To check it or uncheck it; that's the question. It's the "Contiguous" option you see in Photoshop when you use the "Magic Wand," the "Magic Eraser," the "Background Eraser," the "Paint Bucket" and other tools. Basically, "contiguous" means touching or connected. In Photoshop, it describes pixels of the same color touching each other.
Magic Wand
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When you drag the "Magic Wand" to an area of color in a Photoshop image and click, it magically selects a shape with contiguous pixels of the same color. However, if you uncheck "Contiguous," the "Magic Wand" will select every pixel of that color, whether touching or not. You can also raise the "Tolerance" setting to select a broader range of contiguous colors.
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Magic Eraser
Click the area you want to erase in your image, and select "Contiguous" to erase only those pixels actually touching the pixels you clicked. Deselect, or uncheck, "Contiguous" to erase all similar pixels in the image. If you also check "Anti-aliased," it will smooth the edges of your selection. Keep "Opacity" slider at 100 percent to erase pixels completely; lower it if you just want to partially erase pixels.
Paint Bucket
Before you dump that "Paint Bucket" into your image, take a look at the "Contiguous" setting. If you want to paint only those same-colored pixels contiguous to the ones you click, leave "Contiguous" checked. If you want to paint all the pixels that are the same color as the one you checked, uncheck the "Contiguous" setting. Then, the "Paint Bucket" will fill them all with the foreground color shown in the small boxes on the toolbar at your left.
Background Eraser
Be sure you are on the layer with the background you want to erase. Then, in the top toolbar, select "Limits" for erasing. "Contiguous" will erase only the pixels of the sampled color that are connected to each other. "Discontiguous" will erase the sampled color wherever it occurs when you drag the eraser across the background.