Hi Folks,
I have a little query which someone may be able to clarify.
Background: I download photos off my digital camera. These are 2304x1728 (4:3) JPEGS, 8pits/pixel, around 2MB in size.
As well as storing the originals, I also re-size them to 1024x768 (also 4:3, linear reduction by factor 2.25) because (a) at that geometry they exactly fit my screen, (b) they then come out much smaller in byte-size, convenient for emailing for instance.
I do the re-sizing as follows, in ImageMagick:
1. display photo.jpg 2. View --> Resize --> 1024x768 --> Resize 3. File --> Save --> new filename --> Save
and here's where my query comes. At this point I get a popup window which asks me the "Choose JPEG quality" with default option 75%.
I've wondered how much this "quality" matters, so I've done a bit of experimentation, saving the re-sized file both at 75% (the default) and 100%. Then I put both of them up (on different "workspace" panels) and flip between them.
Try as I may, I can't visually perceive any difference between them.
Yet, for example, one 75% file has 171064 bytes, and its 100% counterpart has 694833 bytes, 4 times larger.
At that difference in byte size, I'm surprised that there's no perceptible difference!
Any ideas?
With thanks, Ted.
-------------------------------------------------------------------- E-Mail: (Ted Harding) ted.harding@nessie.mcc.ac.uk Fax-to-email: +44 (0)870 094 0861 Date: 11-Mar-07 Time: 18:00:49 ------------------------------ XFMail ------------------------------
Ted,
On 11 Mar 2007, at 6:00 pm, (Ted Harding) wrote:
Hi Folks,
I have a little query which someone may be able to clarify.
Background: I download photos off my digital camera. These are 2304x1728 (4:3) JPEGS, 8pits/pixel, around 2MB in size.
As well as storing the originals, I also re-size them to 1024x768 (also 4:3, linear reduction by factor 2.25) because (a) at that geometry they exactly fit my screen, (b) they then come out much smaller in byte-size, convenient for emailing for instance.
I do the re-sizing as follows, in ImageMagick:
- display photo.jpg
- View --> Resize --> 1024x768 --> Resize
- File --> Save --> new filename --> Save
and here's where my query comes. At this point I get a popup window which asks me the "Choose JPEG quality" with default option 75%.
I've wondered how much this "quality" matters, so I've done a bit of experimentation, saving the re-sized file both at 75% (the default) and 100%. Then I put both of them up (on different "workspace" panels) and flip between them.
Try as I may, I can't visually perceive any difference between them.
Yet, for example, one 75% file has 171064 bytes, and its 100% counterpart has 694833 bytes, 4 times larger.
Bear in mind that monitors are at best, 72dpi so the perceived difference on screen may not be as noticeable as it would be if you were to print it out. 75% is probably a good place to start, but I would then play with the quality to get the lowest size (if diskspace is an issue) to the best on screen quality. I guess it also depends on whether the people you email the photos to want to print them out or not.
Hope that helps a bit,
Dave
On 11-Mar-07 David Reynolds wrote:
Ted, [...] Bear in mind that monitors are at best, 72dpi so the perceived difference on screen may not be as noticeable as it would be if you were to print it out. 75% is probably a good place to start, but I would then play with the quality to get the lowest size (if diskspace is an issue) to the best on screen quality. I guess it also depends on whether the people you email the photos to want to print them out or not.
Hope that helps a bit,
Dave
Interesting point! Thanks.
As it happens, I'm planning to have some of them printed (using the original full-size files), so I'll take the opportunity to print a couple of reduced ones at both 75% and 100% as a comparison.
Best wishes, Ted.
-------------------------------------------------------------------- E-Mail: (Ted Harding) Ted.Harding@manchester.ac.uk Fax-to-email: +44 (0)870 094 0861 Date: 11-Mar-07 Time: 18:42:22 ------------------------------ XFMail ------------------------------
(Ted Harding) ted.harding@nessie.mcc.ac.uk wrote: [...]
Yet, for example, one 75% file has 171064 bytes, and its 100% counterpart has 694833 bytes, 4 times larger.
At that difference in byte size, I'm surprised that there's no perceptible difference!
Any ideas?
JPEG uses a discrete cosine transform so that the early waveforms account for as much of the picture data as possible. As you move closer and closer to 100%, each extra wave adds less and less visible.
Secondly, the quality scale is neither linear or standardised.
Thirdly, screens might not show all the detail in the picture. When I got a better screen a few years ago, I was amazed how many ripples I noticed in images. Also, decoder performance varies massively. Never use JPEG on the web if hard edges are important, like on most company logos. It's great for photos, though.
Finally, the JPEG FAQ is online: http://www.faqs.org/faqs/jpeg-faq/ or there's more detail in Part 2, question 75 of the comp.compression FAQ http://www.faqs.org/faqs/compression-faq/
Hope that helps,
-
David Reynolds -
MJ Ray -
ted.harding＠nessie.mcc.ac.uk