jpeg default quality for saving

[doug]

Mike ....
I guess we both received value from this thread.
I have never posted an image to this or any other board. My downsizing has been limited to e-mailed photos. I just assumed that downsizing to a max number of pixels was sufficient. But now that I am aware of the 50Kb file size constraint, I can see that a combination of pixel dimension AND save quality may be required to "fit" within the constraints.
In my original post, I asked for enlightenment on my "pixel dimension only" approach, and I certainly got that.

Doug

[den]

Somethings to keep in mind when PWP downsizing [Resize] images and setting jpeg quality for file size saves:
(1) PWP's default resampling is Bicubic with 16.7% Sharpening. This sharpening is different from the Sharpen transform in Sharpen mode. See JS's Resampling white paper [ http://ftp2.bmtmicro.com/dlc/Resampling.pdf ].
(2) Bicubic downsizing will soften the Resized image, so sharpening should be performed before the jpeg quality save to restore 'crispness'.
(3) PWP4 and PWP5 use different jpeg libraries so file sizes will differ for the same jpeg quality setting. It appears that the file size will be approximately same for a 98% quality jpeg save in PWP4 as a 95% quality jpeg save in PWP5.

While other software is available, consider developing a PWP workflow for batch downsizing of images to fixed pixel boundries and specific jpeg quality saves.

A suggested 'message board image posting' Workflow is:
(1) Resize widget whose Operation is 'Bounding rectangle'; Width is 400; Height is 400; Method is Bicubic; 'Maintain Resolution', and Sharpening is 16.7%.
(2) Sharpen widget whose Method is Sharpen and Amount is 62.5%.
(3) Rename/File Save widge whose Output File Type is JPEG; JPEG Quality is 95% [PWP5] or 98% [PWP4]; Rename and Destination Folder set to preferences.

While the above suggested workflow will not always result in jpeg file sizes less then 50KB, it should be close depending upon the amount of detail contained in the image file being processed.

For files exceeding 50KB, the workflow could be modified to have a tiff Output File type, then manually 'File Save As' to jpeg with subsequent reduced quality until a below 50KB file size is realized.

[MikeG]

Den,

I've used your idea of using a workflow to prepare images for posting to this forum for my last two posts. Nice not having to worry about resetting my default jpeg quality setting.

I've also used BD Sizer for batch resizing and squeezing into 20kb ID photos for a mobile phone. BD Sizer doesn't have 'bounding rectangle, which is a great feature of resize in PWP5.

Mike.

[keithrj]

Mike, BD Sizer has an option to "Resize longest side to" This should do 'bounding rectangle' as you require.

[MikeG]

Thanks, Keith, that's the setting I've been using.

Mike.

[MikeG]

Another consideration to bear in mind when trying to cram as much image quality as possible into a small file size is the EXIF data.
BD Sizer strips this out by default.
I think that unchecking 'Include profile' and 'Include comments' in the Jpeg options in the File Save dialog in PWP 5 has a similar effect.

Mike.

[MikeG]

An afterthought to my last post:

As Den suggested, best to use a workflow if stripping out the EXIF to avoid changing the default File Save settings - and then forgetting to undo the changes.

Mike.

[tomczak]

There is a portable freeware called RIOT, which can be used to optimize size vs. quality of jpg, png and gif. One particularly useful feature of it, other than preview of the output image, is the ability to set the desired JPG size, which is than accomplished by the programme automatically adjusting the JPG quality parameter. Being able to preview the result in 1:1 and manually readjusting the compression parameters makes it quite useful a combination.

When you watch it working, it appears that this is done interactively - would anyone have an idea how this is accomplished (e.g. is a file compressed to jpeg, then the final size is estimated, quality parameter changed, then the file compressed again until the the given size is reached?).

http://riot.en.softonic.com/

p.s. I haven't checked how good the programme really is in all other aspects: JPG quality, subsumpling, Exif, ICC, etc. I'm not even sure if it handles 16bit files, but it seems to be able to read a lot of formats.

[MikeG]

Here's a link to the Author's web site:

http://luci.criosweb.ro/riot/

Mike.