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 Post subject: Pixel puzzle
PostPosted: July 19th, 2010, 12:32 pm 
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Here's one for the techies: I have 2 versions of the same pix(a) the original 3998pix by 2666 pix, 1755 KB Jpeg (b) version 'touched up' in Paint, 3998pix by 2666 pix, 737 KB Jpeg.

If the number of pixels are the same and it's the same file type, shouldn't the KB's be the same? When making a print, will one have a better resolution than the other?
Thx in advance for your help.

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 Post subject: Re: Pixel puzzle
PostPosted: July 19th, 2010, 1:07 pm 
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I, too, await this answer with bated breath!!!

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 Post subject: Re: Pixel puzzle
PostPosted: July 19th, 2010, 2:09 pm 
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The answer is it depends upon the amount of data compression applied when the image is generated. Programs like Photoshop allow you to specify the amount of compression (image quality) when you save your image. This is typically a value between 1 and 10 (10 being the best quality / largest file). Keep in mind that the jpeg format is not a lossless compression so an image saved with higher compression will not look as good as one saved with lower compression even though the resolution is the same.
Jeff

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 Post subject: Re: Pixel puzzle
PostPosted: July 19th, 2010, 2:13 pm 
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http://photo.net/equipment/digital/basics/

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 Post subject: Re: Pixel puzzle
PostPosted: July 19th, 2010, 4:52 pm 
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OK, maybe I'm slow but I still don't get it! :doh: I didn't deliberately compress the file, just opened it with Paint then saved it as a JPEG.

L2K: If I compressed the file wouldn't the number of pixels be reduced?

Barbara: Thx for trying to help. Was there any particular wording in that article that explained the apparent puzzle? I read it but it didn't seem to.

Thx

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 Post subject: Re: Pixel puzzle
PostPosted: July 19th, 2010, 8:08 pm 
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Hi Wotrock,
I'm not sure why paint changed the size of your image file.
Perhaps paint has some default settings for the compression which reduced your file size when you saved the image. With a program like Photoshop, you have more control over this.
In simplified form regarding the jpeg compression, the algorithm looks for adjacent pixels which are "similar" and stores them as one entry in the file. By increasing the compression, you expand the definition of what pixels are considered "similar", thus requiring a smaller number of KB to store the information for a given size of photo. Images with large areas of the same colour or pattern compress to a smaller size. The trade off is that image will not be a sharp and you cannot distinguish the finer tonal variations in colour that were in the original image even though the number of pixels remains the same.
With a compressed image there is no longer a one to one mapping between each byte in the file and each pixel so you can have a smaller file which represents the same number of pixels in the image.
Hope that I have helped explain it a little better and not just confused you more!
Cheers,
Jeff

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 Post subject: Re: Pixel puzzle
PostPosted: July 20th, 2010, 12:38 pm 
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L2K:

I think I get it now. So let's say a pic is 1000 pixels by 1000 pixels. and say it's compressed so that, on the average, the surrounding 9 pixels are deemed to be the same color as a given pixel so it is then stored as one piece of info (1 bit, 16 bits or whatever). So the file is compressed by a factor of 10 and takes up 1/10th of the KB's of the unedited file. But regardless of the compression the file is still deemed to be 1000 pixels by 1000 pixels? Is that correct?
Thx

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 Post subject: Re: Pixel puzzle
PostPosted: July 20th, 2010, 3:26 pm 
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The way I look at it is to think of the number of pixels as a unit of measurement like inches. Say a frame size of 1000 x 1000 pixels or inches or centimeters.

If a photo has a lot of detail and lots of different colours, then the compression within that 1000px by 1000px frame size wouldn't be that much. If there is a lot of the same colour, say white or black for example, then there can a lot of compression and a smaller file size. But it still has a frame size of 1000 by 1000

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 Post subject: Re: Pixel puzzle
PostPosted: July 21st, 2010, 11:20 am 
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Hi Wotrock,
Looks like you have nailed the concept!

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 Post subject: Re: Pixel puzzle
PostPosted: July 21st, 2010, 2:08 pm 
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And if you have a really good pic you want to keep save a copy of it as a tiff.
the tiff won't be compressed.
and burn your good pics on a cd for back up.
Jeff

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 Post subject: Re: Pixel puzzle
PostPosted: July 22nd, 2010, 4:16 pm 
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Isn't Paint the age-old MS app ? Isn't this more or less a joke of an app in 2010 ?


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 Post subject: Re: Pixel puzzle
PostPosted: July 23rd, 2010, 8:56 am 
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wotrock wrote:
L2K:I think I get it now. So let's say a pic is 1000 pixels by 1000 pixels. and say it's compressed so that, on the average, the surrounding 9 pixels are deemed to be the same color as a given pixel so it is then stored as one piece of info (1 bit, 16 bits or whatever). So the file is compressed by a factor of 10 and takes up 1/10th of the KB's of the unedited file. But regardless of the compression the file is still deemed to be 1000 pixels by 1000 pixels? Is that correct?


Close enough.

Drop that Paint app and download a trial version of PS Elements. It is all you'll ever need and a whole lot more. You need to get more control of your file saving options than Paint allows.

Anytime you save a photo as a JPEG, the application is compressing it without you asking it to. You need to be able to set the amount of compression based on the task at hand. If I'm e-mailing a photo of the new grandson to a buddy, I make a duplicate and use the lowest setting (highest compression) that still looks good on my monitor, then I immediately drop the dup in the trash after I attach it to the e-mail to get rid of the file. However, if I'm sending something to the printer, I use the highest quality (lowest compression) that PS offers. In each case, the actually pixel dimensions of the image stays the same, and the images will display as the same size when viewed full size on devices of the same resolution (ppi on a display, dpi on a printed image).

Complicating matters even more, images with lots of detail and a wide range of colors yield much larger file sizes than a low contrast, simple image (like a blue sky, for example) of the same pixel dimensions. The algorithms in the software have to specify and save both the pixel color info and mapped pixel locations for many more pixels in order to get the image to look good. More info needed means more data must be saved, hence the larger file sizes. Each saved pixel needs 24 bits of info to recreate it in whichever of the 16.8 millions possible colors it happens to be. That's 3 bytes. So a completely uncompressed image file has a file size that is three times the number of bytes as it has pixels. That's pretty unwieldy, so in real life, some sort of compression is always applied in-camera to an image as it is being saved by the camera software.

Hope that hasn't confused you more like it has me. :doh:

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 Post subject: Re: Pixel puzzle
PostPosted: July 23rd, 2010, 10:10 am 
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Battenkiller said: "That's pretty unwieldy, so in real life, some sort of compression is always applied in-camera to an image as it is being saved by the camera software."
That is correct but only if the camera saves in jpeg. DSLRs images can be and are most often saved in raw format which is not compressed.

When working on a jpeg, don't keep saving the image over and over again. Due to the continued lossy compression, the image will seriously degrade. Make a copy in a lossless compression like PNG, and then, when finished, do your final save as jpeg.

ted

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 Post subject: Re: Pixel puzzle
PostPosted: July 23rd, 2010, 5:02 pm 
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Ted wrote:
DSLRs images can be and are most often saved in raw format which is not compressed.


All RAW formats compress the data before writing it to the card, and by a lot. Otherwise, a 24-bit 15 megapixel image would create a 45 MB file right in the camera. They usually use a proprietary lossless or nearly lossless compression that creates files much smaller in size than a TIFF file, but then you might have trouble with these if you are using software to convert them other than that written and provided by the camera maker. Adobe is attempting to standardize the RAW format to DNG, but AFAIK, it is not receiving universal acceptance.

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 Post subject: Re: Pixel puzzle
PostPosted: July 23rd, 2010, 5:18 pm 
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Battenkiller wrote:

Drop that Paint app and download a trial version of PS Elements. It is all you'll ever need and a whole lot more. You need to get more control of your file saving options than Paint allows.


That's the problem with PS elements---it's a whole lot more so it has a whole lot more ways to confuse me. I have tried Elements but just got frustrated. I find Paint quite useful in marking up such things as topos to indicate a route, campsites or whatever. As you know, it's 'toolbox' is quite small so it's easy to pick the right one.
.

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