Demos & Docs

Demos & Docs -- hq3x

hq3x Magnification Filter

by Maxim Stepin

Test Case 1:

original image

nearest neighbour 3x


Test Case 2:

original image nearest neighbour 3x hq3x

Test Case 3:

original image nearest neighbour 3x hq3x


Executable hq3x.zip29Kb07.25.2003
Source code (recommended)hq3x_src.zip19Kb07.25.2003

Optimized Source code (100% asm)hq3x_src_asm.zip26Kb11.21.2003
C++ Source code (no asm)hq3x_src_c.zip20Kb08.05.2003


hq3x is a fast, high-quality 3x magnification filter.

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.

Currently the hq3x filter is implemented in emulators ZSNES, ScummVM, FCE Ultra, AdvanceMAME and others.

Related links: hq2x / hq3x / hq4x

Copyright (c) 2003 Maxim Stepin aka MaxSt