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[libpng16] Describe new simplified API in the manual pages

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and cleaned up some indentation problems.
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Glenn Randers-Pehrson 2012-01-11 14:00:52 -06:00
parent 1531bd61a0
commit 0501c1c570
4 changed files with 726 additions and 657 deletions
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5
ANNOUNCE
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@ -1,5 +1,5 @@
Libpng 1.6.0beta05 - December 30, 2011
Libpng 1.6.0beta05 - January 11, 2012
This is not intended to be a public release. It will be replaced
within a few weeks by a public version or by another test version.
@ -104,7 +104,8 @@ Version 1.6.0beta04 [December 30, 2011]
Revised contrib/pngminus/pnm2png.c to avoid warnings when png_uint_32
and unsigned long are of different sizes.
Version 1.6.0beta05 [December 30, 2011]
Version 1.6.0beta05 [January 11, 2012]
Updated manual with description of the simplified API (copied from png.h)
Send comments/corrections/commendations to png-mng-implement at lists.sf.net
(subscription required; visit

3
CHANGES
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@ -3855,7 +3855,8 @@ Version 1.6.0beta04 [December 30, 2011]
Revised contrib/pngminus/pnm2png.c to avoid warnings when png_uint_32
and unsigned long are of different sizes.
Version 1.6.0beta05 [December 30, 2011]
Version 1.6.0beta05 [January 11, 2012]
Updated manual with description of the simplified API (copied from png.h)
Send comments/corrections/commendations to png-mng-implement at lists.sf.net
(subscription required; visit

417
libpng-manual.txt
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@ -1,6 +1,6 @@
libpng-manual.txt - A description on how to use and modify libpng
libpng version 1.6.0beta05 - December 30, 2011
libpng version 1.6.0beta05 - January 11, 2012
Updated and distributed by Glenn Randers-Pehrson
<glennrp at users.sourceforge.net>
Copyright (c) 1998-2011 Glenn Randers-Pehrson
@ -11,7 +11,7 @@ libpng-manual.txt - A description on how to use and modify libpng
Based on:
libpng versions 0.97, January 1998, through 1.6.0beta05 - December 30, 2011
libpng versions 0.97, January 1998, through 1.6.0beta05 - January 11, 2012
Updated and distributed by Glenn Randers-Pehrson
Copyright (c) 1998-2011 Glenn Randers-Pehrson
@ -34,17 +34,18 @@ libpng-manual.txt - A description on how to use and modify libpng
II. Structures
III. Reading
IV. Writing
V. Modifying/Customizing libpng
VI. MNG support
VII. Changes to Libpng from version 0.88
VIII. Changes to Libpng from version 1.0.x to 1.2.x
IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
X. Changes to Libpng from version 1.4.x to 1.5.x
XI. Changes to Libpng from version 1.5.x to 1.6.x
XII. Detecting libpng
XIII. Source code repository
XIV. Coding style
XV. Y2K Compliance in libpng
V. Simplified API
VI. Modifying/Customizing libpng
VII. MNG support
VIII. Changes to Libpng from version 0.88
IX. Changes to Libpng from version 1.0.x to 1.2.x
X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
XI. Changes to Libpng from version 1.4.x to 1.5.x
XII. Changes to Libpng from version 1.5.x to 1.6.x
XIII. Detecting libpng
XIV. Source code repository
XV. Coding style
XVI. Y2K Compliance in libpng
I. Introduction
@ -707,7 +708,8 @@ assumes that the PNG data matches your system, to keep this default call:
or you can use the fixed point equivalent:
png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma, PNG_FP_1/screen_gamma);
png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
PNG_FP_1/screen_gamma);
If you don't know the gamma for your system it is probably 2.2 - a good
approximation to the IEC standard for display systems (sRGB). If images are
@ -719,12 +721,15 @@ display driver, a few systems, including older Macs, change the response by
default. As of 1.5.4 three special values are available to handle common
situations:
PNG_DEFAULT_sRGB: Indicates that the system conforms to the IEC 61966-2-1
standard. This matches almost all systems.
PNG_GAMMA_MAC_18: Indicates that the system is an older (pre Mac OS 10.6)
Apple Macintosh system with the default settings.
PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates that the
system expects data with no gamma encoding.
PNG_DEFAULT_sRGB: Indicates that the system conforms to the
IEC 61966-2-1 standard. This matches almost
all systems.
PNG_GAMMA_MAC_18: Indicates that the system is an older
(pre Mac OS 10.6) Apple Macintosh system with
the default settings.
PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
that the system expects data with no gamma
encoding.
You would use the linear (unencoded) value if you need to process the pixel
values further because this avoids the need to decode and reencode each
@ -741,11 +746,11 @@ Libpng only supports composing onto a single color (using png_set_background;
see below). Otherwise you must do the composition yourself and, in this case,
you may need to call png_set_alpha_mode:
#if PNG_LIBPNG_VER >= 10504
png_set_alpha_mode(png_ptr, mode, screen_gamma);
#else
png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
#endif
#if PNG_LIBPNG_VER >= 10504
png_set_alpha_mode(png_ptr, mode, screen_gamma);
#else
png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
#endif
The screen_gamma value is the same as the argument to png_set_gamma; however,
how it affects the output depends on the mode. png_set_alpha_mode() sets the
@ -1165,27 +1170,32 @@ pointer into the info_ptr is returned for any complex types.
int_file_gamma - 100,000 times the gamma at which the
file is written
png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x, &red_y,
&green_x, &green_y, &blue_x, &blue_y)
png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z, &green_X,
&green_Y, &green_Z, &blue_X, &blue_Y, &blue_Z)
png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x, &int_white_y,
&int_red_x, &int_red_y, &int_green_x, &int_green_y,
&int_blue_x, &int_blue_y)
png_get_cHRM(png_ptr, info_ptr, &white_x, &white_y, &red_x,
&red_y, &green_x, &green_y, &blue_x, &blue_y)
png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
&green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
&blue_Z)
png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
&int_white_y, &int_red_x, &int_red_y,
&int_green_x, &int_green_y, &int_blue_x,
&int_blue_y)
png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
&int_red_Z, &int_green_X, &int_green_Y, &int_green_Z,
&int_blue_X, &int_blue_Y, &int_blue_Z)
&int_red_Z, &int_green_X, &int_green_Y,
&int_green_Z, &int_blue_X, &int_blue_Y,
&int_blue_Z)
{white,red,green,blue}_{x,y}
A color space encoding specified using the chromaticities
of the end points and the white point. (PNG_INFO_cHRM)
A color space encoding specified using the
chromaticities of the end points and the
white point. (PNG_INFO_cHRM)
{red,green,blue}_{X,Y,Z}
A color space encoding specified using the encoding end
points - the CIE tristimulus specification of the intended
color of the red, green and blue channels in the PNG RGB
data. The white point is simply the sum of the three end
points. (PNG_INFO_cHRM)
A color space encoding specified using the
encoding end points - the CIE tristimulus
specification of the intended color of the red,
green and blue channels in the PNG RGB data.
The white point is simply the sum of the three
end points. (PNG_INFO_cHRM)
png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
@ -1589,17 +1599,19 @@ Within the matrix,
"A" means the transformation is obtained by png_set_add_alpha().
"X" means the transformation is obtained by png_set_expand().
"1" means the transformation is obtained by
png_set_expand_gray_1_2_4_to_8() (and by png_set_expand() if there
is no transparency in the original or the final format).
png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
if there is no transparency in the original or the final
format).
"C" means the transformation is obtained by png_set_gray_to_rgb().
"G" means the transformation is obtained by png_set_rgb_to_gray().
"P" means the transformation is obtained by
png_set_expand_palette_to_rgb().
"p" means the transformation is obtained by png_set_packing().
"Q" means the transformation is obtained by png_set_quantize().
"T" means the transformation is obtained by png_set_tRNS_to_alpha().
"B" means the transformation is obtained by png_set_background(), or
png_strip_alpha().
"T" means the transformation is obtained by
png_set_tRNS_to_alpha().
"B" means the transformation is obtained by
png_set_background(), or png_strip_alpha().
When an entry has multiple transforms listed all are required to cause the
right overall transformation. When two transforms are separated by a comma
@ -1684,8 +1696,8 @@ 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);
png_set_rgb_to_gray(png_ptr, error_action,
double red_weight, double green_weight);
error_action = 1: silently do the conversion
@ -1707,7 +1719,8 @@ with alpha.
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_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
@ -3450,7 +3463,283 @@ if you transfer responsibility for free'ing text_ptr from libpng to your
application, your application must not separately free those members.
For a more compact example of writing a PNG image, see the file example.c.
V. Modifying/Customizing libpng
V. Simplified API
The simplified API, which became available in libpng-1.6.0, hides the details
of both libpng and the PNG file format itself.
It allows PNG files to be read into a very limited number of
in-memory bitmap formats or to be written from the same formats. If these
formats do not accomodate your needs then you can, and should, use the more
sophisticated APIs above - these support a wide variety of in-memory formats
and a wide variety of sophisticated transformations to those formats as well
as a wide variety of APIs to manipulate ancilliary information.
To read a PNG file using the simplified API:
1) Declare a 'png_image' structure (see below) on the
stack and memset() it to all zero.
2) Call the appropriate png_image_begin_read... function.
3) Set the png_image 'format' member to the required
format and allocate a buffer for the image.
4) Call png_image_finish_read to read the image into
your buffer.
There are no restrictions on the format of the PNG input itself; all valid
color types, bit depths, and interlace methods are acceptable, and the
input image is transformed as necessary to the requested in-memory format
during the png_image_finish_read() step.
To write a PNG file using the simplified API:
1) Declare a 'png_image' structure on the stack and memset()
it to all zero.
2) Initialize the members of the structure that describe the
image, setting the 'format' member to the format of the
image in memory.
3) Call the appropriate png_image_write... function with a
pointer to the image to write the PNG data.
png_image is a structure that describes the in-memory format of an image
when it is being read or define the in-memory format of an image that you
need to write. The "png_image" structure contains the following members:
png_uint_32 width Image width in pixels (columns)
png_uint_32 height Image height in pixels (rows)
png_uint_32 format Image format as defined below
png_uint_32 flags A bit mask containing informational flags
png_controlp opaque Initialize to NULL, free with png_image_free
In the event of an error or warning the following field will be set to a
non-zero value and the 'message' field will contain a '\0' terminated
string with the libpng error or warning message. If both warnings and
an error were encountered, only the error is recorded. If there
are multiple warnings, only the first one is recorded.
As of libpng-1.6.0 the values are
0 - no warning or error
1 - error
2 - warning
png_uint_32 warning_or_error;
char message[64];
The pixels (samples) of the image have one to four channels whose components
have original values in the range 0 to 1.0:
1: A single gray or luminance channel (G).
2: A gray/luminance channel and an alpha channel (GA).
3: Three red, green, blue color channels (RGB).
4: Three color channels and an alpha channel (RGBA).
The channels are encoded in one of two ways:
a) As a small integer, value 0..255, contained in a (png_byte). For the
alpha channel the original value is simply value/255. For the color or
luminance channels the value is encoded according to the sRGB specification
and matches the 8-bit format expected by typical display devices.
The color/gray channels are not scaled (pre-multiplied) by the alpha
channel and are suitable for passing to color management software.
b) As a value in the range 0..65535, contained in a (png_uint_16). All
channels can be converted to the original value by dividing by 65535; all
channels are linear. Color channels use the RGB encoding (RGB end-points) of
the sRGB specification. This encoding is identified by the
PNG_FORMAT_FLAG_LINEAR flag below.
When an alpha channel is present it is expected to denote pixel coverage
of the color or luminance channels and is returned as an associated alpha
channel: the color/gray channels are scaled (pre-multiplied) by the alpha
value.
PNG_FORMAT_*
The #defines to be used in png_image::format. Each #define identifies a
particular layout of channel data and, if present, alpha values. There are
separate defines for each of the two channel encodings.
A format is built up using single bit flag values. Not all combinations are
valid: use the bit flag values below for testing a format returned by the
read APIs, but set formats from the derived values.
NOTE: libpng can be built with particular features disabled, if you see
compiler errors because the definition of one of the following flags has been
compiled out it is because libpng does not have the required support. It is
possible, however, for the libpng configuration to enable the format on just
read or just write; in that case you may see an error at run time. You can
guard against this by checking for the definition of:
PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED
PNG_FORMAT_FLAG_ALPHA 0x01 format with an alpha channel
PNG_FORMAT_FLAG_COLOR 0x02 color format: otherwise grayscale
PNG_FORMAT_FLAG_LINEAR 0x04 png_uint_16 channels else png_byte
PNG_FORMAT_FLAG_BGR 0x08 BGR colors, else order is RGB
PNG_FORMAT_FLAG_AFIRST 0x10 alpha channel comes first
Supported formats are as follows. Future versions of libpng may support more
formats; for compatibility with older versions simply check if the format
macro is defined using #ifdef. These defines describe the in-memory layout
of the components of the pixels of the image.
First the single byte formats:
PNG_FORMAT_GRAY 0
PNG_FORMAT_GA PNG_FORMAT_FLAG_ALPHA
PNG_FORMAT_AG (PNG_FORMAT_GA|PNG_FORMAT_FLAG_AFIRST)
PNG_FORMAT_RGB PNG_FORMAT_FLAG_COLOR
PNG_FORMAT_BGR (PNG_FORMAT_FLAG_COLOR|PNG_FORMAT_FLAG_BGR)
PNG_FORMAT_RGBA (PNG_FORMAT_RGB|PNG_FORMAT_FLAG_ALPHA)
PNG_FORMAT_ARGB (PNG_FORMAT_RGBA|PNG_FORMAT_FLAG_AFIRST)
PNG_FORMAT_BGRA (PNG_FORMAT_BGR|PNG_FORMAT_FLAG_ALPHA)
PNG_FORMAT_ABGR (PNG_FORMAT_BGRA|PNG_FORMAT_FLAG_AFIRST)
Then the linear (png_uint_16) formats. When naming these "Y" is used to
indicate a luminance (gray) channel. The component order within the pixel
is always the same - there is no provision for swapping the order of the
components in the linear format.
PNG_FORMAT_LINEAR_Y PNG_FORMAT_FLAG_LINEAR
PNG_FORMAT_LINEAR_Y_ALPHA
(PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_ALPHA)
PNG_FORMAT_LINEAR_RGB
(PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR)
PNG_FORMAT_LINEAR_RGB_ALPHA
(PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR|
PNG_FORMAT_FLAG_ALPHA)
PNG_IMAGE macros
These are convenience macros to derive information from a png_image structure:
PNG_IMAGE_CHANNELS(fmt)
Returns the total number of channels in a given format: 1..4
PNG_IMAGE_COMPONENT_SIZE(fmt)
Returns the size in bytes of a single component of a pixel
in the image.
PNG_IMAGE_PIXEL_SIZE(fmt)
Returns the size in bytes of a single pixel in the image.
PNG_IMAGE_ROW_STRIDE(image)
Returns the total number of components in a single row
of the image; this is the minimum 'row stride', the minimum
count of components between each row.
PNG_IMAGE_BUFFER_SIZE(image, row_stride)
Returns the size, in bytes, of an image buffer given
a png_image and a row stride - the number of components to
leave space for in each row.
PNG_IMAGE_SIZE(image)
Returns the size, in bytes, of the image in memory given
just a png_image; the row stride is the minimum stride
required for the image.
PNG_IMAGE_FLAG_
Flags containing additional information about the image are held
in the 'flags' field of png_image.
PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB
This indicates the the RGB values of the in-memory
bitmap do not correspond to the red, green and blue
end-points defined by sRGB.
READ APIs
The png_image passed to the read APIs must have been initialized by setting
the png_controlp field 'opaque' to NULL (or, better, memset the whole thing.)
int png_image_begin_read_from_file( png_imagep image,
const char *file_name)
The named file is opened for read and the image header
is filled in from the PNG header in the file.
int png_image_begin_read_from_stdio (png_imagep image,
FILE* file)
The PNG header is read from the stdio FILE object.
int png_image_begin_read_from_memory(png_imagep image,
png_const_voidp memory, png_size_t size)
The PNG header is read from the given memory buffer.
int png_image_finish_read(png_imagep image,
png_colorp background, void *buffer, png_int_32 row_stride));
Finish reading the image into the supplied buffer and
clean up the png_image structure.
row_stride is the step, in png_byte or png_uint_16 units
as appropriate, between adjacent rows. A positive stride
indicates that the top-most row is first in the buffer -
the normal top-down arrangement. A negative stride
indicates that the bottom-most row is first in the buffer.
background need only be supplied if an alpha channel must
be removed from a png_byte format and the removal is to be
done by compositing on a solid color; otherwise it may be
NULL and any composition will be done directly onto the
buffer. The value is an sRGB color to use for the
background, for grayscale output the green channel is used.
For linear output removing the alpha channel is always done
by compositing on black.
void png_image_free(png_imagep image)
Free any data allocated by libpng in image->opaque,
setting the pointer to NULL. May be called at any time
after the structure is initialized.
WRITE APIS
For write you must initialize a png_image structure to describe the image to
be written:
opaque: must be initialized to NULL
width: image width in pixels
height: image height in rows
format: the format of the data you wish to write
flags: set to 0 unless one of the defined flags applies; set
PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images
where the RGB values do not correspond to the colors in sRGB.
int png_image_write_to_file, (png_imagep image,
const char *file, int convert_to_8bit, const void *buffer,
png_int_32 row_stride));
Write the image to the named file.
int png_image_write_to_stdio(png_imagep image, FILE *file,
int convert_to_8_bit, const void *buffer, png_int_32 row_stride)
Write the image to the given (FILE*).
With all write APIs if image is in one of the linear formats with
(png_uint_16) data then setting convert_to_8_bit will cause the output to be
a (png_byte) PNG gamma encoded according to the sRGB specification, otherwise
a 16-bit linear encoded PNG file is written.
With all APIs row_stride is handled as in the read APIs - it is the spacing
from one row to the next in component sized units (float) and if negative
indicates a bottom-up row layout in the buffer.
Note that the write API does not support interlacing, sub-8-bit pixels,
and indexed (paletted) images.
VI. Modifying/Customizing libpng
There are two issues here. The first is changing how libpng does
standard things like memory allocation, input/output, and error handling.
@ -3871,7 +4160,7 @@ When PNG_DEBUG = 1, the macros are defined, but only png_debug statements
having level = 0 will be printed. There aren't any such statements in
this version of libpng, but if you insert some they will be printed.
VI. MNG support
VII. MNG support
The MNG specification (available at http://www.libpng.org/pub/mng) allows
certain extensions to PNG for PNG images that are embedded in MNG datastreams.
@ -3898,7 +4187,7 @@ or any other MNG chunks; your application must provide its own support for
them. You may wish to consider using libmng (available at
http://www.libmng.com) instead.
VII. Changes to Libpng from version 0.88
VIII. Changes to Libpng from version 0.88
It should be noted that versions of libpng later than 0.96 are not
distributed by the original libpng author, Guy Schalnat, nor by
@ -3950,7 +4239,7 @@ application:
png_uint_32 application_vn = PNG_LIBPNG_VER;
VIII. Changes to Libpng from version 1.0.x to 1.2.x
IX. Changes to Libpng from version 1.0.x to 1.2.x
Support for user memory management was enabled by default. To
accomplish this, the functions png_create_read_struct_2(),
@ -4047,7 +4336,7 @@ which also expands tRNS to alpha was replaced with
png_set_expand_gray_1_2_4_to_8()
which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.
IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
Private libpng prototypes and macro definitions were moved from
png.h and pngconf.h into a new pngpriv.h header file.
@ -4155,7 +4444,7 @@ was renamed to PNG_READ_QUANTIZE_SUPPORTED.
We removed the trailing '.' from the warning and error messages.
X. Changes to Libpng from version 1.4.x to 1.5.x
XI. Changes to Libpng from version 1.4.x to 1.5.x
From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
function) incorrectly returned a value of type png_uint_32.
@ -4445,7 +4734,10 @@ pngusr.h a system builder may also define equivalent options in pngusr.dfa
DFA_XTRA to the file name. The makefiles in contrib/pngminim illustrate
how to do this, and a case where pngusr.h is still required.
XI. Changes to Libpng from version 1.5.x to 1.6.x
XII. Changes to Libpng from version 1.5.x to 1.6.x
A "simplified API" has been added (see documentation in png.h and a simple
example in contrib/examples/pngtopng.c)
We no longer include string.h in png.h. The include statement has been moved
to pngpriv.h, where it is not accessible by applications. Applications that
@ -4466,10 +4758,7 @@ The following has been removed:
function returns a 32-bit integer instead of
a string.
A "simplified API" has been added (see documentation in png.h and a simple
example in contrib/examples/pngtopng.c)
XII. Detecting libpng
XIII. Detecting libpng
The png_get_io_ptr() function has been present since libpng-0.88, has never
changed, and is unaffected by conditional compilation macros. It is the
@ -4478,7 +4767,7 @@ libpng version since 0.88. In an autoconf "configure.in" you could use
AC_CHECK_LIB(png, png_get_io_ptr, ...
XIII. Source code repository
XV. Source code repository
Since about February 2009, version 1.2.34, libpng has been under "git" source
control. The git repository was built from old libpng-x.y.z.tar.gz files
@ -4502,7 +4791,7 @@ simple verbal discriptions of bug fixes, reported either to the
SourceForge bug tracker, to the png-mng-implement at lists.sf.net
mailing list, or directly to glennrp.
XIV. Coding style
XV. Coding style
Our coding style is similar to the "Allman" style, with curly
braces on separate lines:
@ -4614,9 +4903,9 @@ Lines do not exceed 80 characters.
Other rules can be inferred by inspecting the libpng source.
XV. Y2K Compliance in libpng
XVI. Y2K Compliance in libpng
December 30, 2011
January 11, 2012
Since the PNG Development group is an ad-hoc body, we can't make
an official declaration.

958
libpng.3
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