Grayscale Mode

Grayscale mode is a color mode in Photoshop. In grayscale mode, images are represented using only shades of gray rather than the full pallet of colors.

In RGB mode, the color of each pixel is stored as three numbers, or channels (nnn, nnn, nnn). If all three channels have the same value then the resulting color is a shade of gray.

Black (0,0,0)
Gray (80, 80, 80)
Gray (160, 160, 160)
White (255, 255, 255)

RGB mode stores all three channels for each pixel. In this case RGB would store (0, 0, 0), (80, 80, 80), (160, 160, 160), (255, 255, 255). Notice that 2/3 of the information stored is redundant.

Grayscale mode stores only one channel called the K value. K value represents the percentage of black in the pixel.


Demonstration

RGB Mode to Grayscale Mode

For our demo we’ll use a two pixel image.

Two pixel color image
The numbers in brackets are the pixel’s RGB value.

To convert an image to grayscale mode:

Image > Mode > Grayscale

Converting to grayscale mode

Photoshop will ask you if you want to discard the color information.

Photoshop warning

Photoshop will apply an algorithm that converts the RGB value of each pixel to a K value.

TwoPixelGrayscale2

Use the eyedropper tool to see their K values:

5% black
K value of 5% black
78% black
K value of 78% black

If we use the color picker tool we can see their equivalent RGB values:

RGB value of (246, 246, 246)
RGB value of (76, 76, 76)

Grayscale Mode to RGB Mode

We now change the mode back to RGB

Image > Mode > RGB Color

ScreenHunter_11 Mar. 20 22.23

Even though the mode is RGB the colors are not restored. The RGB values are simply the RGB equivalent of the grayscale K values.

TwoPixelRGBRevert
Revered back to RGB image.

This is why Photoshop asks you if you want to discard the color information. Grayscale mode conversion is a destructive way to create black and white images because it destroys the color information.

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