Microsoft Office Tutorials and References
In Depth Information
Color Depth
However, if you think you might want to use those same pictures for some other purpose
in the future, such as printing them in a magazine or newsletter, then go ahead and take
them with the camera’s highest setting, but then compress them in PowerPoint or resize
copies of them in a third-party image-editing program. See the section “Compressing
Images” later in this chapter to learn how.
Color Depth
Color depth is the number of bits required to describe the color of a single pixel in the
image. For example, in 1-bit color, a single binary digit represents each pixel. Each pixel is
either black (1) or white (0). In 4-bit color, there are 16 possible colors because there are
16 combinations of 1s and 0s in a four-digit binary number. In 8-bit color there are 256
For most fi le formats, the highest number of colors you can have in an image is 16.7 million
colors, which is 24-bit color (also called true color ). It uses 8 bits each for red, green, and
There is also 32-bit color, which has the same number of colors as 24-bit but adds 8 more
bits for an alpha channel. The alpha channel describes the amount of transparency for each
pixel. This is not so much an issue for single-layer graphics, but in multilayer graphics,
such as the ones you can create in high-end graphics programs like Photoshop, the extent
to which a lower layer shows through an upper one is important.
For a great article on alpha channel usage in PowerPoint by Geetesh Bajaj, go to
ucts/ powerpoint/ppalpha.html .
A color depth of 48-bit is fairly new, and it’s just like 24-bit color except it uses 16 rather
than 8 bits to defi ne each of the three channels: red, green, and blue. It does not have an
alpha channel bit. Because the human eye cannot detect the small differences it intro-
duces, 48-bit color depth is not really necessary. Of the graphics formats that PowerPoint
supports, only PNG and TIFF support 48-bit color depth.
Normally, you should not decrease the color depth of a photo to less than 24-bit unless
there is a major issue with lack of disk space that you cannot resolve any other way. To
decrease the color depth, you would need to open the graphic fi le in a third-party image-
editing program and use the command in that program for decreasing the number of colors.
Before going through that, try compressing the images in the presentation (see the section
“Compressing Images” later in the chapter) to see if that solves the problem.
File Format
Many scanners scan in JPEG format by default, but most also support TIF, and some also
support other formats. Images you acquire from a digital camera are almost always JPEG.
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