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                                                341
             SECTION 4.8                                                                  Images



KEY                  TYPE       VALUE

ImageMask            boolean    (Optional) A flag indicating whether the image is to be treated as an image
                                mask (see Section 4.8.5, “Masked Images”). If this flag is true, the value of
                                BitsPerComponent must be 1 and Mask and ColorSpace should not be
                                specified; unmasked areas are painted using the current nonstroking col-
                                or. Default value: false.

Mask                 stream     (Optional except for image masks; not allowed for image masks; PDF 1.3)
                     or array   An image XObject defining an image mask to be applied to this image (see
                                “Explicit Masking” on page 351), or an array specifying a range of colors
                                to be applied to it as a color key mask (see “Color Key Masking” on page
                                351). If ImageMask is true, this entry must not be present. (See
                                implementation note 52 in Appendix H.)

Decode               array      (Optional) An array of numbers describing how to map image samples
                                into the range of values appropriate for the image’s color space (see
                                “Decode Arrays” on page 344). If ImageMask is true, the array must be
                                either [ 0 1 ] or [ 1 0 ]; otherwise, its length must be twice the number of
                                color components required by ColorSpace. If the image uses the
                                JPXDecode filter and ImageMask is false, Decode is ignored.

                                Default value: see “Decode Arrays” on page 344.

Interpolate          boolean    (Optional) A flag indicating whether image interpolation is to be per-
                                formed (see “Image Interpolation” on page 346). Default value: false.

Alternates           array      (Optional; PDF 1.3) An array of alternate image dictionaries for this image
                                (see “Alternate Images” on page 347). The order of elements within the
                                array has no significance. This entry may not be present in an image XOb-
                                ject that is itself an alternate image.

SMask                stream     (Optional; PDF 1.4) A subsidiary image XObject defining a soft-mask
                                image (see “Soft-Mask Images” on page 553) to be used as a source of
                                mask shape or mask opacity values in the transparent imaging model. The
                                alpha source parameter in the graphics state determines whether the mask
                                values are interpreted as shape or opacity.
                                If present, this entry overrides the current soft mask in the graphics state,
                                as well as the image’s Mask entry, if any. (However, the other transparency-
                                related graphics state parameters—blend mode and alpha constant—
                                remain in effect.) If SMask is absent, the image has no associated soft
                                mask (although the current soft mask in the graphics state may still ap-
                                ply).

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