/* Copyright (C) 1997 Aladdin Enterprises.  All rights reserved.
  
  This file is part of GNU Ghostscript.
  
  GNU Ghostscript is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY.  No author or distributor accepts responsibility
  to anyone for the consequences of using it or for whether it serves any
  particular purpose or works at all, unless he says so in writing.  Refer
  to the GNU General Public License for full details.
  
  Everyone is granted permission to copy, modify and redistribute GNU
  Ghostscript, but only under the conditions described in the GNU General
  Public License.  A copy of this license is supposed to have been given
  to you along with GNU Ghostscript so you can know your rights and
  responsibilities.  It should be in a file named COPYING.  Among other
  things, the copyright notice and this notice must be preserved on all
  copies.
  
  Aladdin Enterprises supports the work of the GNU Project, but is not
  affiliated with the Free Software Foundation or the GNU Project.  GNU
  Ghostscript, as distributed by Aladdin Enterprises, does not require any
  GNU software to build or run it.
*/

/*$Id: gxalpha.h $ */
/* Internal machinery for alpha channel support */

#ifndef gxalpha_INCLUDED
#  define gxalpha_INCLUDED

/*
 * As discussed in the classic Porter & Duff paper on compositing,
 * supporting alpha channel properly involves premultiplying color values
 * that are associated with non-unity alpha values.  After considerable
 * thrashing around trying to read between the lines of the spotty NeXT
 * documentation, we've concluded that the correct approach is to
 * premultiply towards whatever the color value 0 represents in the device's
 * native color space: black for DeviceGray and DeviceRGB (displays and some
 * file formats), white for DeviceCMYK (color printers), with a special hack
 * for monochrome printers TBD.  This makes things very easy internally, at
 * the expense of some inconsistency at the boundaries.
 *
 * For the record, the only places apparently affected by this decision
 * are the following:
 *      - alphaimage, if it doesn't assume premultiplication (see below)
 *      - readimage
 *      - The cmap_rgb_alpha_ procedures in gxcmap.c
 *      - [color]image, if they are supposed to use currentalpha (see below)
 *      - The compositing code in gsalphac.c
 *
 * The NeXT documentation also is very unclear as to how readimage,
 * alphaimage, and [color]image are supposed to work.  Our current
 * interpretation is the following:
 *
 *      - readimage reads pixels exactly as the device stores them
 *      (converted into DeviceGray or DeviceRGB space if the device
 *      uses a palette).  Pixels with non-unity alpha come out
 *      premultiplied, however the device stores them.
 *
 *      - alphaimage assumes the pixels are premultiplied as appropriate
 *      for the relevant color space.  This makes alphaimage and
 *      readimage complementary, i.e., the output of readimage is
 *      suitable as the input of alphaimage.
 *
 *      - [color]image disregard currentalpha, and treat all input as
 * opaque (alpha = 1).  */
/*
 * Just in case we ever change our minds about the direction of
 * premultiplication, uncommenting the following preprocessor definition is
 * supposed to produce premultiplication towards white.
 */
/*#define PREMULTIPLY_TOWARDS_WHITE */

#endif /* gxalpha_INCLUDED */
