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 combinations.

For most file 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 Blue.

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 multi-layer 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.

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 define each of the three channels: Red, Green, and Blue. It does not have an Alpha channel bit. Forty-eightbit color depth is not really necessary, because the human eye cannot detect the small differences it introduces.

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 file 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.

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