libpng.txt

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                         keyword.  Can be empty.
    text_ptr[i].text_length - length of text string,
                 after decompression, 0 for iTXt
    text_ptr[i].itxt_length - length of itxt string,
                 after decompression, 0 for tEXt/zTXt
    text_ptr[i].lang  - language of comment (empty
                         string for unknown).
    text_ptr[i].lang_key  - keyword in UTF-8
                         (empty string for unknown).
    num_text       - number of comments (same as
                     num_comments; you can put NULL here
                     to avoid the duplication)
    Note while png_set_text() will accept text, language,
    and translated keywords that can be NULL pointers, the
    structure returned by png_get_text will always contain
    regular zero-terminated C strings.  They might be
    empty strings but they will never be NULL pointers.

    num_spalettes = png_get_sPLT(png_ptr, info_ptr,
       &palette_ptr);
    palette_ptr    - array of palette structures holding
                     contents of one or more sPLT chunks
                     read.
    num_spalettes  - number of sPLT chunks read.

    png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
       &unit_type);
    offset_x       - positive offset from the left edge
                     of the screen
    offset_y       - positive offset from the top edge
                     of the screen
    unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

    png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
       &unit_type);
    res_x          - pixels/unit physical resolution in
                     x direction
    res_y          - pixels/unit physical resolution in
                     x direction
    unit_type      - PNG_RESOLUTION_UNKNOWN,
                     PNG_RESOLUTION_METER

    png_get_sCAL(png_ptr, info_ptr, &unit, &width,
       &height)
    unit        - physical scale units (an integer)
    width       - width of a pixel in physical scale units
    height      - height of a pixel in physical scale units
                 (width and height are doubles)

    png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
       &height)
    unit        - physical scale units (an integer)
    width       - width of a pixel in physical scale units
    height      - height of a pixel in physical scale units
                 (width and height are strings like "2.54")

    num_unknown_chunks = png_get_unknown_chunks(png_ptr,
       info_ptr, &unknowns)
    unknowns          - array of png_unknown_chunk
                        structures holding unknown chunks
    unknowns[i].name  - name of unknown chunk
    unknowns[i].data  - data of unknown chunk
    unknowns[i].size  - size of unknown chunk's data
    unknowns[i].location - position of chunk in file

    The value of "i" corresponds to the order in which the
    chunks were read from the PNG file or inserted with the
    png_set_unknown_chunks() function.

The data from the pHYs chunk can be retrieved in several convenient
forms:

    res_x = png_get_x_pixels_per_meter(png_ptr,
       info_ptr)
    res_y = png_get_y_pixels_per_meter(png_ptr,
       info_ptr)
    res_x_and_y = png_get_pixels_per_meter(png_ptr,
       info_ptr)
    res_x = png_get_x_pixels_per_inch(png_ptr,
       info_ptr)
    res_y = png_get_y_pixels_per_inch(png_ptr,
       info_ptr)
    res_x_and_y = png_get_pixels_per_inch(png_ptr,
       info_ptr)
    aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
       info_ptr)

   (Each of these returns 0 [signifying "unknown"] if
       the data is not present or if res_x is 0;
       res_x_and_y is 0 if res_x != res_y)

The data from the oFFs chunk can be retrieved in several convenient
forms:

    x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
    y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
    x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
    y_offset = png_get_y_offset_inches(png_ptr, info_ptr);

   (Each of these returns 0 [signifying "unknown" if both
       x and y are 0] if the data is not present or if the
       chunk is present but the unit is the pixel)

For more information, see the png_info definition in png.h and the
PNG specification for chunk contents.  Be careful with trusting
rowbytes, as some of the transformations could increase the space
needed to hold a row (expand, filler, gray_to_rgb, etc.).
See png_read_update_info(), below.

A quick word about text_ptr and num_text.  PNG stores comments in
keyword/text pairs, one pair per chunk, with no limit on the number
of text chunks, and a 2^31 byte limit on their size.  While there are
suggested keywords, there is no requirement to restrict the use to these
strings.  It is strongly suggested that keywords and text be sensible
to humans (that's the point), so don't use abbreviations.  Non-printing
symbols are not allowed.  See the PNG specification for more details.
There is also no requirement to have text after the keyword.

Keywords should be limited to 79 Latin-1 characters without leading or
trailing spaces, but non-consecutive spaces are allowed within the
keyword.  It is possible to have the same keyword any number of times.
The text_ptr is an array of png_text structures, each holding a
pointer to a language string, a pointer to a keyword and a pointer to
a text string.  The text string, language code, and translated
keyword may be empty or NULL pointers.  The keyword/text
pairs are put into the array in the order that they are received.
However, some or all of the text chunks may be after the image, so, to
make sure you have read all the text chunks, don't mess with these
until after you read the stuff after the image.  This will be
mentioned again below in the discussion that goes with png_read_end().

Input transformations

After you've read the header information, you can set up the library
to handle any special transformations of the image data.  The various
ways to transform the data will be described in the order that they
should occur.  This is important, as some of these change the color
type and/or bit depth of the data, and some others only work on
certain color types and bit depths.  Even though each transformation
checks to see if it has data that it can do something with, you should
make sure to only enable a transformation if it will be valid for the
data.  For example, don't swap red and blue on grayscale data.

The colors used for the background and transparency values should be
supplied in the same format/depth as the current image data.  They
are stored in the same format/depth as the image data in a bKGD or tRNS
chunk, so this is what libpng expects for this data.  The colors are
transformed to keep in sync with the image data when an application
calls the png_read_update_info() routine (see below).

Data will be decoded into the supplied row buffers packed into bytes
unless the library has been told to transform it into another format.
For example, 4 bit/pixel paletted or grayscale data will be returned
2 pixels/byte with the leftmost pixel in the high-order bits of the
byte, unless png_set_packing() is called.  8-bit RGB data will be stored
in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
is called to insert filler bytes, either before or after each RGB triplet.
16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
byte of the color value first, unless png_set_strip_16() is called to
transform it to regular RGB RGB triplets, or png_set_filler() or
png_set_add alpha() is called to insert filler bytes, either before or
after each RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data can
be modified with
png_set_filler(), png_set_add_alpha(), or png_set_strip_16().

The following code transforms grayscale images of less than 8 to 8 bits,
changes paletted images to RGB, and adds a full alpha channel if there is
transparency information in a tRNS chunk.  This is most useful on
grayscale images with bit depths of 2 or 4 or if there is a multiple-image
viewing application that wishes to treat all images in the same way.

    if (color_type == PNG_COLOR_TYPE_PALETTE)
        png_set_palette_to_rgb(png_ptr);

    if (color_type == PNG_COLOR_TYPE_GRAY &&
        bit_depth < 8) png_set_gray_1_2_4_to_8(png_ptr);

    if (png_get_valid(png_ptr, info_ptr,
        PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);

These three functions are actually aliases for png_set_expand(), added
in libpng version 1.0.4, with the function names expanded to improve code
readability.  In some future version they may actually do different
things.

PNG can have files with 16 bits per channel.  If you only can handle
8 bits per channel, this will strip the pixels down to 8 bit.

    if (bit_depth == 16)
        png_set_strip_16(png_ptr);

If, for some reason, you don't need the alpha channel on an image,
and you want to remove it rather than combining it with the background
(but the image author certainly had in mind that you *would* combine
it with the background, so that's what you should probably do):

    if (color_type & PNG_COLOR_MASK_ALPHA)
        png_set_strip_alpha(png_ptr);

In PNG files, the alpha channel in an image
is the level of opacity.  If you need the alpha channel in an image to
be the level of transparency instead of opacity, you can invert the
alpha channel (or the tRNS chunk data) after it's read, so that 0 is
fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
images) is fully transparent, with

    png_set_invert_alpha(png_ptr);

PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
they can, resulting in, for example, 8 pixels per byte for 1 bit
files.  This code expands to 1 pixel per byte without changing the
values of the pixels:

    if (bit_depth < 8)
        png_set_packing(png_ptr);

PNG files have possible bit depths of 1, 2, 4, 8, and 16.  All pixels
stored in a PNG image have been "scaled" or "shifted" up to the next
higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] to
8 bits/sample in the range [0, 255]).  However, it is also possible to
convert the PNG pixel data back to the original bit depth of the image.
This call reduces the pixels back down to the original bit depth:

    png_color_8p sig_bit;

    if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
        png_set_shift(png_ptr, sig_bit);

PNG files store 3-color pixels in red, green, blue order.  This code
changes the storage of the pixels to blue, green, red:

    if (color_type == PNG_COLOR_TYPE_RGB ||
        color_type == PNG_COLOR_TYPE_RGB_ALPHA)
        png_set_bgr(png_ptr);

PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
into 4 or 8 bytes for windowing systems that need them in this format:

    if (color_type == PNG_COLOR_TYPE_RGB)
        png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);

where "filler" is the 8 or 16-bit number to fill with, and the location is
either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
you want the filler before the RGB or after.  This transformation
does not affect images that already have full alpha channels.  To add an
opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
will generate RGBA pixels.

Note that png_set_filler() does not change the color type.  If you want
to do that, you can add a true alpha channel with

    if (color_type == PNG_COLOR_TYPE_RGB ||
           color_type == PNG_COLOR_TYPE_GRAY)
    png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);

where "filler" contains the alpha value to assign to each pixel.
This function was added in libpng-1.2.7.

If you are reading an image with an alpha channel, and you need the
data as ARGB instead of the normal PNG format RGBA:

    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_fixed(png_ptr, error_action,
             int red_weight, int 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 times 100000
    green_weight:     weight of green component times 100000
                      If either weight is negative, default
                      weights (21268, 71514) are used.

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.  bKGD and sBIT data
will be silently converted to grayscale, using the green channel
data, regardless of the error_action setting.

With red_weight+green_weight<=100000,
the normalized graylevel is computed:

    int rw = red_weight * 65536;
    int gw = green_weight * 65536;
    int bw = 65536 - (rw + gw);