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Adobe Photoshop 7 Tutorials
Color Modes / Bitdepth
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Subtitles of the Movie (Adobe Photoshop 7 / Color Modes / Bitdepth)
Why are certain files so large and take up space on your hard drive, while other files are fairly small. It has to do with two basic factors, and that is the amount of pixels in your document, how many pixels across versus how many pixels down. And as you know a raster image type of graphic, those are the kind of images that we generally work with in Photoshop, are based on a grid of pixels. And the other factor is the bit depth of your document. Bit depth, of course, is the amount of information in any one pixel. Notice my 3 pictures of the dandelion here. My RGB is about 26k in size. While my gray scale image is about 1/3 that size, since it's made up of only one channel of information. And this bitmap version, which any one pixel is either black or white, is very small indeed - it's 1 kilobyte in size. Of course this is because of the different bit depths of these three different images. Well, I have created this cool little graphic here to demonstrate some more information about bit depth. And you can see that I zoomed way in to the petal of my dandelion and extracted this graphic, and converted this into various different versions to demonstrate various different bit depths information. You will probably here the phrase 8-bit or 24-bit color, and I want to talk a little bit about that. An RGB image is said to be 24-bit color. A bit is an on and off state in your pixel graphic, and of course computers work in on and off states. An 8-bit document (which generally can look like a gray scale document) is called that because 2 to the 8th power equals 256 values or in this case 256 levels of gray which will create a continuous tone gray scale image. If you multiply that times 3 for 3 channels, one for the red, green and blue content, 3 times 8 equals 24 bits, and giving us a 24 bit image. Bitmap mode in Photoshop will create a type of image which the pixel bit depth is 1 bit, and that means that any pixel is either black or white. So this will create the smallest file size, and as you see it will create a very graphic stark representation - either black or white. Then we have the gray scale version giving us the black and white essentially version. And you can see that this will be much larger since any one pixel can be any of 256 values. So it can either be black or white or anywhere in between gray. Then another type of 8-bit image is called index color. So instead of having any pixel be a value of gray, it can be any of 256 colors. And this is a popular bit depth used for GIF images, which you might see on the World Wide Web. Full color images of course will contain three 8-bit gray scale channels pushed through red, green and blue, generating a full colored image. A 24-bit RGB image, any one pixel can be any of 16-million colors. CMYK can be said to be 36-bits. So 8-bit image is a continuous tone gray scale, or it could also be, as I mentioned, this index color. A 24-bit image is generally considered to be continuous tone color. 36-bit may be CMYK or often it is an RGB image, with one extra channel for a mask. And you can see that very often in television or even certain types of web formats, like the PNG format can contain its own transparency mask. So remember, whenever you choose a different mode here, we are actually increasing or decreasing our file size, because we are adding or decreasing the bit depth information of the pixels.
Tutorial Information
| Course: | Adobe Photoshop 7 |
| Author: | Andrew J. Hathaway |
| SKU: | 33329 |
| ISBN: | 1889347272 |
| Release Date: | 2002-09-05 |
| Duration: | 11 hrs / 152 lessons |
| Work Files: |
Yes |
| Captions: | No |
| Compatibility: |
Vista/XP/2000, OS X, Linux QuickTime 7, Flash 8 |
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