Photoshop's threshold effect adjusts images by converting all colors and shades of grey to either black or white. Designers use the adjustment to create stark, high-contrast images. The effect is often used to give images a stylized, illustrative look, and can be applied to any one of an image's layers or to its own layer. It can also be applied in multiple layers to give the user more control over what parts of the image to keep and what to discard.
You can apply threshold to an existing layer or in its own layer. Applying it to an existing layer changes the layer forever unless you go into the image's history and undo it. Applying threshold in a new layer allows you to work with the effect while preserving all other layers. You can then show or hide the layer with the threshold according to your needs. To apply threshold to an existing layer, go to the image menu at the top of the screen, choose adjustments, then select threshold. To apply it in its own layer choose the threshold option in the adjustments panel on the right-hand side of the screen.
Adjusting the threshold is a matter of specifying a level of brightness. All pixels lighter than that level turn white, while all darker pixels turn black. Thus, you can make your threshold primarily black, primarily white or somewhere in between. When you apply threshold, Photoshop displays a window with a histogram and an adjustment bar. A histogram is a graphical representation of the distribution of dark versus light colored pixels. Dragging the adjustment bar toward one end of the graph adds more white, while dragging it to the other adds more black.
It is common for designers to apply threshold to a series of identical layers to make an image appear more like an illustration. To do this, choose the layer to which you want to apply the effect and copy it three to six times. Apply threshold to the top layer and adjust it so it's mostly white. Apply threshold to each following layer, adjusting it each time so there is more and more black. Remove the white from each layer. Change the black in the top layer to a dark shade of the color of your choice. Change the black in each following layer to a lighter shade of the that color, each time choosing a lighter and lighter shade. The result is multiple shades of the same color taking the place of the black, giving the image a "layered" appearance.
If applying threshold to a color image that you intend to keep purely black and white, you can decrease the file size of the image by changing its mode. This is advantageous because the image will take up less memory. To do this, apply the threshold and adjust it to your liking. Go to the image option in the top menu bar and select the mode option. Choose Grayscale from the list. Photoshop will prompt you to flatten the image and discard the color information. Do this, then save the file as either a JPEG or PNG. The file's size will be reduced.
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