Artefacts

[andybarton]

Do you resize at 72 dpi before posting?

[jaapv]

Good point, William. I am actually unsure whether it should be before or after colour space conversion, that being a non-destructive move. I always convert to 8-bits as last step before saving, just to be sure. There is certainly a quality difference between a 8-bits and 16-bits reduction imo, and I feel sharpening gives more smooth results as well.

[sthan]

Hi, Andy,
Never resized at 72 since I post on the forum.
In the same series, all pics have the same
specs, and some are noisy, others are not.
Here a re-try using "Convert to : sRGB" as advised by William.
On my HD, image is 240 ppi, 1000 x 738 and 204 Kb.
Same mess around the tatoo/hair/ear. Same image, same resolution,
a little bigger (1000 x 750) posted on LFI
is absolutely clear!

[jaapv]

Originally Posted by sthan

Thanks a lot, William,

You learned me something.
In fact, I don't sharpen my pictures, only high-pass it,
which is far less destructive.
Regarding 16-bit / 8-bit, I will be more cautious.
Regarding your previous post, I'll check the difference
between the two conversions.
What puzzles me is that 2 pics are noisy in the whole
series, and not the others. They all have been shot in
abundant available light, no underexposition (hi, robx2004).
Why some are artefacted, and not others ? Especially
when I posted the same pics in another website, where
they're perfectly clean (hi again, robx2004 ).

PS : I'm impatient to see your portfolio in next LFI. Congrats.

If you look at the size of max-quality jpegs, there is a wide variation between images of the same pixel number, depending on the number of similar pixels next to one another, basically on the amount of information stored inside the image and the compressability by the Jpeg algorithm. Obviously the site will compress the larger jpeg more.

[sthan]

The same again, smaller (950 x 701).
Slightly better, but still not so clear.
Thx to all, I'll post postage stamp from
now on.

[andybarton]

Given that computer screens display at 72ppi, that's the ppi you should present your images at in order to be seen properly on the internet.

If you do anything else, then it seems to me that the browser is doing the resizing, not your image editing program and you are doing yourself a disservice.

I ALWAYS resize to 930 wide (or 800 tall) and at 72 ppi before saving when I want to display here. I will post a screenshot of the settings in the Image Size Adjustment box when I get home.

[jaapv]

Originally Posted by andybarton

Do you resize at 72 dpi before posting?

Is that of any relevance for web use, Andy? I always thought only the pixel number counts (and, of course, the jpeg compression). DPI only comes into play when outputting for print.900 pixels is 900 pixels, regardless of the DPI setting.

[sthan]

Andy,
You're more experienced than me in all things of the forum,
but when posting, I never resized to 72, and especially
not here. Meanwhile, it's the first time I experiment these difficulties.
Reason why I'm perplexed.
Yours,
Johan

[andybarton]

Originally Posted by jaapv

Is that of any relevance for web use, Andy? I always thought only the pixel number counts (and, of course, the jpeg compression). DPI only comes into play when outputting for print.900 pixels is 900 pixels, regardless of the DPI setting.

Yes.

Resize an image at 300 dpi to 900 x 900 and it shrinks down to a very small image on screen. Set it up to 72 dpi, and it becomes the right size to view onscreen.

Try it and see.

I will show you the difference later.

Oh, and converting to 8 bit and jpg should be the last thing you do. Never ever work on jpgs and save them during your working process. Each time you do, more and more info is lost. This is not the case when working on tiff or psd format files.

[jaapv]

Only in your photoshop screen, Andy, not after saving and uploading, not even to sites that don't resize. In photoshop you can hit ctrl-alt-0 and it will show the same image at the normal size again, without altering the DPI setting, as that has no relevance for the screen display.

You are of course, 100% right about working files in 16 bits in photoshop. Actually I have taken to working DMR files, that are a native 16 bits, in 32 bits and I have convinced myself I can see a difference.

[andybarton]

I will have a play when I get home and post the results

[wattsy]

Originally Posted by andybarton

Yes.

Resize an image at 300 dpi to 900 x 900 and it shrinks down to a very small image on screen. Set it up to 72 dpi, and it becomes the right size to view onscreen.

Only in programmes (e.g. Preview) which recognise the DPI setting. I've been optimising images for web browsers for a dozen years or so and DPI has never come into play in this context (browsers simply display images at the actual pixel size).

[stunsworth]

Originally Posted by andybarton

Given that computer screens display at 72ppi, that's the ppi you should present your images at in order to be seen properly on the internet.

No, all that's important is pixel width. Doesn't matter if it's 1 ppi or 1000 ppi, the screen will just show the pixels.

[andybarton]

I have just had a play with two versions of the same image. One at 72 ppi and one at 337 ppi

I stand corrected with regards the 72 ppi proposal. Thanks and apologies for the confusion. This has just saved me a step in jpg presentation

72 ppi
porsche72.jpg


337 ppi
porsche337.jpg