levels and curves

[marieD]

HI everyone,
I was playing around with PS tonight and finally found the levels and curves that are often mentioned in the forum. I understand a little about the levels and how they change the tone (great for B&W, since G3 seems to take flat BW images) But i do not fully understand the curves. I do not have a manuel, since i recieved it as a copy.

Any help would be appreciated, since both of them seem to be a staple in the digital post production part. :P

[nomel]

This is what it does, not how to use it :)

background, just in case: for 24 bit images (8 bit * 3 colors), each color channel ( red, green, and blue) has 256 different levels of brightness (2^8). So, each color channel can be given a value between 0 and 255. So, for black, all three channels would be 0, and white, all would be 255. All red, red would be 255, and others 0. etc. Higher bit images would just have more than 255 levels (2^n, where n is the bits per color channel)

Curves allows you to change one brightness value of a color channel (or, to the brightness channel or whatever I'm assuming) to another. As you can see in the curves dialog, there is a graph with a line. The x axis is the input value, the y axis the output. 0,0 of the graph is located at the bottom left. The line is at a 45 because it is set to output = input by default. If you click and move your mouse around on the graph, you can see the input and output level for the current place your mouse is under. Moving this curve changes the brightness value from whatever input value the curve is on to whatever the output value the line is on.

This allows you to do things like add contrast without clipping (upping contrast just increases brightness of bright pixels, and decreases brightness of dark pixels about some reference level). This can be done by making an s shaped curve. The darks are made darker, and the brights brighter, just not in a linear fashion (if below reference, subtract x, if above, add x) like most contrast algorithms.

This also allows you to do things such as darken the darks, or brighten the bright without touching the brights and darks, respectively.

You can also do things like invert the picture. Since this involves making the darks bright and brights dark, you can just use a straight line, but with a negative slope.

Besides that, I'm not sure and can't wait to find out. Everyone seems to use it, I guess I should know how.

[riverdance]

Thanks Nomel

That expanation is one of the shortest and easy to understand I've seen - thanks for sharing it.

Riverdance
County Kerry
Ireland

[msol]

Check out these great tutorials on curves:

http://www.luminous-landscape.com/tutorials/command_primer.shtml
http://www.gurusnetwork.com/tutorials/photoshop/curves1.html

... or google for "totorial Curves Photoshop". These really help in understanding the subject.

Good luck!

[kreego]

Curves allows you to change one brightness value of a color channel (or, to the brightness channel or whatever I'm assuming) to another

Really good explanation, Nomel! FWIW, this explanation applies to any image editing software - off hand I recall that Jasc Paintshop Pro, and Macromedia Fireworks have the same kind of tool...

K

[marieD]

This all helps a lot. Thank you all for the info. I am so glad taht i found the levels. like i said before, they seem to be a staple in post production.

It seems that the g3 takes flat B&W images? Is that the camera, or is it me?

[marieD]

This all helps a lot. Thank you all for the info. I am so glad taht i found the levels. like i said before, they seem to be a staple in post production.

It seems that the g3 takes flat B&W images? Is that the camera, or is it me?