the worst possible time.

Some people really never notice much difference, so then about anything is "good enough" (until it bites them bad, the only copy of their prized image is now full of JPG artifacts - from repeated edits and saves, and/or due to specifying a Quality factor too low). There are other more critical photographers (we all probably have been bitten before, once, at first) thinking that even the very best quality is barely sufficient for THEIR images. We cannot reason with either group. :) Certainly we cannot have the same discussion with both. Risk of course depends on how important your images are, and there are times and places for both approaches, but I lean towards the latter group. Photography is too much time and money invested trying for greater quality, so I really don't see any reason to intentionally select less than maximum quality. $200 for a huge disk is nothing compared to the (tens of) thousands of dollars of photo gear (plan on disk backup too). Certainly there are always a few exceptions, but regarding my more important images, I'm not interested in any debate at all about how little it might hurt. Why accept any risk? Internet transmission speed certainly is a size consideration, but which does not involve any of my archived master copies. Today, inexpensive disk space rules out any reason to compromise my image quality, certainly not my original copies. Indeed, there are better ways to go about it.

The Best Plan When We Need JPG

For those who critically care about their images, the best plan, and actually, the easy way, is first, to always keep and archive your unedited original image as the best master you will ever have. Otherwise, you can never get it back, so never write over it - always keep that original intact for just in case (whatever it is, especially if it is a JPG from the camera - and the camera should of course be set to create the finest possible image it can.)

Then second, when editing (saving only to a copy with a different file name, always preserving the original intact), always save your in-work image ONLY to a lossless format (TIF or PNG), EVERY time, UNTIL your last necessary FINAL one JPG save (at high quality level). For example, Photoshop, and the free editors Irfanview and Faststone (see Google), have batch modes to copy many files from JPG to TIF in one easy operation. This would be pointless unless you are intending to edit them, because this TIF step will NOT remove any existing JPG artifacts, the data will of course still contain those original JPG artifacts - but subsequent TIF Saves will not add any more. Computers and disks are big and fast and inexpensive today, this larger file size is a small issue today (and that is in fact simply how big your data is). Then as lossless TIF files, you can edit away, red eye and color and contrast adjustments, straightening and cropping and resampling, and saving again with wild abandon (just NOT as JPG), until finished, at that needed One Final JPG Save. So if it was JPG previously, now the total is only two saves as JPG (original camera save, and this one final save, is two), which is worse than one, but much better than six. Using camera RAW images would also eliminate the first JPG, and offer other advantages too.

And then third, when if any further need to edit it again comes up (resample size, whatever, anything requiring another Save As JPG), discard this second JPG (as an expendable copy), and start over from your better archived master you kept. Avoid using any image saved repeated times as JPG. JPG is lossy, which means we do not get back the same quality we put in. There are more losses every time it is saved as JPG. With lossless formats (PNG or TIF LZW), it does not matter if you save a jillion times. But it matters if saving to JPG. An extra unthinking save as JPG is never a good plan. And if it overwrites your only original copy, it is a terrible plan, you can never get it back.

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