Pixel size, file size and file compression.

Pixel size, file size and file compression.

PostBy: Richard S. On: Fri May 20, 2005 4:16 am

File size is determined first by the format of the image, second by how much it is compressed if the format supports compression and third by the content and pixel measurement of the image (e.g. 800x600).

A pixel is a single dot of color. The pixel measurement of an image is determined by how many pixels or dots of color it contains, if the image has 300 horizontal dots(pixels) and 300 vertical dots(pixels) it would be a 300 x 300 pixel image. This is the actual size of the image.

You can find out the pixel size, file size or format of an image a few different ways, the easiest is by hovering your mouse pointer over the file and a information box will appear to give you the pixel dimensions and file size. The format can be determined this way also or by looking at the file extension at the end of the file, the two common ones are .bmp and .jpg . For more detailed information right click the image and select properties.

.bmp's are uncompressed images and are only suitable for images to be stored on your computer because of their large file size. .jpg's can be compressed producing a smaller file size which makes them ideal for the internet and other things such as e-mail. Remeber though that the compression comes with a cost which is quality loss, how much depends on the image and what it's compressed at. For the novice though who won't be concerning themselves with compression levels we can disregard this loss since the quality loss is minimal. One thing to note is you want to try and avoid the pitfall of opening an image in a photoediting application and constantly saving it as a .jpg, each time you save it the image will be recompressed at the default value which will lead to considerable quality loss over time, this doesn't apply to resaving an image in non-editing applications such as a e-mail program.
Last edited by Richard S. on Sat May 21, 2005 8:21 am, edited 1 time in total.
Richard S.
 
Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite

PostBy: Bunky On: Fri May 20, 2005 1:06 pm

Well that starts me off a bit. I guess I'll have to take a class at some pont, eh? Too bad I quit before I thought about changing my major. So for someone that's dumber than a bottle of water: If I made a picture w/91 x 91 dots per inch, how big should the width & height be? It gives no indication on what type of measurements used. :(
Bunky
 
Stove/Furnace Make: Alaskan
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PostBy: Richard S. On: Fri May 20, 2005 3:16 pm

Bunkyhead wrote:Well that starts me off a bit. I guess I'll have to take a class at some pont, eh? Too bad I quit before I thought about changing my major. So for someone that's dumber than a bottle of water: If I made a picture w/91 x 91 dots per inch, how big should the width & height be? It gives no indication on what type of measurements used. :(


No you don't need a class for basics such as this. Digital images are measured in pixels. Don't confuse pixels measurements with DPI (dots per inch) also called resolution, it has no bearing on the image size except if your scanning or printing it. If the image or part of the image you scanned was 3 inches by 3 inches and you scanned it at 91 DPI you'd create an image that is 273 x 273. All DPI does is scale the image.

If you take that 273 x 273 image and change the DPI in an image editing application for example to 200 it's still going to be 273 by 273. What you have done is changed the deafult size it prints at.

See this post for a longer explanation about DPI and Resolution. http://nepacrossroads.com/viewtopic.php?t=140
Richard S.
 
Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite


PostBy: Capmaster On: Fri May 20, 2005 6:43 pm

Printing digital images also has an often-neglected parameter. It's called line frequency and is a measurement of how many lines/inch the image is rasterized at for printing. Unless you're using a continuous-tone printer like a dye sublimation rig, the image will be rasterized. Regardless of the resolution or size of the image in pixels you will need to use the proper line frequency when it's printed or it won't look good.

This is especially important for grayscale images. There you want to have as many shades of gray as is practical, but you have to trade off gray shades with line frequency to find the correct balance.

To find the resuting gray shades given a certain line frequency is found by:

(Printer DPI/Line Freq.)^2

a 300 DPI Laser Jet at a line freq. of 53 (default for this printer in many DTP apps) = 32 shades of gray. You can see how as you increase the line freq. for a sharper image, you are trading off gray shades which is equally important ;)

Typical newspapers may run line frequencies from 50 - 100. Magazines will run 100 - 200 usually. You can see where a high resolution printer is a necessity :)

This isn't an issue if you're starting out. And there are many online places where you can upload your digicam files and have them printed on silver halide paper, the same stuff film negatives are printed on, and extremely stable compared to dye sub ;)
Capmaster
 

PostBy: Bunky On: Sun May 22, 2005 1:09 pm

So, say if I take a pic w/a cheap cell phone & mail it to my email & save it under documents, that may have less dots per inch & be postable? OR.... If I scan a teenie pic that may work? I'm just shooting for a small enough pic to use for an avatar in your forums for now. I love taking pictures but I'm not even close to begin understanding this stuff. Or would a small drawing be better w/only a couple different colors? A dull pic w/not too much details? I can come up w/ something. It's better than nothing. But my brain is freekin dead when it comes to technology guys! So sorry :oops:
Bunky
 
Stove/Furnace Make: Alaskan
Stove/Furnace Model: Kast Console II

PostBy: Capmaster On: Sun May 22, 2005 6:07 pm

Bunkyhead wrote:So, say if I take a pic w/a cheap cell phone & mail it to my email & save it under documents, that may have less dots per inch & be postable? OR.... If I scan a teenie pic that may work? I'm just shooting for a small enough pic to use for an avatar in your forums for now. I love taking pictures but I'm not even close to begin understanding this stuff. Or would a small drawing be better w/only a couple different colors? A dull pic w/not too much details? I can come up w/ something. It's better than nothing. But my brain is freekin dead when it comes to technology guys! So sorry :oops:

Bunkyhead,

Very simply - it is the pixel count that determines the size of a pic. You can select how you want it printed in an app and that could affect the final size. Same with your display settings on your monitor (frequently 72 dots/inch).

If you take a pic with your phone it'll have a certain pixel width and height. That won't change unless you change it manually. You can probably change it in Windows Paint in Start>All Programs>Accessories.

If you get in a pinch just e-mail it to me and I'll resize it to avatar size for you and mail it back ;) I'll PM you the address.
Capmaster
 

PostBy: Bunky On: Sun May 22, 2005 11:45 pm

I really do want to try it myself, bless you for your help. I may get PO'd & send it along, but I'll need a week to get frustrated enough to give up on it! :study: :-k : Thanks for your help :salute:
Bunky
 
Stove/Furnace Make: Alaskan
Stove/Furnace Model: Kast Console II

PostBy: Richard S. On: Mon May 23, 2005 2:22 am

This isn't the best program for this, it's fairly limited but it's more than adequate for basic stuff..... right click the file and select "open with". You should now have a pop-up box containing programs that can open the image. If it's present select "Microsoft Photo Editor". This will open the image in that editor. On the top menu bar select image>resize. This will bring up a dialog box:

Image

On the bottom where it say units switch it to pixels, the width and height in pixels will now be displayed. If you change the width you will see that the height will automatically change too, this is to maintain the correct aspect, it automatically adjusts so your image doesn't become skewed or squished in one direction or the other. If you want to allow the aspect to change put a check next to "allow distortion", you can chnge the width anf height independent of each other. Using a different editor will have a similar workflow.

When your done select OK. On the top menu bar select "save as", type a !different! name in the filename box . This is important if you want to keep your original larger file, if you save with the same name it will overwrite the original. If you want to overwrite the original you can skip all that and instead of selecting "save as" select "save".

Also note..... and this is probably the button I use the most :oops: ....if you make a mistake there's a undo button. You can access it either from the menu.... edit>undo..... or more easily from the tool bar located directly below the menu bar. It's a blue circular arrow.
Richard S.
 
Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite

PostBy: Bunky On: Mon May 23, 2005 6:58 pm

:thumbleft:
Bunky
 
Stove/Furnace Make: Alaskan
Stove/Furnace Model: Kast Console II

PostBy: Capmaster On: Tue May 24, 2005 10:50 am

Bunkyhead wrote::thumbleft:
Success?

8) 8) 8)
Capmaster
 

PostBy: Bunky On: Tue May 24, 2005 10:24 pm

I don't have Micro Photo Editor as an otpion- used paint & got distortaion? Still trying :lol:
Bunky
 
Stove/Furnace Make: Alaskan
Stove/Furnace Model: Kast Console II

PostBy: Bunky On: Tue May 24, 2005 10:50 pm

Tell me- Is this a good file size? Ok for uploading to yawl? I did 100 pixels per inch in Dell Photo Editor when I scanned it the first time.
So it's what 300x300 for thisn & 100x100 for dother? Can you tell the size of it because it now doesn't look like it is the size I thought it was. The paint app messes it up if I change the hieght to 100 x 100 but I got the KB down to 14-16 or so though :dontknow:
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Bunky
 
Stove/Furnace Make: Alaskan
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PostBy: Richard S. On: Wed May 25, 2005 10:04 am

If you right click the image... the one above, and select properties it will tell you what the actual size is. Again DPI and resolution have no bearing on how it appears on your monitor in most instances. An example of where dpi would make a difference is if you imported it into a word document. Word uses the dpi to set the default scale.

You have lowered the file size but as you can see it looks like crap, lowering the compression settings too far will give you results like that.
Richard S.
 
Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite

PostBy: Bunky On: Wed May 25, 2005 5:20 pm

Shall I keep them as an example?
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Last edited by Bunky on Wed May 25, 2005 11:21 pm, edited 1 time in total.
Bunky
 
Stove/Furnace Make: Alaskan
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PostBy: Richard S. On: Wed May 25, 2005 6:56 pm

Bunkyhead wrote:As long as I keep it under 50Kb, I'm not breakin the rules though, right?
Thx for the education. I bet it's annoying to have to work w/a bunkyhead, isn't it? I'm a slowwwww learner but I don't quit easily. :wink:


I forget what the limit is but it won't allow you to post anything bigger, you can delete them yourselve. Click the profile button the top of one of your posts. There's a link there for the image control panel.
Richard S.
 
Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite