/* * jdmainct.c * * This file was part of the Independent JPEG Group's software: * Copyright (C) 1994-1996, Thomas G. Lane. * libjpeg-turbo Modifications: * Copyright (C) 2010, 2016, D. R. Commander. * For conditions of distribution and use, see the accompanying README.ijg * file. * * This file contains the main buffer controller for decompression. * The main buffer lies between the JPEG decompressor proper and the * post-processor; it holds downsampled data in the JPEG colorspace. * * Note that this code is bypassed in raw-data mode, since the application * supplies the equivalent of the main buffer in that case. */ #include "jinclude.h" #include "jdmainct.h" /* * In the current system design, the main buffer need never be a full-image * buffer; any full-height buffers will be found inside the coefficient or * postprocessing controllers. Nonetheless, the main controller is not * trivial. Its responsibility is to provide context rows for upsampling/ * rescaling, and doing this in an efficient fashion is a bit tricky. * * Postprocessor input data is counted in "row groups". A row group * is defined to be (v_samp_factor * DCT_scaled_size / min_DCT_scaled_size) * sample rows of each component. (We require DCT_scaled_size values to be * chosen such that these numbers are integers. In practice DCT_scaled_size * values will likely be powers of two, so we actually have the stronger * condition that DCT_scaled_size / min_DCT_scaled_size is an integer.) * Upsampling will typically produce max_v_samp_factor pixel rows from each * row group (times any additional scale factor that the upsampler is * applying). * * The coefficient controller will deliver data to us one iMCU row at a time; * each iMCU row contains v_samp_factor * DCT_scaled_size sample rows, or * exactly min_DCT_scaled_size row groups. (This amount of data corresponds * to one row of MCUs when the image is fully interleaved.) Note that the * number of sample rows varies across components, but the number of row * groups does not. Some garbage sample rows may be included in the last iMCU * row at the bottom of the image. * * Depending on the vertical scaling algorithm used, the upsampler may need * access to the sample row(s) above and below its current input row group. * The upsampler is required to set need_context_rows TRUE at global selection * time if so. When need_context_rows is FALSE, this controller can simply * obtain one iMCU row at a time from the coefficient controller and dole it * out as row groups to the postprocessor. * * When need_context_rows is TRUE, this controller guarantees that the buffer * passed to postprocessing contains at least one row group's worth of samples * above and below the row group(s) being processed. Note that the context * rows "above" the first passed row group appear at negative row offsets in * the passed buffer. At the top and bottom of the image, the required * context rows are manufactured by duplicating the first or last real sample * row; this avoids having special cases in the upsampling inner loops. * * The amount of context is fixed at one row group just because that's a * convenient number for this controller to work with. The existing * upsamplers really only need one sample row of context. An upsampler * supporting arbitrary output rescaling might wish for more than one row * group of context when shrinking the image; tough, we don't handle that. * (This is justified by the assumption that downsizing will be handled mostly * by adjusting the DCT_scaled_size values, so that the actual scale factor at * the upsample step needn't be much less than one.) * * To provide the desired context, we have to retain the last two row groups * of one iMCU row while reading in the next iMCU row. (The last row group * can't be processed until we have another row group for its below-context, * and so we have to save the next-to-last group too for its above-context.) * We could do this most simply by copying data around in our buffer, but * that'd be very slow. We can avoid copying any data by creating a rather * strange pointer structure. Here's how it works. We allocate a workspace * consisting of M+2 row groups (where M = min_DCT_scaled_size is the number * of row groups per iMCU row). We create two sets of redundant pointers to * the workspace. Labeling the physical row groups 0 to M+1, the synthesized * pointer lists look like this: * M+1 M-1 * master pointer --> 0 master pointer --> 0 * 1 1 * ... ... * M-3 M-3 * M-2 M * M-1 M+1 * M M-2 * M+1 M-1 * 0 0 * We read alternate iMCU rows using each master pointer; thus the last two * row groups of the previous iMCU row remain un-overwritten in the workspace. * The pointer lists are set up so that the required context rows appear to * be adjacent to the proper places when we pass the pointer lists to the * upsampler. * * The above pictures describe the normal state of the pointer lists. * At top and bottom of the image, we diddle the pointer lists to duplicate * the first or last sample row as necessary (this is cheaper than copying * sample rows around). * * This scheme breaks down if M < 2, ie, min_DCT_scaled_size is 1. In that * situation each iMCU row provides only one row group so the buffering logic * must be different (eg, we must read two iMCU rows before we can emit the * first row group). For now, we simply do not support providing context * rows when min_DCT_scaled_size is 1. That combination seems unlikely to * be worth providing --- if someone wants a 1/8th-size preview, they probably * want it quick and dirty, so a context-free upsampler is sufficient. */ /* Forward declarations */ METHODDEF(void) process_data_simple_main(j_decompress_ptr cinfo, JSAMPARRAY output_buf, JDIMENSION *out_row_ctr, JDIMENSION out_rows_avail); METHODDEF(void) process_data_context_main(j_decompress_ptr cinfo, JSAMPARRAY output_buf, JDIMENSION *out_row_ctr, JDIMENSION out_rows_avail); #ifdef QUANT_2PASS_SUPPORTED METHODDEF(void) process_data_crank_post(j_decompress_ptr cinfo, JSAMPARRAY output_buf, JDIMENSION *out_row_ctr, JDIMENSION out_rows_avail); #endif LOCAL(void) alloc_funny_pointers(j_decompress_ptr cinfo) /* Allocate space for the funny pointer lists. * This is done only once, not once per pass. */ { … } LOCAL(void) make_funny_pointers(j_decompress_ptr cinfo) /* Create the funny pointer lists discussed in the comments above. * The actual workspace is already allocated (in main_ptr->buffer), * and the space for the pointer lists is allocated too. * This routine just fills in the curiously ordered lists. * This will be repeated at the beginning of each pass. */ { … } LOCAL(void) set_bottom_pointers(j_decompress_ptr cinfo) /* Change the pointer lists to duplicate the last sample row at the bottom * of the image. whichptr indicates which xbuffer holds the final iMCU row. * Also sets rowgroups_avail to indicate number of nondummy row groups in row. */ { … } /* * Initialize for a processing pass. */ METHODDEF(void) start_pass_main(j_decompress_ptr cinfo, J_BUF_MODE pass_mode) { … } /* * Process some data. * This handles the simple case where no context is required. */ METHODDEF(void) process_data_simple_main(j_decompress_ptr cinfo, JSAMPARRAY output_buf, JDIMENSION *out_row_ctr, JDIMENSION out_rows_avail) { … } /* * Process some data. * This handles the case where context rows must be provided. */ METHODDEF(void) process_data_context_main(j_decompress_ptr cinfo, JSAMPARRAY output_buf, JDIMENSION *out_row_ctr, JDIMENSION out_rows_avail) { … } /* * Process some data. * Final pass of two-pass quantization: just call the postprocessor. * Source data will be the postprocessor controller's internal buffer. */ #ifdef QUANT_2PASS_SUPPORTED METHODDEF(void) process_data_crank_post(j_decompress_ptr cinfo, JSAMPARRAY output_buf, JDIMENSION *out_row_ctr, JDIMENSION out_rows_avail) { … } #endif /* QUANT_2PASS_SUPPORTED */ /* * Initialize main buffer controller. */ GLOBAL(void) jinit_d_main_controller(j_decompress_ptr cinfo, boolean need_full_buffer) { … }
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