71 lines
2.4 KiB
Plaintext
71 lines
2.4 KiB
Plaintext
hqx Library README
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==================
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Introduction
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------------
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hqx is a fast, high-quality magnification filter designed for pixel art.
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Install
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-------
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NOTE: DevIL library and development headers are required.
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./configure
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make && make install
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For more information refer to INSTALL.
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Usage
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-----
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hqx -s scaleBy input output
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Where scaleBy is either 2, 3 or 4
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For example:
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hqx -s 4 test.png out.png
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Example
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-------
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#include <stdint.h>
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#include <hqx.h>
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uint32_t *src; // Pointer to source bitmap in RGB format
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size_t width, height; // Size of source bitmap
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/*
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* Code to init src, width & height
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*/
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uint32_t *dest = (uint32_t *) malloc(width * 4 * height * 4 * sizeof(uint32_t));
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hqxInit();
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hq4x_32(src, dest, width, height);
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Implementation
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--------------
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The first step is an analysis of the 3x3 area of the source pixel. At first, we
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calculate the color difference between the central pixel and its 8 nearest
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neighbors. Then that difference is compared to a predefined threshold, and these
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pixels are sorted into two categories: "close" and "distant" colored. There are
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8 neighbors, so we are getting 256 possible combinations.
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For the next step, which is filtering, a lookup table with 256 entries is used,
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one entry per each combination of close/distant colored neighbors. Each entry
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describes how to mix the colors of the source pixels from 3x3 area to get
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interpolated pixels of the filtered image.
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The present implementation is using YUV color space to calculate color
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differences, with more tolerance on Y (brightness) component, then on color
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components U and V. That color space conversion is quite easy to implement if
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the format of the source image is 16 bit per pixel, using a simple lookup table.
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It is also possible to calculate the color differences and compare them to a
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threshold very fast, using MMX instructions.
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Creating a lookup table was the most difficult part - for each combination the
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most probable vector representation of the area has to be determined, with the
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idea of edges between the different colored areas of the image to be preserved,
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with the edge direction to be as close to a correct one as possible. That vector
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representation is then rasterised with higher (3x) resolution using
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anti-aliasing, and the result is stored in the lookup table.
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The filter was not designed for photographs, but for images with clear sharp
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edges, like line graphics or cartoon sprites. It was also designed to be fast
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enough to process 256x256 images in real-time.
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