Please Explain dpi

[bloodhound31]

To protect my images on my website, I have just resized a 11Mb image, to a 46Kb image, and the properties tells me that although the size has changed, the Dpi is still 350.

How is that possible? I obviously don't understand.

Perhaps someone can tell me how is the best way to save my watermarked image for the web, including how to? What file size and resolution is optimum, that the viewer gets a nice look at the image, without it being good enough to rip off and print to any large size.:shrug:

I tried to resize and the image has become so jpg artefacted and pixellated, i would be ashamed to put it on my site. (See below)

This is really confusing me.:screwy:

Baz.

[Octane]

Watermarking your images like that takes away any visual impact it may have had.

Keep your images to 750-800 pixels on the longest edge and that will suffice.

H

[bloodhound31]

Thanks Humayun. Where you been mate? I sent you a text today.

Baz.

[bojan]

DPI (dots per inch) property is irrelevant when displaying images on the screen, only the number of pixels is important... I think DPI may be relevant when printing them by using some software packages and/or printers.. but then again I am just a pixel peeper ;) .. I have never printed any of my digital images (!).

[iceman]

46kb is very compressed.

Like H says, if you keep the longest edge to 750 or 800px, then save for jpeg so that the image size is around 150-250kb, you shouldn't have any (or many) compression artefacts.

If people are going to steal a small image like that, there's not much they can do with it. I agree that large watermarks plastered across images take the impact away.

[Barrykgerdes]

Dpi is simply dots per inch. and is irrelevant when the picture is displayed on your screen. The native dpi for my screen is 72 dpi but this is governed by size of the screen as well as the number of pixels it contains. So if your image is 1024 x768 pixlels (dots) at 350 dpi is 2.92 inches wide x 2.19 inches high. But if you change the size to 200 dpi the picture size will now be 5.12 inches wide x 3.84 inches high but the absolute resolution will still be the same.

If you wish to print out this picture then dpi becomes important particulaly if you want a particular size of picture. The dpi can be set in most graphics programs to allow printout on a particular size.

Now regarding your origiinal 11MB picture I assume it was something like 2560 x 1920 pixels in a bitmap form. If stored as a *.Jpg the size will be around 1 MB (depending on detail) and how much compression you use. This compression does reduce the quality of the picture but in general it won't be noticed unless you wish to print it commercially. If you use maximum compression on the original picture before loading it to the web it will be closer to 300KB this will still look pretty good but useless to anyone trying to plagerise it. You can then provide your original image in Tiff form to anyone that needs it (at a price).

When you reduced the picture to a smaller size the reducing program used a special algorithm to combine or drop pixels so that the overall resolution will be much less and will be forever lost.

As for watermarking pictures for copy protection. This will degrade the picture for most purposes and anyone well versed in graphic manipulation can remove it in any case. Also there is not much point in copy protecting an image that has been manipulated to change its size to something much smaller.

Incidently when reducing the size of a picture try to keep the change to a simple percentage like 50%,33.3% or 25%. This will preserve the best detail with simple binning of pixels in standard groups. If you are just cropping the image to make it smaller also try to keep the size to the 2nth size eg 256,512,384,768,1024 etc. to get the best results from standard screen sizes.

Barry

[Octane]

Well, at the very least you're honest enough to admit it, Bojan.

H

[bojan]

Had that ever been questionable ? ;)

[White Rabbit]

One way of stopping someone from stealing you images off your site is to disable right click on your site.

There are ways around it I'm sure but most people wouldnt know how to do so.

[bloodhound31]

There are some fantastic gold nuggets of knowledge and experience coming out here. Thanks guys, it really is good to pick up on all these little techy tricks that make the difference.

Barry, my images are all taken in Canon RAW, but when I process them I save them as a maximum quality JPEG, around 11 or 12Mb, and measuring 3888 x 2592 at 350DPI.

What is the maximum size that format could be printed?

I do want to print them at some stage and I have heard it would be better to save them as a TIFF for this purpose. Does that sound right?

[Barrykgerdes]

Hi Baz

I am no expert in imagary but I have a reasonable working knowledge gained by experience over many years. The Canon can be a little problem because not all graphics programs will read them. It is best to convert them into Tiff for storage of the masters. This is the form that most comercial printers like and will retain all the data.

For general use the size can be reduced by storing as jpeg which can use varying amounts of compression. The problem with the compression is you lose some data, and each time you load and save you lose some more.

For picture of that size in pixels some typical file sizes would be:-
tiff 28MB with no compression
png 15MB
bmp 29MB as 24 bit image
bmp 38MB as 32 bit image
Jpg 3.4MB with minimum compression
jpg 1.13MB with medium compression
jpg 822KB with maximum compression

You can see from this that there is a large reduction in size for jpg even at minimum compression. But you do lose some picture quality.

Hope this paints a better picture of what you need to do for best storage results.

When printing your pictures the more pixels you have in a given space the better resolution you will get. With the size of picture you have, printing at 300 dpi 12.5 ins x 8.6 ins, if you print at 600 dpi the picture will be 6.25 x 4.3 and so on. On good photo quality paper the smaller image would probably look crisper but the actual resolution would be the same as the larger picture. It is more a matter of what pleases you the most.


Barry

[bloodhound31]

So, a couple more questions..

Dots per inch...how big is a dot? Does each dot get bigger as you reduce the DPI?

Per inch....300 dots in an inch (Square inch of print?) doesn't seem like a lot. Or does it mean the inch is 300 dots hight by 300 dots wide?

Or to ask another way, are there 300 dots in a linear inch? (300 x 300 = 90,000 dots in a square inch of printed area?)

Cheers.

Baz.

[Barrykgerdes]

Dpi is a nominal value that is used mainly for printing and is the number of dots per linear inch in both planes so in a square inch there is technically 90000 dots.

As for the size of the dots technically the dot is the same dot no matter how many you have in the inch. However depending on the algorithm used by the computer or printer to display a dot the actual dot size will be expanded by many more dots to fill the available space. Watch the effect as you zoom in on a picture. You will eventually see the individual pixels displayed with many pixels for each of the originals.

If your picture is 600 x 600 pixels and your printer is set to print at 300 dpi the printed picture will be 2" x2" but if you print it at 100 dpi the picture will be 6" x 6". However there won't be any noticeable gaps in the picture because the printer will convert each dot into a mattrix of 3 x 3.
This may not be the same for all printers. Some advertise printing resolutions of 1200 by 600 dpi. So the printer will work out how many dots it will need to print to represent each pixel of the original picture.

Barry

[bloodhound31]

Thanks Barry, it's making a lot of sense now. I'm not sure how it benefits me in my printing ambitions yet, but I'm sure it will help. It's good to have an understanding of it.

Cheers,

Baz.