prepping files for forum display

[stormy]

Maybe you have one of these already, but I don't know where to find it...

How about a sticky (so it doesn't get lost in an old thread that originally started on another subject) on "How to Best Prepare Your Files for Viewing on f295"?

The only hard and fast rule seems to be "Make it less than 100K", but this is still a big file and I've seen Tom ask many many times that people make their files even smaller than that.

I'd love to oblige, but find that compression leads to visible blocks and jaggies in the image. Other people don't seem to have this problem - their files look fine at small sizes, so obviously I'm missing something, but I would really like to learn.

Cheers,
-=S=-

[taco]

I have the same problems with the "make it less then 100K" but I always thought that it's the bad quality of my scanner in the first place preventing a good resize. I'm happy to hear that I'm not the only one.

[ImageMaker]

Most of my images suffer pretty badly if I try to make them much smaller than 90k -- the plethora of detail means they don't compress well with JPG, and decreasing the quality setting just means throwing away some of that detail (which is hard enough to get into the file at a reasonable pixel count in the first place that I'm not very happy to waste it).

Important to remember that an image with big swatches of softly gradated tone or color compress to a much smaller file at the same quality setting, and look better at lower quality, than a file with a bunch of fine detail. Hence why Narsuitus's image looked good at 30k and a reasonable pixel count, while some of mine are difficult to bring in under 100k at a pixel count big enough to see what's going on. Hundreds of twisty little branches and vines and bark scales and so forth just don't compress well, even in B&W (which, other factors equal, ought to produce about 1/3 the file size of color).

[moot]

For what it's worth, my method is to use PS "image size" to set the resolution to 96 pixels/inch (about 38 pixels/cm) and then set the long dimension of the image to around 6 inches (15 cm) or less. I then save the result as a jpeg (quality around 9, usually), and the file size always comes in under 100k, but sometimes not by much. More often the result seems to be something like 50k -60k if my (lousy) memory serves.

When I resized John's image I set it to 7" because I liked the image enough I thought it deserved to be as big as possible. The file ended up tiny anyway.

[gneissgirl]

Stormy-
I'm sure Tom has removed them now, but my very first posts were HUGE! I had no idea what I was doing. I think it was Earl who gave me this tip:
Do you ever use photoshop elements? It has a command called "Save for Web" which allows you to save your image as a .jpg of high, medium, or low quality, and at the same time to determine the dimensions, in pixels, of your compressed image. I usually choose medium quality and 400 or 450 pixels for the longest dimension of my image. I normally end up with a compressed image of 25 to 50k, and they seem to look OK.

My original 4 x 5 B&W images are sometimes 30-50M (depending on the resolution use to scan), but this normally gets them down to under 50k with no trouble.

[stormy]

Does the size/dimensions/resolution of the original scan matter at all? For example - should the scanned density (ppi) be a multiple of the desired result? Does that make a difference?

[dvoracek]

the size times the resolution yields the number of pixels.

2 inches at 100 pixels per inch will be 200 pixels.

the size and resolution doesn't really matter to the browser, only the dimensions in pixels are taken into consideration when it's displayed.

Nick

[stormy]

Thanks, but I guess my question wasn't clear.

Does it make a difference if you go from an original scan at 360ppi to 72ppi (a factor of 5), as opposed to 300ppi to 72ppi (a factor of 4.1666667) Will this make a difference in the dithering, smoothness, what-have-you? (72ppi being used for discussion purposes only.)


Tom P - if you're reading this, I still think it would be helpful to have guidelines/suggestions as a sticky (or something similar) somewhere so that people can find them more readily.

[ImageMaker]

Does it make a difference if you go from an original scan at 360ppi to 72ppi (a factor of 5), as opposed to 300ppi to 72ppi (a factor of 4.1666667) Will this make a difference in the dithering, smoothness, what-have-you? (72ppi being used for discussion purposes only.)

It depends on your software and settings. I usually use GIMP for this step, and I have it set to use bicubic interpolation in resizing; this gives a very smooth result. I think it also helps to have considerable oversampling; if you're going from, say, 150 ppi to 72 ppi, being a little off a clean 2:1 will cause considerable artifacting, but when you're reducing by a factor or around 10 on length, as I usually am after scanning a 4x5 at 600 ppi, or a 6x6 cm at 1200 ppi, the number of original pixels that get combined into a final pixel is enough that artifacting shouldn't normally be a problem.

So, I guess for resizing purposes, you want your original scan to either be exactly your final size (so no resizing required), or at least 4-5 times the final size (in each dimension), to avoid bad effects from resizing, and then you want bicubic interpolation to get the best quality reduced image -- and then I find I almost always like about a 30 (40, if from 4x5) sharpen filter applied after the resize to restore the "look and feel" of the original image.

[buggy]

For what it's worth I use PS and save my images in 'Save for Web'. I save as a jpg and usually it's set to 40% quality. I go to image size and set the percentage so that the long side is around 580. Usually this is around 10-20%. That way the entire image will show on the screen without the need to be scrolling around. By doing this I usually end up with around a 20-30K image.

The image always displays at 72 ppi on the computer screen so a setting other than 72 ppi is not necessary.

[ImageMaker]

The image always displays at 72 ppi on the computer screen so a setting other than 72 ppi is not necessary.

Buggy, my screen is at 96 ppi -- 17" monitor with 1152x864 resolution (highest pixel count that doesn't cause eyestrain from small text and icons, and lets me keep 75 Hz scan rate). If my eyes were a little younger or my video card a little newer, I might run at 1280x960 (the more common 1280x1024 has non-square pixels on a standard 4:3 screen), which would be, I think, about 102 ppi.

[buggy]

ImageMaker, Oh, I didn't know that. I'm on a 3 year old laptop so I'm not up with the latest technology. Maybe that's why my eyes are going bad...:)

[stormy]

Hey, IM - thanks for the good advice!

[murrayatuptowngallery]

I think SVGA has been 96 dpi for many years, but books still talk about 72 which was VGA.

There's an article on the web somewhere (Photo.net maybe) that has suggestions for scanning higher than needed resolution and how to do unsharp mask. I can't remember if it's before or after downsizing...I think before, so the reduction doesn't do as much 'harm.

This may offend purists, but compression to put online is hardly 'au naturale'.

[ImageMaker]

Generally, a USM is best applied with the maximum of existing information (i.e. before resizing) and then a simple sharpen filter (a simpler algorithm that uniformly boosts local contrast) applied after resizing. Some folks like to resize in steps and sharpen after each step, but I don't see the utility in that; if the original scan is genuinely sharp to begin with, I have in the past often not bothered to sharpen at all.

I've also noticed that if I sharpen more and then use more JPG compression (i.e. lower quality setting) to get the same file size, I get almost no visible difference in the image -- with JPG, a given amount of information (in the form of what's where) will give a certain file size, amost without regard for how you got there (except that if you try to use JPG compression to make the file too small, the artifacts of the wavelet compression algorithm get annoying).

[callum moffat]

I USE RESIZE IMAGE (UNDER THE HELP DROP DOWN IN PS) - CHOOSE ONLINE DISPLAY-THEN MAKE PIC 500PIXELS ALONG ITS LONGEST EDGE- PRESS FINISH. THEN CHOOSE SAVE}SAVE FOR WEB AND SLIDE THE QUALITY SLIDER ALONG AND BACK UNTIL THE SIZE IS UNDER 100K THEN FINALLY SAVE AS } AND YOUR FILE NAME

MY ORIGINALS ARE ALL TIFF FILES, DONT KNOW IF THAT MAKES ANY DIFFERNCE TO RESIZING :-/