As objects recede in space they not only appear to shrink in size, but tend to lose detail, contrast of values, intensity of color, and their edges appear less distinct the greater the distance from the viewer's eyes. This is the principle of Atmospheric Perspective. Some writers call it "aerial perspective," but this is misleading, as the term, "aerial" usually pertains to flying.
The visual alteration of images over distance is the direct result of "X" amount of atmosphere between the eye and the object or plane in view. The atmosphere contains water vapor and its own density, which renders it somewhat less than totally transparent, adding a certain degree of whiteness to the air. Light renders the atmosphere white. The more air we must look through to view something, the more atmosphere we see between it and our eyes, and the more the image is altered by it. An optical illusion is created by the presence of a semitransparent white between the eye and any color darker than white, which alters the color in question in the direction of blue, as well as lightening the value. This is precisely why the sky appears to be blue. The sky's blue is created by the blackness of space being viewed through a layer of semitransparent white atmosphere. The white is the atmosphere illuminated by the sun.
At night, without the sun's light, the atmosphere is no longer white, and the blackness of space becomes visible. Note that the sky is always lightest just above the horizon. This is the greatest distance we can see at ground level, which is where the atmosphere contains the most (white) water vapor and the greatest density. At the horizon, the density of the atmosphere renders it more opaque, and thus, whiter. As we look up, we look through thinner air, which is less opaque, and the sky is bluer and darker. This is why distant objects and planes appear lighter, bluer, and less distinct. The same phenomenon can be produced with paint. The process is called scumbling and is accomplished by applying a thin veil of white paint semitransparently over a layer of (dry) darker paint. The optical result in paint is the same as in the air. Translucent white over black reads bluish, just as light grey smoke against dark trees reads blue. Note that the same smoke may appear to be brown when a white cloud is behind it, a reverse of the scumble phenomenon. Dark over light increases apparent warmth. This is the principle at work in glazing, that is, the application of darker transparent paint over a lighter passage. Glazing and scumbling are discussed at length in Chapter Six and Chapter Ten.
In painting, atmospheric perspective can be rendered directly, in one step, using opaque paint exclusively, at least in the distance and middle ground, following the principle, by adding white, and sometimes blue, increasing the white (and blue) to indicate greater distance, softening edges by working wet paint into wet paint, suppressing detail and diminishing contrast between light and shadow to indicate greater distance. The effect can also be gotten, perhaps slightly more convincingly, in a two-stage process whereby the same procedure is used as in the one-step method except that the area of greatest distance is rendered very slightly darker than the desired final effect. The illusion is completed in the second step by scumbling a thin film of white or light grey over the dried paint of the first step in the areas of the greatest distance. The illusion of depth can be further enhanced by painting the deepest foreground shadows, and only these foreground shadows, in transparent glazes over a relatively lighter underpainting or primer. This creates the highest degree of clarity, as would be the case when the least amount of atmosphere is present between the shadow and the viewer's eye, appropriate for the immediate foreground only. The combined, systematic use of glazing, scumbling, and opaque painting allows the painter to create the illusion of depth to the highest degree possible. However, the successful rendering of spatial recession depends even more heavily on observance of the principles of geometric and atmospheric perspective than it does on expert paint handling.
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