HqxCli-Java

What is this fork for?

I did not touch a single line of the algorithm code but I did some modification:

• The original project is a lib, this is lib and a command line application that uses it
• The packages names are now coherent
• This is a maven project, not an Eclipse project

Original Introduction

hqx ("hq" stands for "high quality" and "x" stands for magnification) is one of the pixel art scaling algorithms developed by Maxim Stepin, used in emulators such as Nestopia, bsnes, ZSNES, Snes9x, FCE Ultra and many more. There are 3 hqx filters: hq2x, hq3x, and hq4x, which magnify by factor of 2, 3, and 4 respectively.

Port Introduction

hqx-java is a Java port of the excellent hqxSharp C# port, which itself is a port of the original hqx C project

Like the hqxSharp project, the focus of this code is asset creation and usage in tools, so no optimizations were done, just an almost-direct copy of the code.

Usage

	Darshan@Darshan-HP:/Codice/Java/hqx-java/target$ java -jar .\hqx-java.jar
	hqx image converter
	Usage -> hqx.jar [options] inputFile
			 the input file can also be specified with an option
			 If not overridden output file name will be inputfile.hq2x.png of hq2x
			 hq3x.png for hq3x and so on

	Option             Description
	------             -----------
	-?, -h, --help     show help
	--all              Upscale the input file with hq2x,hq3x,hq4x
	--hq2x             Upscale the input file with hq2x
	--hq3x             Upscale the input file with hq3x
	--hq4x             Upscale the input file with hq4x
	--input            Specify input file
	--output           Override the default naming convention for output file

Examples

Original links are dead (hq2x and hq3x)

• Wikipedia entry for hq2x
• Cached version of the original links
  • www.hiend3d.com/hq2x.html
  • www.hiend3d.com/hq3x.html
  • www.hiend3d.com/hq4x.html

How does it works?

This question is not easy to answer. Basically no one knows it exactly. The code is just a big lookup table that do a pattern-based interpolation. The only clue we have is the description within the original code.

A description from the original implementation

Verbatim from https://code.google.com/archive/p/hqx/source/default/source

The first step is an analysis of the 3x3 area of the source pixel.

At first, we calculate the color difference between the central pixel and its 8 nearest neighbors. Then that difference is compared to a predefined threshold, and these pixels are sorted into two categories: "close" and "distant" colored. There are 8 neighbors, so we are getting 256 possible combinations.

For the next step, which is filtering, a lookup table with 256 entries is used, one entry per each combination of close/distant colored neighbors. Each entry describes how to mix the colors of the source pixels from 3x3 area to get interpolated pixels of the filtered image.

The present implementation is using YUV color space to calculate color differences, with more tolerance on Y (brightness) component, then on color components U and V. That color space conversion is quite easy to implement if the format of the source image is 16 bit per pixel, using a simple lookup table. It is also possible to calculate the color differences and compare them to a threshold very fast, using MMX instructions.

Creating a lookup table was the most difficult part - for each combination the most probable vector representation of the area has to be determined, with the idea of edges between the different colored areas of the image to be preserved, with the edge direction to be as close to a correct one as possible.

That vector representation is then rasterised with higher (3x) resolution using anti-aliasing, and the result is stored in the lookup table. The filter was not designed for photographs, but for images with clear sharp edges, like line graphics or cartoon sprites. It was also designed to be fast enough to process 256x256 images in real-time.

Some modern implementation ( like the one inside FFmpeg ) extract the logic from the original code with machine learning.

Take a look at this implementation http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavfilter/vf_hqx.c;hb=HEAD

and at this blog post http://blog.pkh.me/p/19-butchering-hqx-scaling-filters.html

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License

This software is released under the GNU Lesser General Public License

GNU Lesser General Public License

Version 3, 29 June 2007
Copyright © 2007 Free Software Foundation, Inc. <http://fsf.org/>

Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

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