Pixel dimensions in CS2

JC
Posted By
Jack_Cane
May 4, 2008
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448
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8
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I opened a jpg image in PS CS2, and opened the Image size panel. I found the following: Width 2304 px., height 1728 px., doc size 32" x 24" at 72 px/in, and Pixel Dimensions 11.4M.

Can anyone explain the relationship of the pixel dimensions to the other data, i.e., how the pixel dimensions are calculated.
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P
Phosphor
May 4, 2008
I’m gonna rip my fkn hair out, I swear.

No, Jack, that is in no way directed at you personally, but at the number of times this same sort of question gets asked. It’s one of those things that—in this day and age of digital imagery—they should start teaching kids about resolution in about 3rd grade. Might be good for helping with learning math, too.

Go to <http://www.scantips.com>

First, you bookmark it, because it’s a great resource. Then, scroll down to the "*Start" link, and start exploring and reading. It’s essential info, and Wayne Fulton explains it as well as anyone.

It’s helped MANY, MANY people with the question, myself included.

🙂
J
Jim
May 4, 2008
wrote in message
I opened a jpg image in PS CS2, and opened the Image size panel. I found the following: Width 2304 px., height 1728 px., doc size 32" x 24" at 72 px/in, and Pixel Dimensions 11.4M.

Can anyone explain the relationship of the pixel dimensions to the other data, i.e., how the pixel dimensions are calculated.
There are 2304 x 1728 pixels in the image file.
Using 8 bits per color and since it will take one byte per color per pixel, that means the the number of bytes is 2304 * 1728 * 3 bytes in the image file. The obscure part is how many bytes make a megabyte. As computers use base 2 numbers, what CS2 reported is about correct.
The document size and the pixels per inch are not used in computing the size of the image.
Jim
JC
Jack_Cane
May 4, 2008
Thanks Phos.

From <http://firesign3.com/glossary.html>, Pixel Dimension is defined as…"The size of a computer display screen expressed in horizontal pixels by vertical pixels (H x V, or 800 x 600, for example)".

From <http://www.scantips.com/basics1b.html>: "If the image size were say 1000×750 pixels (written as width x height by convention), then there would be 1000 columns and 750 rows of data values, or 1000×750 = 750,000 pixels total. For 24 bit color, each pixel’s data contains three 8-bit RGB byte values, or 750,000 x 3 = 2,250,000 bytes".

In my example I have a 2304×1728 image. If I multiply those and take 3 times the product, the answer is 11.9 Mpx, while Photoshop gives "Pixel dimension" as 11.4M.

So now I have three questions

1. Why the difference between 11.9 and 11.4?

2. Are the three colors included in the definition of pixel dimensions? Are there actually 3 pixels (i.e., red, green, blue) for every point in the 1024×1728 array of an image?

If you blow up an image on the screen, you see each pixel having one out of 16.777M colors, based on the relative weights of the three colors. But it is one pixel.

3. What am I missing?
JJ
John Joslin
May 4, 2008
What am I missing?

Lots — it would take too long to explain it all here but more reading on that Scantips site and this:

<http://en.wikipedia.org/wiki/Raster_image>

might help.
EG
Ed_Grenzig
May 4, 2008
Jack

This 2304 x 1728 = 3,981,312 pixels
PS has an error in there listing. 11.4M, should really be 11.4MB (left out B)and applies to the size of the uncompressed RGB file. The pixel dimensions are listed below the words.

But 1 MB is not 1,000,000 bytes.
1 MB = 1,048,576 bytes. (in computer talk, 1kB = 1024 bytes)

so 2304 x 1728 x 3 / 1,048,576 = 11.39 MB

Ed
JC
Jack_Cane
May 6, 2008
Thanks Ed. I knew it had to be something simple that I was missing. The whole thing makes much more sense now.
JR
John_R_Nielsen
May 6, 2008
But 1 MB is not 1,000,000 bytes. 1 MB = 1,048,576 bytes. (in computer talk, 1kB = 1024 bytes)

Unless you’re talking about disks,of course.
EG
Ed_Grenzig
May 6, 2008
Good Point!

Yes…
I forgot that disk manufacturers love to advertise the higher number.

example: my advertised 120GB hard drive: has 120,030,994,432 bytes or 111.78 GB my advertised 2GB thumb drive has: 2,063,269,888 bytes or 1.92GB

Ed

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