Go Back   Steve's Digicams Forums > Digicam Help > General Discussion

Reply
 
Thread Tools Search this Thread
Old Dec 28, 2002, 8:50 AM   #1
Senior Member
 
Join Date: Sep 2002
Posts: 659
Default Edge distortion on Internet image

The following unmodified image shows, to my eye, a distinct edge distortion at the top of the picture that I can only describe as "puckering". This dosn't show in the original image from the camera, in prints or even in the Photoshop "Publish for Internet" preview. I suspect it relates to the contrast between the subject and the background but I'd appreciate some advice on whether it can be fixed. This isn't the only time that I've found this problem....

checklg is offline   Reply With Quote
Sponsored Links
Old Dec 28, 2002, 9:22 AM   #2
Senior Member
 
Join Date: Aug 2002
Posts: 1,910
Default

The only thing I see is limitations of the flash, in darkening around the edges of the photo. You're the only one who has the benefit of having the original image.

There's always going to be some distortion between the original and resized image as you are removing pixels, and also depends on how much JPEG compression you are using.
Mike_PEAT is offline   Reply With Quote
Old Dec 28, 2002, 9:39 AM   #3
Senior Member
 
Klaus DK's Avatar
 
Join Date: Aug 2002
Posts: 2,216
Default

I agree! You could try to use the BLURTOOL in PS and carefully draw on the line, maybe use the LIGHTENTOOL on the line first.
Klaus DK is offline   Reply With Quote
Old Dec 29, 2002, 7:21 AM   #4
Senior Member
 
Join Date: Sep 2002
Posts: 659
Default

Thanks for the replies. I've been doing some more experiments with another image and I've concluded that the distortion seems to relate to a combination of modification of contrast etc in the original image and the subsequent re-sampling logic applied in Photoshop.

Compare -

Bicubic - www.pbase.com/image/9750652/original.jpg

Bilinear - www.pbase.com/image/9750684/original.jpg

Nearest Neighbour - www.pbase.com/image/9750700/original.jpg

They are all "70 percent" JPEGS from a Photoshop 2.0 file with slightly modified contrast, lighting level and a light application of unsharp mask. The distortion disapears in the last one, but at the cost of an increase in background "grain" of the shot.

I believe I've reduced this grain on the fourth version. This was by leaving the original image from the camera completely unmodified - simply cropped, resized using Nearest Neighbour and then saved for the web at 70 percent.

www.pbase.com/image/9752120/original.jpg

A similar result can be achieved by applying Bicubic to the unmodified image.

By the look of things I'm creating an artifact in the image that is only visible over the Internet......
checklg is offline   Reply With Quote
Old Dec 29, 2002, 4:07 PM   #5
Senior Member
 
Join Date: Aug 2002
Posts: 2,162
Default

I noticed your 640x423 test image offers 'save as bitmap'. It occurs to me that for small web images, uploading uncompressed bitmaps aligned to multiples of most monitor settings (say 800x600) could avoid much of the argument about JPEG. The gain of JPEG is for posting large images.

I remember doing similar tests to post an image from an old 1Mpix cam, finding uncompressed BMP was superior, with not much increase in file size, for smallish images, reducing the number of colour samples say from 32 or 24 to 16 helps as well.
voxmagna is offline   Reply With Quote
Old Jan 12, 2003, 10:11 AM   #6
Senior Member
 
Join Date: Sep 2002
Posts: 659
Default

Doing some further tests in the new year this problem also seems to be related to the type of monitor. I normaly use a Packard-Bell LCD, but on a Compaq V70 the problem disapears entirely !

As a result I'm now posting to a test gallery and then checking out on both monitors before deciding whether to keep the image on-line.
checklg is offline   Reply With Quote
Old Jan 12, 2003, 11:10 AM   #7
Senior Member
 
Join Date: Aug 2002
Posts: 2,162
Default

If monitor 'convergence' or 'pin cushion' is the problem, move the edges of your pic to the central area of your monitor where there should be less (but not always) errors.

If you need to check this out, I once used Corel Draw to produce a test file image 'cross hatch', consisting of many (use repeat tool) horizontal and vertical lines with equal fine spacing. Reduce the brightness and you can expect to see colour fringing and defocussing at the corners, but it's all a question of degree. Watch what happens as your monitor warms up from cold as well!

Health warning: don't try this test on your pride and joy mega bucks monitor, you might see worse than you expected!
voxmagna is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 6:57 PM.