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Diffstat (limited to 'libpng-manual.txt')
| -rw-r--r-- | libpng-manual.txt | 259 |
1 files changed, 59 insertions, 200 deletions
diff --git a/libpng-manual.txt b/libpng-manual.txt index 73e09abd0..b291ef14b 100644 --- a/libpng-manual.txt +++ b/libpng-manual.txt @@ -1,6 +1,6 @@ libpng-manual.txt - A description on how to use and modify libpng - libpng version 1.6.10 - March 6, 2014 + libpng version 1.6.11 - June 5, 2014 Updated and distributed by Glenn Randers-Pehrson <glennrp at users.sourceforge.net> Copyright (c) 1998-2014 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 1.6.10 - March 6, 2014 + libpng versions 0.97, January 1998, through 1.6.11 - June 5, 2014 Updated and distributed by Glenn Randers-Pehrson Copyright (c) 1998-2014 Glenn Randers-Pehrson @@ -54,7 +54,7 @@ This file describes how to use and modify the PNG reference library file, example.c is a good starting point for using the library, as it is heavily commented and should include everything most people will need. We assume that libpng is already installed; see the -INSTALL file for instructions on how to install libpng. +INSTALL file for instructions on how to configure and install libpng. For examples of libpng usage, see the files "example.c", "pngtest.c", and the files in the "contrib" directory, all of which are included in @@ -2636,7 +2636,7 @@ png_infop info_ptr; png_progressive_combine_row(png_ptr, old_row, new_row); - /* where old_row is what was displayed for + /* where old_row is what was displayed previously for the row. Note that the first pass (pass == 0, really) will completely cover the old row, so the rows do not have to be @@ -3793,8 +3793,9 @@ 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 2-byte integer. All -channels can be converted to the original value by dividing by 65535; all + b) As a value in the range 0..65535, contained in a 2-byte integer, in +the native byte order of the platform on which the application is running. +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. @@ -3861,7 +3862,9 @@ First the single byte formats: Then the linear 2-byte 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. +components in the linear format. The components are 16-bit integers in +the native byte order for your platform, and there is no provision for +swapping the bytes to a different endian condition. PNG_FORMAT_LINEAR_Y PNG_FORMAT_FLAG_LINEAR PNG_FORMAT_LINEAR_Y_ALPHA @@ -3926,7 +3929,7 @@ First the information about the samples. * * png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)]; * - * Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to use the + * Alternatively, use the PNG_IMAGE_COLORMAP_SIZE macro below to use the * information from one of the png_image_begin_read_ APIs and dynamically * allocate the required memory. */ @@ -3955,9 +3958,16 @@ Information about the whole row, or whole image row. For a color-mapped image this is the minimum number of bytes in a row. + If you need the stride measured in bytes, row_stride_bytes is + PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt) + plus any padding bytes that your application might need, for example + to start the next row on a 4-byte boundary. + 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. + stride - the number of components to leave space for in each row. This + macro takes care of multiplying row_stride by PNG_IMAGE_PIXEL_COMONENT_SIZE + when the image has 2-byte components. PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01 This indicates the the RGB values of the in-memory bitmap do not @@ -4086,14 +4096,11 @@ clears the newly allocated memory to zero; note that png_calloc(png_ptr, size) is not the same as the calloc(number, size) function provided by stdlib.h. There is limited support for certain systems with segmented memory architectures and the types of pointers declared by png.h match this; you -will have to use appropriate pointers in your application. Since it is -unlikely that the method of handling memory allocation on a platform -will change between applications, these functions must be modified in -the library at compile time. If you prefer to use a different method -of allocating and freeing data, you can use png_create_read_struct_2() or -png_create_write_struct_2() to register your own functions as described -above. These functions also provide a void pointer that can be retrieved -via +will have to use appropriate pointers in your application. If you prefer +to use a different method of allocating and freeing data, you can use +png_create_read_struct_2() or png_create_write_struct_2() to register your +own functions as described above. These functions also provide a void +pointer that can be retrieved via mem_ptr=png_get_mem_ptr(png_ptr); @@ -4236,29 +4243,6 @@ the simpler ones to get an idea of how they work. Try to find a similar transformation to the one you want to add and copy off of it. More details can be found in the comments inside the code itself. -Configuring for 16-bit platforms - -You will want to look into zconf.h to tell zlib (and thus libpng) that -it cannot allocate more then 64K at a time. Even if you can, the memory -won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K. - -Configuring for DOS - -For DOS users who only have access to the lower 640K, you will -have to limit zlib's memory usage via a png_set_compression_mem_level() -call. See zlib.h or zconf.h in the zlib library for more information. - -Configuring for Medium Model - -Libpng's support for medium model has been tested on most of the popular -compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets -defined, and FAR gets defined to far in pngconf.h, and you should be -all set. Everything in the library (except for zlib's structure) is -expecting far data. You must use the typedefs with the p or pp on -the end for pointers (or at least look at them and be careful). Make -note that the rows of data are defined as png_bytepp, which is -an "unsigned char far * far *". - Configuring for gui/windowing platforms: You will need to write new error and warning functions that use the GUI @@ -4268,18 +4252,6 @@ in order to have them available during the structure initialization. They can be changed later via png_set_error_fn(). On some compilers, you may also have to change the memory allocators (png_malloc, etc.). -Configuring for compiler xxx: - -All includes for libpng are in pngconf.h. If you need to add, change -or delete an include, this is the place to do it. -The includes that are not needed outside libpng are placed in pngpriv.h, -which is only used by the routines inside libpng itself. -The files in libpng proper only include pngpriv.h and png.h, which -in turn includes pngconf.h and, as of libpng-1.5.0, pnglibconf.h. -As of libpng-1.5.0, pngpriv.h also includes three other private header -files, pngstruct.h, pnginfo.h, and pngdebug.h, which contain material -that previously appeared in the public headers. - Configuring zlib: There are special functions to configure the compression. Perhaps the @@ -4321,6 +4293,8 @@ zlib.h for more information on what these mean. png_set_compression_method(png_ptr, method); +This controls the size of the IDAT chunks (default 8192): + png_set_compression_buffer_size(png_ptr, size); As of libpng version 1.5.4, additional APIs became @@ -4419,46 +4393,6 @@ Note that the numbers above were invented purely for this example and are given only to help explain the function usage. Little testing has been done to find optimum values for either the costs or the weights. -Removing unwanted object code - -There are a bunch of #define's in pngconf.h that control what parts of -libpng are compiled. All the defines end in _SUPPORTED. If you are -never going to use a capability, you can change the #define to #undef -before recompiling libpng and save yourself code and data space, or -you can turn off individual capabilities with defines that begin with -PNG_NO_. - -In libpng-1.5.0 and later, the #define's are in pnglibconf.h instead. - -You can also turn all of the transforms and ancillary chunk capabilities -off en masse with compiler directives that define -PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS, -or all four, -along with directives to turn on any of the capabilities that you do -want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra -transformations but still leave the library fully capable of reading -and writing PNG files with all known public chunks. Use of the -PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces a library -that is incapable of reading or writing ancillary chunks. If you are -not using the progressive reading capability, you can turn that off -with PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING -capability, which you'll still have). - -All the reading and writing specific code are in separate files, so the -linker should only grab the files it needs. However, if you want to -make sure, or if you are building a stand alone library, all the -reading files start with "pngr" and all the writing files start with "pngw". -The files that don't match either (like png.c, pngtrans.c, etc.) -are used for both reading and writing, and always need to be included. -The progressive reader is in pngpread.c - -If you are creating or distributing a dynamically linked library (a .so -or DLL file), you should not remove or disable any parts of the library, -as this will cause applications linked with different versions of the -library to fail if they call functions not available in your library. -The size of the library itself should not be an issue, because only -those sections that are actually used will be loaded into memory. - Requesting debug printout The macro definition PNG_DEBUG can be used to request debugging @@ -4496,17 +4430,6 @@ 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. -Prepending a prefix to exported symbols - -Starting with libpng-1.6.0, you can configure libpng (when using the -"configure" script) to prefix all exported symbols by means of the -configuration option "--with-libpng-prefix=FOO_", where FOO_ can be any -string beginning with a letter and containing only uppercase -and lowercase letters, digits, and the underscore (i.e., a C language -identifier). This creates a set of macros in pnglibconf.h, so this is -transparent to applications; their function calls get transformed by -the macros to use the modified names. - VII. MNG support The MNG specification (available at http://www.libpng.org/pub/mng) allows @@ -4833,8 +4756,6 @@ does not account for any palette index used by ancillary chunks such as the bKGD chunk; you must check those separately to determine the maximum palette index actually used. -A. Changes that affect users of libpng - There are no substantial API changes between the non-deprecated parts of the 1.4.5 API and the 1.5.0 API; however, the ability to directly access members of the main libpng control structures, png_struct and png_info, @@ -4973,26 +4894,6 @@ limits are now The png_set_option() function (and the "options" member of the png struct) was added to libpng-1.5.15. -B. Changes to the build and configuration of libpng - -Details of internal changes to the library code can be found in the CHANGES -file and in the GIT repository logs. These will be of no concern to the vast -majority of library users or builders; however, the few who configure libpng -to a non-default feature set may need to change how this is done. - -There should be no need for library builders to alter build scripts if -these use the distributed build support - configure or the makefiles - -however, users of the makefiles may care to update their build scripts -to build pnglibconf.h where the corresponding makefile does not do so. - -Building libpng with a non-default configuration has changed completely. -The old method using pngusr.h should still work correctly even though the -way pngusr.h is used in the build has been changed; however, library -builders will probably want to examine the changes to take advantage of -new capabilities and to simplify their build system. - -B.1 Specific changes to library configuration capabilities - The library now supports a complete fixed point implementation and can thus be used on systems that have no floating point support or very limited or slow support. Previously gamma correction, an essential part @@ -5003,27 +4904,7 @@ independent of the choice of fixed versus floating point APIs and all the missing fixed point APIs have been implemented. The exact mechanism used to control attributes of API functions has -changed. A single set of operating system independent macro definitions -is used and operating system specific directives are defined in -pnglibconf.h - -As part of this the mechanism used to choose procedure call standards on -those systems that allow a choice has been changed. At present this only -affects certain Microsoft (DOS, Windows) and IBM (OS/2) operating systems -running on Intel processors. As before, PNGAPI is defined where required -to control the exported API functions; however, two new macros, PNGCBAPI -and PNGCAPI, are used instead for callback functions (PNGCBAPI) and -(PNGCAPI) for functions that must match a C library prototype (currently -only png_longjmp_ptr, which must match the C longjmp function.) The new -approach is documented in pngconf.h - -Despite these changes, libpng 1.5.0 only supports the native C function -calling standard on those platforms tested so far (__cdecl on Microsoft -Windows). This is because the support requirements for alternative -calling conventions seem to no longer exist. Developers who find it -necessary to set PNG_API_RULE to 1 should advise the mailing list -(png-mng-implement) of this and library builders who use Openwatcom and -therefore set PNG_API_RULE to 2 should also contact the mailing list. +changed, as described in the INSTALL file. A new test program, pngvalid, is provided in addition to pngtest. pngvalid validates the arithmetic accuracy of the gamma correction @@ -5099,46 +4980,6 @@ even though the default is to use the macros - this allows applications to choose at app buildtime whether or not to use macros (previously impossible because the functions weren't in the default build.) -B.2 Changes to the configuration mechanism - -Prior to libpng-1.5.0 library builders who needed to configure libpng -had either to modify the exported pngconf.h header file to add system -specific configuration or had to write feature selection macros into -pngusr.h and cause this to be included into pngconf.h by defining -PNG_USER_CONFIG. The latter mechanism had the disadvantage that an -application built without PNG_USER_CONFIG defined would see the -unmodified, default, libpng API and thus would probably fail to link. - -These mechanisms still work in the configure build and in any makefile -build that builds pnglibconf.h, although the feature selection macros -have changed somewhat as described above. In 1.5.0, however, pngusr.h is -processed only once, when the exported header file pnglibconf.h is built. -pngconf.h no longer includes pngusr.h, therefore pngusr.h is ignored after the -build of pnglibconf.h and it is never included in an application build. - -The rarely used alternative of adding a list of feature macros to the -CPPFLAGS setting in the build also still works; however, the macros will be -copied to pnglibconf.h and this may produce macro redefinition warnings -when the individual C files are compiled. - -All configuration now only works if pnglibconf.h is built from -scripts/pnglibconf.dfa. This requires the program awk. Brian Kernighan -(the original author of awk) maintains C source code of that awk and this -and all known later implementations (often called by subtly different -names - nawk and gawk for example) are adequate to build pnglibconf.h. -The Sun Microsystems (now Oracle) program 'awk' is an earlier version -and does not work; this may also apply to other systems that have a -functioning awk called 'nawk'. - -Configuration options are now documented in scripts/pnglibconf.dfa. This -file also includes dependency information that ensures a configuration is -consistent; that is, if a feature is switched off dependent features are -also removed. As a recommended alternative to using feature macros in -pngusr.h a system builder may also define equivalent options in pngusr.dfa -(or, indeed, any file) and add that to the configuration by setting -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. - 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 @@ -5197,11 +5038,25 @@ Error detection in some chunks has improved; in particular the iCCP chunk reader now does pretty complete validation of the basic format. Some bad profiles that were previously accepted are now accepted with a warning or rejected, depending upon the png_set_benign_errors() setting, in particular the -very old broken Microsoft/HP 3144-byte sRGB profile. The PNG spec requirement -that only grayscale profiles may appear in images with color type 0 or 4 and -that even if the image only contains gray pixels, only RGB profiles may appear -in images with color type 2, 3, or 6, is now enforced. The sRGB chunk -is allowed to appear in images with any color type. +very old broken Microsoft/HP 3144-byte sRGB profile. Starting with +libpng-1.6.11, recognizing and checking sRGB profiles can be avoided by +means of + + #ifdef PNG_SKIP_sRGB_CHECK_PROFILE + png_set_option(ping, PNG_SKIP_sRGB_CHECK_PROFILE, + PNG_OPTION_ON); + #endif + +It's not a good idea to do this if you are using the "simplified API", +which needs to be able to recognize an sRGB profile conveyed via the iCCP +chunk. + +The PNG spec requirement that only grayscale profiles may appear in images +with color type 0 or 4 and that even if the image only contains gray pixels, +only RGB profiles may appear in images with color type 2, 3, or 6, is now +enforced. The sRGB chunk is allowed to appear in images with any color type +and is interpreted by libpng to convey a one-tracer-curve profile or a +three-tracer-curve profile as appropriate. Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk contained an empty language field or an empty translated keyword. Both of these @@ -5235,16 +5090,16 @@ Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning of the IDAT stream to set the size of the sliding window for reading instead of using the default 32-kbyte sliding window size. It was discovered that there are hundreds of PNG files in the wild that have incorrect CMF bytes that caused -libpng to issue a "too far back" error and reject the file. Libpng-1.6.3 and -later calculate their own safe CMF from the image dimensions, provide a way -to revert to the libpng-1.5.x behavior (ignoring the CMF bytes and using a -32-kbyte sliding window), by using +zlib to issue the "invalid distance too far back" error and reject the file. +Libpng-1.6.3 and later calculate their own safe CMF from the image dimensions, +provide a way to revert to the libpng-1.5.x behavior (ignoring the CMF bytes +and using a 32-kbyte sliding window), by using png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW, PNG_OPTION_ON); -and provide a tool (contrib/tools/pngfix) for optimizing the CMF bytes -correctly. +and provide a tool (contrib/tools/pngfix) for rewriting a PNG file while +optimizing the CMF bytes in its IDAT chunk correctly. Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the wrong length, which resulted in PNG files that cannot be read beyond the bad iTXt @@ -5286,7 +5141,8 @@ mailing list, or directly to glennrp. XV. Coding style -Our coding style is similar to the "Allman" style, with curly +Our coding style is similar to the "Allman" style +(See http://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly braces on separate lines: if (condition) @@ -5408,6 +5264,9 @@ with an even number of lower-case hex digits (e.g., 0x00, 0xff, 0x0100). We prefer to use underscores in variable names rather than camelCase, except for a few type names that we inherit from zlib.h. +We prefer "if (something != 0)" and "if (something == 0)" +over "if (something)" and if "(!something)", respectively. + We do not use the TAB character for indentation in the C sources. Lines do not exceed 80 characters. @@ -5416,13 +5275,13 @@ Other rules can be inferred by inspecting the libpng source. XVI. Y2K Compliance in libpng -March 6, 2014 +June 5, 2014 Since the PNG Development group is an ad-hoc body, we can't make an official declaration. This is your unofficial assurance that libpng from version 0.71 and -upward through 1.6.10 are Y2K compliant. It is my belief that earlier +upward through 1.6.11 are Y2K compliant. It is my belief that earlier versions were also Y2K compliant. Libpng only has two year fields. One is a 2-byte unsigned integer |