4.10.6 Masked Images
In LanguageLevel 1 or 2, images painted with the image or colorimage operator mark all affected areas of the page as if with opaque paint (see Section 4.1, “Imag-ing Model”). All portions of the image, whether black, white, gray, or color, com-pletely obscure any marks that may previously have existed in the same place on the page.
• Type 4 image dictionaries (LanguageLevel 3) specify a color or range of colors to be masked out wherever they occur within the image; this technique is known as color key masking.
• The number of components per sample is always 1, regardless of the current color space, because sample values represent masking properties rather than colors.
• The number of bits per component is always 1. In its five-operand form, imagemask has no bits/sample operand; in the one-operand (image dictionary) form, the dictionary’s BitsPerComponent value must be 1.