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    if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
       png_set_swap_alpha(png_ptr);

For some uses, you may want a grayscale image to be represented as
RGB.  This code will do that conversion:

    if (color_type == PNG_COLOR_TYPE_GRAY ||
        color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
       png_set_gray_to_rgb(png_ptr);

Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
with alpha.

    if (color_type == PNG_COLOR_TYPE_RGB ||
        color_type == PNG_COLOR_TYPE_RGB_ALPHA)
       png_set_rgb_to_gray(png_ptr, error_action,
          double red_weight, double green_weight);

    error_action = 1: silently do the conversion

    error_action = 2: issue a warning if the original
                      image has any pixel where
                      red != green or red != blue

    error_action = 3: issue an error and abort the
                      conversion if the original
                      image has any pixel where
                      red != green or red != blue

    red_weight:       weight of red component

    green_weight:     weight of green component
                      If either weight is negative, default
                      weights are used.

In the corresponding fixed point API the red_weight and green_weight values are
simply scaled by 100,000:

    png_set_rgb_to_gray(png_ptr, error_action,
       png_fixed_point red_weight,
       png_fixed_point green_weight);

If you have set error_action = 1 or 2, you can
later check whether the image really was gray, after processing
the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
It will return a png_byte that is zero if the image was gray or
1 if there were any non-gray pixels.  Background and sBIT data
will be silently converted to grayscale, using the green channel
data for sBIT, regardless of the error_action setting.

The default values come from the PNG file cHRM chunk if present; otherwise, the
defaults correspond to the ITU-R recommendation 709, and also the sRGB color
space, as recommended in the Charles Poynton's Colour FAQ,
<http://www.poynton.com/>, in section 9:

   <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>

    Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

Previous versions of this document, 1998 through 2002, recommended a slightly
different formula:

    Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

Libpng uses an integer approximation:

    Y = (6968 * R + 23434 * G + 2366 * B)/32768

The calculation is done in a linear colorspace, if the image gamma
can be determined.

The png_set_background() function has been described already; it tells libpng to
composite images with alpha or simple transparency against the supplied
background color.  For compatibility with versions of libpng earlier than
libpng-1.5.4 it is recommended that you call the function after reading the file
header, even if you don't want to use the color in a bKGD chunk, if one exists.

If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
you may use this color, or supply another color more suitable for
the current display (e.g., the background color from a web page).  You
need to tell libpng how the color is represented, both the format of the
component values in the color (the number of bits) and the gamma encoding of the
color.  The function takes two arguments, background_gamma_mode and need_expand
to convey this information; however, only two combinations are likely to be
useful:

    png_color_16 my_background;
    png_color_16p image_background;

    if (png_get_bKGD(png_ptr, info_ptr, &image_background))
       png_set_background(png_ptr, image_background,
           PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
    else
       png_set_background(png_ptr, &my_background,
           PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);

The second call was described above - my_background is in the format of the
final, display, output produced by libpng.  Because you now know the format of
the PNG it is possible to avoid the need to choose either 8-bit or 16-bit
output and to retain palette images (the palette colors will be modified
appropriately and the tRNS chunk removed.)  However, if you are doing this,
take great care not to ask for transformations without checking first that
they apply!

In the first call the background color has the original bit depth and color type
of the PNG file.  So, for palette images the color is supplied as a palette
index and for low bit greyscale images the color is a reduced bit value in
image_background->gray.

If you didn't call png_set_gamma() before reading the file header, for example
if you need your code to remain compatible with older versions of libpng prior
to libpng-1.5.4, this is the place to call it.

Do not call it if you called png_set_alpha_mode(); doing so will damage the
settings put in place by png_set_alpha_mode().  (If png_set_alpha_mode() is
supported then you can certainly do png_set_gamma() before reading the PNG
header.)

This API unconditionally sets the screen and file gamma values, so it will
override the value in the PNG file unless it is called before the PNG file
reading starts.  For this reason you must always call it with the PNG file