Photoshop
Colour and Tonal Adjustments
Creative Image Adjustments
This lecture covers the remaining adjustment commands: Invert, Threshold, Posterize, and Gradient Map. These are used to create special effects rather than as tonal or colour correction tools because the resulting effects are extreme.
Invert
This command inverts all colours in your image. The result is like looking at a photographic negative. It can be applied as an adjustment layer. To apply it to a layer's pixels, use the Image > Adjustments > Invert command. The keyboard shortcut is Ctrl-I (Cmd-I). Invert has no options.
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Before inverting |
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After inverting |
The inversion is based on a scale between 0 and 255. A colour value that is 255 before inverting becomes 0 after inverting. A value of 50 before inverting becomes 205 after inverting. Not only are dark and light values reversed, the colours will change to their complementary equivalents. In the example above, the blue sky became yellow.
Threshold
The Threshold command converts greyscale or colour images to high-contrast, black-and-white images. By default, the midpoint of the tonal range is the threshold. All pixels lighter than the threshold value are converted to white; darker pixels are converted to black. You can set a different threshold value by dragging the slider at the bottom of dialog. The value is displayed as the Threshold Level. This command is useful for determining the lightest and darkest areas of an image.
The Threshold command can be applied using an adjustment layer. You can also apply it to a layer's pixels by going to Image > Adjustments > Threshold.
Here are two examples of using the Threshold command on images used in this course.
Posterize
The Posterize command lets you specify the number of tonal levels (or brightness values) for each channel in an image and then maps pixels to the closest matching level. For example, choosing two tonal levels in an RGB image gives six colours: two for red, two for green, and two for blue.
The Posterize command can be applied using an adjustment layer. You can also apply it to a layer's pixels by going to Image > Adjustments > Posterize. You can change the number of levels in the dialog.
Here are examples of Posterized images.
Gradient Map
The Gradient Map tool applies a gradient set of colours to an image's highlights and shadows. It can be applied as an adjustment layer or to the pixels on a layer by going to Image > Adjustments > Gradient Map. The image doesn't need to be converted to Greyscale mode before using this option.
The gradients are selected from the gradient presets panel. To open it, click on the inverted triangle on the right side of the gradient preview. Dithering can be applied using the Dither option. The gradient can be flipped horizontally by enabling the Reverse option.
Here are two examples of usage. More extreme results are obtained by using high-contrast, multi-coloured gradients. The city image was mapped to the gradient shown above in the dialog image.
Creative Image Adjustments Summary
- The Invert command inverts the tonal and colour values in an image and the result is like a photographic negative.
- Threshold creates a high-contrast image by converting tonal values to white or black based on the threshold value set in the dialog.
- Posterize maps each colour channel to the number of colour levels you specify in the dialog.
- The Gradient Map option maps a selected gradient to the tonal values in the image.
Creative Image Adjustments Keyboard Shortcuts:
- Invert: Ctrl-I (Cmd-I)
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