What does pixel intensity mean?

[Bluetigerx4]

Hi, I'm trying to learn more about photoshop histograms and I cannot for the life of me find out online what is meant by a pixel's intensity.  Could someone please give stupid me this "too obvious to post online answer?"  Thanks so much!

[Guest]

In the histogram the bottom line represents a range of grey tones numbering 256 starting from black and ranging to white.

Each of these steps ( or at least those whos corresponding tones are to found in the image ) may have a number of pixels stacked above it, which creates the shape of the histogram.

The number of pixels stacked above any one tone is the pixel intensity of that tone.

[Bluetigerx4]

Thanks tuftedduck, that really is simple!  So basically the intensity is the number of pixels for a specific level.  Thanks!

[Noel Carboni]

"Mr. President, that's not entiiirely accurate." (to quote a line from "Independence Day"). 

A pixel's intensity is its brightness (I won't get into how that is calculated, but as a rule of thumb this will do).  The histogram shows you how many pixels are at a given intensity level as compared to the others in the image.  Black is on the left, white on the right, and intensities increase as you move to the right.  Think of it as a bar graph with tiny one pixel-wide bars.

There are ways to have Photoshop show you not just the overal intensity, but the intensities of each individual color (e.g., in an RGB image there are three sets of intensities shown - those of the red channel, green channel, and blue channel).  But before we get into that let's make sure you understand the basics.

-Noel

[Bluetigerx4]

Hi Noel, I thought it a bit odd that pixel intensity was pixel count.  So intensity is a given level between 0-255 hehehehe, correct????? or??????

[Noel Carboni]

Yes.   This is a throwback to the 8 bit days, from which Photoshop grew.  Even 16 bit images have pixel intensities that are still expressed from 0-255.

-Noel

[Chris Cox]

You can set the info palette to display 16 bit and 32 bit values.... (same as you can set it to display other color models).