Re: Lost Bytes with rotatation in Windows Photo and Fax viewer
- From: "Michael J. Mahon" <mjmahon@xxxxxxx>
- Date: Sat, 26 Apr 2008 10:15:59 -0700
Yves Alarie wrote:
If you open a photo file to view it and then close it without making any change, the size of the file will not change.
However, if you make a change such as rotation, and you close the file the compression software on your computer is more efficient than the compression software in your camera and when "saving" the change the file size will be slightly smaller but there will be no loss in quality (the term used is lossless rotation).
If you make other changes such as adding text, removing red eye, etc. and you save the file there will "some" loss in quality since the file will need to be compressed again to save it. However this loss in quality is not something you can detect. So no need to worry about it. It is best to make all your changes and then "save as" and give another name so you still keep the original file and best to do all your editing so you will do only one "save as", but don't worry about two or three editing of the same file.
You can read a lot more details about JPG and how this compression work at this site:
http://en.wikipedia.org/wiki/JPEG#Color_space_transformation
"Scott092707" <Scott092707@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message news:0D921179-F9D1-44A1-B1B5-9825EA945DC8@xxxxxxxxxxxxxxxx
Most image rotation algorithms now try to be "lossless", in the
sense that they do not actually decompress, rotate, and recompress
the image, but simply perform a "transpose" operation on the compressed
data, and on the 8x8 pixel macroblocks that comprise the compressed
data.
This operation can only be done perfectly if the pixel dimensions of
the image are divisible by eight in both directions. If this is not
the case, then a few rows or columns of data must be discarded from
the edge(s) to make the image conform.
This will result in a small change in image size, but, once done,
the image dimensions will now be a multiple of 8 pixels and no
further loss should occur if additional lossless rotations are
performed.
The reason for preferring this approach is that JPEG is a lossy
compression algorithm, and, though decompressing and recompressing
an image without alteration using exactly the same compression
parameters does not cause information loss, any change in either
the image or the compression (particularly increasing the compression)
will, in general, cause some loss of information. This loss is
usually practically invisible under normal conditions, but it is
a good reason to avoid sequential decompression, modification, and
recompression cycles on images.
If you need to make multiple, separate modifications to an image,
try to make them either all in one session, with only one decompression
at the beginning and a final re-compression at the end, or use a
lossless compression for storing the intermediate results, like TIFF.
Also, it is good practice never to save a modified image "over" the
original image, since you may wish to return to the original to re-do
some processing.
-michael
Thank you, Yves.
You have relieved me as to potential loss of quality in the print.
And taught me an interesting rule-of-thumb.
I am still curious, tho' :
a)why the pixels initially disappear,
b)why they do not subsequently do so,
and c) why the size bounces back and forth by 5 bytes
depending on the orientation.
(I'm ALWAYS curious...)
"Yves Alarie" wrote:
At 1,823 KB file size you have more bytes than you need a loss of 44 bytes
is nothing.
But really, this is not the way it works for printing, the file size
obviously reflects how large a print you can get but the real numbers you
want are the pixel dimensions.
The "rule of thumb" is if you want a great print you divide the pixel
dimensions by 300.
At 1,823 KB file, I am guessing that the pixel dimensions will be something
like 2048 x 1360. So you can get a print of 2048/300 = 6.8 inches by
1360/300 = 4.5 inches. Now you can reduce 300 to around 200 and still get a
very good print.
Take a look at you file size in a particular photo folder. For a particular
camera, the pixel dimensions will always be the same for all photos in the
folder, but the file size will vary according to how much information you
have in each image. The extremes would be taking a photo of a plain black
and white object (photo # 1) and a landscape photo with lots of details and
colors (photo #2). If you look at the size of the files, photo #1 would be
very small while photo #2 would be very large. However, if you look at the
pixel dimensions, they would be exactly the same for both photos.
To find the pixel dimensions of your photos, just place your mouse pointer
over a thumbnail or file name. A box will open and Dimensions will be the
first item listed. Another way to get it is to use the Details view for your
folder. Open the folder in Details view and right click on the Name column
header, a list will open, click on More and then check Dimension to add this
info as a column.
The bottom line is: pixel dimensions control print quality, not file size.
--
-michael
NadaPong: Network game demo for Apple II computers!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
tool--and it's seriously underused."
.
- References:
- Lost Bytes with roatation in Windows Photo and Fax viewer
- From: Scott092707
- Re: Lost Bytes with roatation in Windows Photo and Fax viewer
- From: Yves Alarie
- Re: Lost Bytes with rotatation in Windows Photo and Fax viewer
- From: Scott092707
- Re: Lost Bytes with rotatation in Windows Photo and Fax viewer
- From: Yves Alarie
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