hqx Library README

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.

Build

autoreconf -vi

Install

NOTE: DevIL library and development headers are required.

./configure
make && make install

For more information refer to INSTALL.

Usage

hqx -s scaleBy input output Where scaleBy is either 2, 3 or 4

For example:

hqx -s 4 test.png out.png

Example

#include <stdint.h>
#include <hqx.h>

uint32_t *src; // Pointer to source bitmap in RGB format
size_t width, height; // Size of source bitmap

/*
 * Code to init src, width & height
 */

uint32_t *dest = (uint32_t *) malloc(width * 4 * height * 4 * sizeof(uint32_t));
hqxInit();
hq4x_32(src, dest, width, height);

Implementation

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.