CS 39J > Schedule & Notes > Session 9 Detailed Notes |
http://inst.eecs.berkeley.edu/~cs39j/session09.html
21 March 2002
The Computer Labs
Photoshopping
Presentation Files
Photoshop is a professional-grade application used in the graphics, publishing, and print industry. It's not limited to digital photography, however; web designers, digital artists, and even comic book artists use Photoshop for their everyday tasks.
To beginners, Photoshop is very daunting because of the wide array of choices, buttons, and things to click on the screen. In session 10, we'll alleviate that problem by walking you step-by-step through a series of tutorials on Photoshop.
A pixel is the smallest unit of a picture file. Each pixel always contains information about the color used; you can assemble these pixels into one giant matrix to form a photograph, just like a mosaic of colored tiles.
We discuss color spaces. Because of the nature of computer storage, we digitally store a pixel's color by using a mathematical representation. One of these representations is RGB (red-green-blue), which deals exclusively with output to computer and television displays. CMYK (cyan-magneta-yellow-black) is used for print output. HSB (hue-saturation-brightness) is simply another representation of colors; it all depends on whatever suits your fancy. There are a smattering of other representations (duotone, tritone, grayscale, indexed, and so forth); if you're curious, you can ask us and we'll be more than happy to educate you.
Histograms graph the number of pixels using a color based on that color itself. In the histogram above, more of the pixels are on the left side of the graph; hence, the picture we're analyzing is a bit dark. This histogram represents the "luminosity"; you can easily change the settings to a particular color representation. For instance, you can retrieve the histogram for pixels with any red information. Same with green. And, same with blue.
The red eye effect illuminates the insides of one's eyes; this happens
when the subject's pupils are wide open in response to lack of light, and the
merciless photographer takes a photo of the subject with his flash on. We can
fix this in Photoshop by decreasing the amount of saturation (color purity)
of the subject's eyeballs, and then add the appropriate color of the person.
Of course, we'll need to know the person's actual eye colors to do this accurately
and to prevent the person from looking like a soulless zombie.
The process is rather arduous because we're using three different operating systems. Make sure to read the handouts (in order!). Most of the content can be learned through practice and experimentation.
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