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 */
#define PNG_ALPHA_PNG           0 /* according to the PNG standard */
#define PNG_ALPHA_STANDARD      1 /* according to Porter/Duff */
#define PNG_ALPHA_ASSOCIATED    1 /* as above; this is the normal practice */
#define PNG_ALPHA_PREMULTIPLIED 1 /* as above */
#define PNG_ALPHA_OPTIMIZED     2 /* 'PNG' for opaque pixels, else 'STANDARD' */
#define PNG_ALPHA_BROKEN        3 /* the alpha channel is gamma encoded */

PNG_FP_EXPORT(227, void, png_set_alpha_mode, (png_structrp png_ptr, int mode,
    double output_gamma))
PNG_FIXED_EXPORT(228, void, png_set_alpha_mode_fixed, (png_structrp png_ptr,
    int mode, png_fixed_point output_gamma))
#endif

#if defined(PNG_GAMMA_SUPPORTED) || defined(PNG_READ_ALPHA_MODE_SUPPORTED)
/* The output_gamma value is a screen gamma in libpng terminology: it expresses
 * how to decode the output values, not how they are encoded.
 */
#define PNG_DEFAULT_sRGB -1       /* sRGB gamma and color space */
#define PNG_GAMMA_MAC_18 -2       /* Old Mac '1.8' gamma and color space */
#define PNG_GAMMA_sRGB   220000   /* Television standards--matches sRGB gamma */
#define PNG_GAMMA_LINEAR PNG_FP_1 /* Linear */
#endif

/* The following are examples of calls to png_set_alpha_mode to achieve the
 * required overall gamma correction and, where necessary, alpha
 * premultiplication.
 *
 * png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
 *    This is the default libpng handling of the alpha channel - it is not
 *    pre-multiplied into the color components.  In addition the call states
 *    that the output is for a sRGB system and causes all PNG files without gAMA
 *    chunks to be assumed to be encoded using sRGB.
 *
 * png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
 *    In this case the output is assumed to be something like an sRGB conformant
 *    display preceeded by a power-law lookup table of power 1.45.  This is how
 *    early Mac systems behaved.
 *
 * png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);
 *    This is the classic Jim Blinn approach and will work in academic
 *    environments where everything is done by the book.  It has the shortcoming
 *    of assuming that input PNG data with no gamma information is linear - this
 *    is unlikely to be correct unless the PNG files where generated locally.
 *    Most of the time the output precision will be so low as to show
 *    significant banding in dark areas of the image.
 *
 * png_set_expand_16(pp);
 * png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);
 *    This is a somewhat more realistic Jim Blinn inspired approach.  PNG files
 *    are assumed to have the sRGB encoding if not marked with a gamma value and
 *    the output is always 16 bits per component.  This permits accurate scaling
 *    and processing of the data.  If you know that your input PNG files were
 *    generated locally you might need to replace PNG_DEFAULT_sRGB with the
 *    correct value for your system.
 *
 * png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);
 *    If you just need to composite the PNG image onto an existing background
 *    and if you control the code that does this you can use the optimization
 *    setting.  In this case you just copy completely opaque pixels to the
 *    output.  For pixels that are not completely transparent (you just skip
 *    those) you do the composition math using png_composite or png_composite_16
 *    below then encode the resultant 8-bit or 16-bit values to match the output
 *    encoding.
 *
 * Other cases
 *    If neither the PNG nor the standard linear encoding work for you because
 *    of the software or hardware you use then you have a big problem.  The PNG
 *    case will probably result in halos around the image.  The linear encoding
 *    will probably result in a washed out, too bright, image (it's actually too
 *    contrasty.)  Try the ALPHA_OPTIMIZED mode above - this will probably
 *    substantially reduce the halos.  Alternatively try:
 *
 * png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);
 *    This option will also reduce the halos, but there will be slight dark
 *    halos round the opaque parts of the image where the background is light.
 *    In the OPTIMIZED mode the halos will be light halos where the background
 *    is dark.  Take your pick - the halos are unavoidable unless you can get
 *    your hardware/software fixed!  (The OPTIMIZED approach is slightly
 *    faster.)
 *
 * When the default gamma of PNG files doesn't match the output gamma.
 *    If you have PNG files with no gamma information png_set_alpha_mode allows
 *    you to provide a default gamma, but it also sets the ouput gamma to the
 *    matching value.  If you know your PNG files have a gamma that doesn't
 *    match the output you can take advantage of the fact that
 *    png_set_alpha_mode always sets the output gamma but only sets the PNG
 *    default if it is not already set:
 *
 * png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
 * png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);
 *    The first call sets both the default and the output gamma values, the
 *    second call overrides the output gamma without changing the default.  This
 *    is easier than achieving the same effect with png_set_gamma.  You must use
 *    PNG_ALPHA_PNG for the first call - internal checking in png_set_alpha will
 *    fire if more than one call to png_set_alpha_mode and png_set_background is
 *    made in the same read operation, however multiple calls with PNG_ALPHA_PNG
 *    are ignored.
 */

#ifdef PNG_READ_STRIP_ALPHA_SUPPORTED
PNG_EXPORT(36, void, png_set_strip_alpha, (png_structrp png_ptr));
#endif

#if defined(PNG_READ_SWAP_ALPHA_SUPPORTED) || \
    defined(PNG_WRITE_SWAP_ALPHA_SUPPORTED)
PNG_EXPORT(37, void, png_set_swap_alpha, (png_structrp png_ptr));
#endif

#if defined(PNG_READ_INVERT_ALPHA_SUPPORTED) || \
    defined(PNG_WRITE_INVERT_ALPHA_SUPPORTED)
PNG_EXPORT(38, void, png_set_invert_alpha, (png_structrp png_ptr));
#endif

#if defined(PNG_READ_FILLER_SUPPORTED) || defined(PNG_WRITE_FILLER_SUPPORTED)
/* Add a filler byte to 8-bit Gray or 24-bit RGB images. */
PNG_EXPORT(39, void, png_set_filler, (png_structrp png_ptr, png_uint_32 filler,
    int flags));
/* The values of the PNG_FILLER_ defines should NOT be changed */
#  define PNG_FILLER_BEFORE 0
#  define PNG_FILLER_AFTER 1