pack4png: CodeVERSION 1.1, JULY 06, 2003 Author: Theo Pavlidis 1. OverviewThis page describes the source code of version 1.1 of pack4png. 2. Available zip filesThere are four files:
There is also a mirror site: http://membled.com/work/mirror/pack4png/ . 3. Code Description3.1. The pack4png Library (p4p)The software has been set up as an MFC project (named p4p) to produce a DLL. If you have a different computing environment you may not want all those files. There are four key files:
The DLL has the following public interface (and two other similar functions that you can find from pack4png.h): extern "C" __declspec(dllimport) int pack4png1( long hist[], // input: histogram int levels, // input: usually 256 int compression // input: should be in the range 1-9 or 11-19 int pxmap[] // output: new pixel values in terms of old ); The function pack4png1() accepts as input an intensity histogram for a particular color that has been computed over an image (or a region of an image), the number of levels per pixel (usually 256), and a quality parameter. The output is the array pxmap[] that maps old pixel values to new ones. The function returns the number of zones found (i.e. number of distinct values in the pxmap array). Returns of 0 or -1 indicate errors. The value of compression%10 is the minimum number of level crossings of the histogram that are required to define a zone (see the paper for details). Minimum value is clearly 1 (minimum compression). The maximum value equals the number of levels used (9 in this imprementation). compression/10 determines whether anti-aliasing intensities should be kept (if the value is 0) or not.(Again, see the paper for details) The latter strategy produces even higher compression. If all this seems too confusing, think of compression as having a range of 1-19 which is close to what is happening. For images that fit well the model under which this method has been developed the value of compression%10 has little effect. It is possible to pass information about whether the image is color (rather than gray) by adding 1000 to the value of compression and whether we are confident that the image is a good candidate for space domain compression (rather than JPEG) by adding 100 to the value of compression. Such information is used in heuristic post-procesing and it may be omitted in future versions. Note: Earlier drafts used the term quality to describe the effect of changes in the strategy of the algorithm specified in the third argument of the function. However image quality is quite subjective and in this draft the third argument is called compression. A corresponding set of functions have been added that do the packing when a 6x6x6 global color table is expected to be used. extern "C" __declspec(dllimport) int pack4gct1( long hist[], // input: histogram int levels, // input: usually 256 int compression // input: should be in the range 1-9 or 11-19 int pxmap[] // output: new pixel values in terms of old ); 3.2. The pack4png Windows Application (p4pApp)The software has been set up as an MFC project (named p4pApp) to produce an executable file. The application has the usual Microsoft Windows Document/View structure and look with a toolbar. It reads a PNG file, displays it, lets the user select the parameters, and produces another PNG file. In order to use this code you also need to have the PNG code library lpng125 or later as well as the zlib. You may download both by going to libpng site. (If you are not familiar with PNG you should also look at the PNG Home Page). Once you install these libraries you should edit the local.h file (it contains definitions that depend on the local system) and you are ready to build the executable. For convenience, lpng.zip on this site contains copies of the definition files of the PNG library that are needed to compile this application. The following information is for those who are adventurous. Besides the libpng files and the Microsoft application Document and View files, the key files of p4pApp are
There is a user's manual for the application. 4. How to build a Batch ApplicationIt is easy to build a batch application using pack4png that does not depend on the Microsoft Windows environment. The main program can be as simple as the one shown below (initialization and error checking not included): char *InputFileName, *OutputlFieName;
int compression; // meaningful values 1-19
CPix pix;
pix.ReadPix(InputFileName); // reads a PNG file
pix.InvokeP4P(compression); // calls pack4png
pix.WritePix(OutputlFieName); // writes a PNG file
In order to build it you need to add files CPix.h, CPix.cpp, pngcall.h, pngcall.cpp (from the p4pApp project), pack4png.h (from the p4p project) and define the compiler flag BATCH_P4P. You also need to have the PNG code library lpng125 or later as well as the zlib. (See Section 3.2. above.) |