/* * jcsample.c * * This file was part of the Independent JPEG Group's software: * Copyright (C) 1991-1996, Thomas G. Lane. * libjpeg-turbo Modifications: * Copyright 2009 Pierre Ossman <[email protected]> for Cendio AB * Copyright (C) 2014, MIPS Technologies, Inc., California. * Copyright (C) 2015, 2019, D. R. Commander. * For conditions of distribution and use, see the accompanying README.ijg * file. * * This file contains downsampling routines. * * Downsampling input data is counted in "row groups". A row group * is defined to be max_v_samp_factor pixel rows of each component, * from which the downsampler produces v_samp_factor sample rows. * A single row group is processed in each call to the downsampler module. * * The downsampler is responsible for edge-expansion of its output data * to fill an integral number of DCT blocks horizontally. The source buffer * may be modified if it is helpful for this purpose (the source buffer is * allocated wide enough to correspond to the desired output width). * The caller (the prep controller) is responsible for vertical padding. * * The downsampler may request "context rows" by setting need_context_rows * during startup. In this case, the input arrays will contain at least * one row group's worth of pixels above and below the passed-in data; * the caller will create dummy rows at image top and bottom by replicating * the first or last real pixel row. * * An excellent reference for image resampling is * Digital Image Warping, George Wolberg, 1990. * Pub. by IEEE Computer Society Press, Los Alamitos, CA. ISBN 0-8186-8944-7. * * The downsampling algorithm used here is a simple average of the source * pixels covered by the output pixel. The hi-falutin sampling literature * refers to this as a "box filter". In general the characteristics of a box * filter are not very good, but for the specific cases we normally use (1:1 * and 2:1 ratios) the box is equivalent to a "triangle filter" which is not * nearly so bad. If you intend to use other sampling ratios, you'd be well * advised to improve this code. * * A simple input-smoothing capability is provided. This is mainly intended * for cleaning up color-dithered GIF input files (if you find it inadequate, * we suggest using an external filtering program such as pnmconvol). When * enabled, each input pixel P is replaced by a weighted sum of itself and its * eight neighbors. P's weight is 1-8*SF and each neighbor's weight is SF, * where SF = (smoothing_factor / 1024). * Currently, smoothing is only supported for 2h2v sampling factors. */ #define JPEG_INTERNALS #include "jinclude.h" #include "jpeglib.h" #include "jsimd.h" /* Pointer to routine to downsample a single component */ downsample1_ptr; /* Private subobject */ my_downsampler; my_downsample_ptr; /* * Initialize for a downsampling pass. */ METHODDEF(void) start_pass_downsample(j_compress_ptr cinfo) { … } /* * Expand a component horizontally from width input_cols to width output_cols, * by duplicating the rightmost samples. */ LOCAL(void) expand_right_edge(JSAMPARRAY image_data, int num_rows, JDIMENSION input_cols, JDIMENSION output_cols) { … } /* * Do downsampling for a whole row group (all components). * * In this version we simply downsample each component independently. */ METHODDEF(void) sep_downsample(j_compress_ptr cinfo, JSAMPIMAGE input_buf, JDIMENSION in_row_index, JSAMPIMAGE output_buf, JDIMENSION out_row_group_index) { … } /* * Downsample pixel values of a single component. * One row group is processed per call. * This version handles arbitrary integral sampling ratios, without smoothing. * Note that this version is not actually used for customary sampling ratios. */ METHODDEF(void) int_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr, JSAMPARRAY input_data, JSAMPARRAY output_data) { … } /* * Downsample pixel values of a single component. * This version handles the special case of a full-size component, * without smoothing. */ METHODDEF(void) fullsize_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr, JSAMPARRAY input_data, JSAMPARRAY output_data) { … } /* * Downsample pixel values of a single component. * This version handles the common case of 2:1 horizontal and 1:1 vertical, * without smoothing. * * A note about the "bias" calculations: when rounding fractional values to * integer, we do not want to always round 0.5 up to the next integer. * If we did that, we'd introduce a noticeable bias towards larger values. * Instead, this code is arranged so that 0.5 will be rounded up or down at * alternate pixel locations (a simple ordered dither pattern). */ METHODDEF(void) h2v1_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr, JSAMPARRAY input_data, JSAMPARRAY output_data) { … } /* * Downsample pixel values of a single component. * This version handles the standard case of 2:1 horizontal and 2:1 vertical, * without smoothing. */ METHODDEF(void) h2v2_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr, JSAMPARRAY input_data, JSAMPARRAY output_data) { … } #ifdef INPUT_SMOOTHING_SUPPORTED /* * Downsample pixel values of a single component. * This version handles the standard case of 2:1 horizontal and 2:1 vertical, * with smoothing. One row of context is required. */ METHODDEF(void) h2v2_smooth_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr, JSAMPARRAY input_data, JSAMPARRAY output_data) { … } /* * Downsample pixel values of a single component. * This version handles the special case of a full-size component, * with smoothing. One row of context is required. */ METHODDEF(void) fullsize_smooth_downsample(j_compress_ptr cinfo, jpeg_component_info *compptr, JSAMPARRAY input_data, JSAMPARRAY output_data) { … } #endif /* INPUT_SMOOTHING_SUPPORTED */ /* * Module initialization routine for downsampling. * Note that we must select a routine for each component. */ GLOBAL(void) jinit_downsampler(j_compress_ptr cinfo) { … }
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