 Adobe Photoshop generates a pseudo-random noise dither pattern on startup, with each pixel location in a 2D raster array assigned a gray value (R=G=B) and an alpha value of 1 (“on”).As the opacity of the top layer is reduced, the alpha value of some of the gray pixels is switched from 1 to 0 ("off"), with the result that image pixels corresponding to a gray-valued pixel in the raster array are either on (visible, opaque) or off (invisible, transparent), with no opacity gradation.Multiply and Screen blend modes are basic blend modes for darkening and lightening images, respectively.