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A raster image is a digital image made up of squares called pixels. Each pixel has a color, and as the pixels get smaller, the detail, or image resolution, increases because the number of so-called mixed pixels grows smaller. A mixed pixel is a square that falls on at least one border between the colors in the frame. Because the pixel can only contain one color, the resolution of the image is degraded since one color must be chosen to fill the whole pixel. This is why raster images with lower resolution look "blocky". As the pixels grow increasingly smaller, the amount of mixed pixels decreases, the resolution increases, and the image grows sharper.