kc3-lang/libjpeg-turbo/simd/arm/jdsample-neon.c

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• Hash : e69dd40c
  Author :
  Date : 2024-01-23T13:26:41
  

  Reorganize source to make things easier to find
  
  - Move all libjpeg documentation, except for README.ijg, into the doc/
    subdirectory.
  
  - Move the TurboJPEG C API documentation from doc/html/ into
    doc/turbojpeg/.
  
  - Move all C source code and headers into a src/ subdirectory.
  
  - Move turbojpeg-jni.c into the java/ subdirectory.
  
  Referring to #226, there is no ideal solution to this problem.  A
  semantically ideal solution would have involved placing all source code,
  including the SIMD and Java source code, under src/ (or perhaps placing
  C library source code under lib/ and C test program source code under
  test/), all header files under include/, and all documentation under
  doc/.  However:
  
  - To me it makes more sense to have separate top-level directories for
    each language, since the SIMD extensions and the Java API are
    technically optional features.  src/ now contains only the code that
    is relevant to the core C API libraries and associated programs.
  - I didn't want to bury the java/ and simd/ directories or add a level
    of depth to them, since both directories already contain source code
    that is 3-4 levels deep.
  - I would prefer not to separate the header files from the C source
    code, because:
    1. It would be disruptive.  libjpeg and libjpeg-turbo have
       historically placed C source code and headers in the same
       directory, and people who are familiar with both projects (self
       included) are used to looking for the headers in the same directory
       as the C source code.
    2. In terms of how the headers are used internally in libjpeg-turbo,
       the distinction between public and private headers is a bit fuzzy.
  - It didn't make sense to separate the test source code from the library
    source code, since there is not a clear distinction in some cases.
    (For instance, the IJG image I/O functions are used by cjpeg and djpeg
    as well as by the TurboJPEG API.)
  
  This solution is minimally disruptive, since it keeps all C source code
  and headers together and keeps java/ and simd/ as top-level directories.
  It is a bit awkward, because java/ and simd/ technically contain source
  code, even though they are not under src/.  However, other solutions
  would have been more awkward for different reasons.
  
  Closes #226
  

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• simd/arm/jdsample-neon.c

• /*
   * jdsample-neon.c - upsampling (Arm Neon)
   *
   * Copyright (C) 2020, Arm Limited.  All Rights Reserved.
   * Copyright (C) 2020, 2024, D. R. Commander.  All Rights Reserved.
   *
   * This software is provided 'as-is', without any express or implied
   * warranty.  In no event will the authors be held liable for any damages
   * arising from the use of this software.
   *
   * Permission is granted to anyone to use this software for any purpose,
   * including commercial applications, and to alter it and redistribute it
   * freely, subject to the following restrictions:
   *
   * 1. The origin of this software must not be misrepresented; you must not
   *    claim that you wrote the original software. If you use this software
   *    in a product, an acknowledgment in the product documentation would be
   *    appreciated but is not required.
   * 2. Altered source versions must be plainly marked as such, and must not be
   *    misrepresented as being the original software.
   * 3. This notice may not be removed or altered from any source distribution.
   */
  
  #define JPEG_INTERNALS
  #include "../../src/jinclude.h"
  #include "../../src/jpeglib.h"
  #include "../../src/jsimd.h"
  #include "../../src/jdct.h"
  #include "../../src/jsimddct.h"
  #include "../jsimd.h"
  
  #include <arm_neon.h>
  
  
  /* The diagram below shows a row of samples produced by h2v1 downsampling.
   *
   *                s0        s1        s2
   *            +---------+---------+---------+
   *            |         |         |         |
   *            | p0   p1 | p2   p3 | p4   p5 |
   *            |         |         |         |
   *            +---------+---------+---------+
   *
   * Samples s0-s2 were created by averaging the original pixel component values
   * centered at positions p0-p5 above.  To approximate those original pixel
   * component values, we proportionally blend the adjacent samples in each row.
   *
   * An upsampled pixel component value is computed by blending the sample
   * containing the pixel center with the nearest neighboring sample, in the
   * ratio 3:1.  For example:
   *     p1(upsampled) = 3/4 * s0 + 1/4 * s1
   *     p2(upsampled) = 3/4 * s1 + 1/4 * s0
   * When computing the first and last pixel component values in the row, there
   * is no adjacent sample to blend, so:
   *     p0(upsampled) = s0
   *     p5(upsampled) = s2
   */
  
  void jsimd_h2v1_fancy_upsample_neon(int max_v_samp_factor,
                                      JDIMENSION downsampled_width,
                                      JSAMPARRAY input_data,
                                      JSAMPARRAY *output_data_ptr)
  {
    JSAMPARRAY output_data = *output_data_ptr;
    JSAMPROW inptr, outptr;
    int inrow;
    unsigned colctr;
    /* Set up constants. */
    const uint16x8_t one_u16 = vdupq_n_u16(1);
    const uint8x8_t three_u8 = vdup_n_u8(3);
  
    for (inrow = 0; inrow < max_v_samp_factor; inrow++) {
      inptr = input_data[inrow];
      outptr = output_data[inrow];
      /* First pixel component value in this row of the original image */
      *outptr = (JSAMPLE)GETJSAMPLE(*inptr);
  
      /*    3/4 * containing sample + 1/4 * nearest neighboring sample
       * For p1: containing sample = s0, nearest neighboring sample = s1
       * For p2: containing sample = s1, nearest neighboring sample = s0
       */
      uint8x16_t s0 = vld1q_u8(inptr);
      uint8x16_t s1 = vld1q_u8(inptr + 1);
      /* Multiplication makes vectors twice as wide.  '_l' and '_h' suffixes
       * denote low half and high half respectively.
       */
      uint16x8_t s1_add_3s0_l =
        vmlal_u8(vmovl_u8(vget_low_u8(s1)), vget_low_u8(s0), three_u8);
      uint16x8_t s1_add_3s0_h =
        vmlal_u8(vmovl_u8(vget_high_u8(s1)), vget_high_u8(s0), three_u8);
      uint16x8_t s0_add_3s1_l =
        vmlal_u8(vmovl_u8(vget_low_u8(s0)), vget_low_u8(s1), three_u8);
      uint16x8_t s0_add_3s1_h =
        vmlal_u8(vmovl_u8(vget_high_u8(s0)), vget_high_u8(s1), three_u8);
      /* Add ordered dithering bias to odd pixel values. */
      s0_add_3s1_l = vaddq_u16(s0_add_3s1_l, one_u16);
      s0_add_3s1_h = vaddq_u16(s0_add_3s1_h, one_u16);
  
      /* The offset is initially 1, because the first pixel component has already
       * been stored.  However, in subsequent iterations of the SIMD loop, this
       * offset is (2 * colctr - 1) to stay within the bounds of the sample
       * buffers without having to resort to a slow scalar tail case for the last
       * (downsampled_width % 16) samples.  See "Creation of 2-D sample arrays"
       * in jmemmgr.c for more details.
       */
      unsigned outptr_offset = 1;
      uint8x16x2_t output_pixels;
  
      /* We use software pipelining to maximise performance.  The code indented
       * an extra two spaces begins the next iteration of the loop.
       */
      for (colctr = 16; colctr < downsampled_width; colctr += 16) {
  
          s0 = vld1q_u8(inptr + colctr - 1);
          s1 = vld1q_u8(inptr + colctr);
  
        /* Right-shift by 2 (divide by 4), narrow to 8-bit, and combine. */
        output_pixels.val[0] = vcombine_u8(vrshrn_n_u16(s1_add_3s0_l, 2),
                                           vrshrn_n_u16(s1_add_3s0_h, 2));
        output_pixels.val[1] = vcombine_u8(vshrn_n_u16(s0_add_3s1_l, 2),
                                           vshrn_n_u16(s0_add_3s1_h, 2));
  
          /* Multiplication makes vectors twice as wide.  '_l' and '_h' suffixes
           * denote low half and high half respectively.
           */
          s1_add_3s0_l =
            vmlal_u8(vmovl_u8(vget_low_u8(s1)), vget_low_u8(s0), three_u8);
          s1_add_3s0_h =
            vmlal_u8(vmovl_u8(vget_high_u8(s1)), vget_high_u8(s0), three_u8);
          s0_add_3s1_l =
            vmlal_u8(vmovl_u8(vget_low_u8(s0)), vget_low_u8(s1), three_u8);
          s0_add_3s1_h =
            vmlal_u8(vmovl_u8(vget_high_u8(s0)), vget_high_u8(s1), three_u8);
          /* Add ordered dithering bias to odd pixel values. */
          s0_add_3s1_l = vaddq_u16(s0_add_3s1_l, one_u16);
          s0_add_3s1_h = vaddq_u16(s0_add_3s1_h, one_u16);
  
        /* Store pixel component values to memory. */
        vst2q_u8(outptr + outptr_offset, output_pixels);
        outptr_offset = 2 * colctr - 1;
      }
  
      /* Complete the last iteration of the loop. */
  
      /* Right-shift by 2 (divide by 4), narrow to 8-bit, and combine. */
      output_pixels.val[0] = vcombine_u8(vrshrn_n_u16(s1_add_3s0_l, 2),
                                         vrshrn_n_u16(s1_add_3s0_h, 2));
      output_pixels.val[1] = vcombine_u8(vshrn_n_u16(s0_add_3s1_l, 2),
                                         vshrn_n_u16(s0_add_3s1_h, 2));
      /* Store pixel component values to memory. */
      vst2q_u8(outptr + outptr_offset, output_pixels);
  
      /* Last pixel component value in this row of the original image */
      outptr[2 * downsampled_width - 1] =
        GETJSAMPLE(inptr[downsampled_width - 1]);
    }
  }
  
  
  /* The diagram below shows an array of samples produced by h2v2 downsampling.
   *
   *                s0        s1        s2
   *            +---------+---------+---------+
   *            | p0   p1 | p2   p3 | p4   p5 |
   *       sA   |         |         |         |
   *            | p6   p7 | p8   p9 | p10  p11|
   *            +---------+---------+---------+
   *            | p12  p13| p14  p15| p16  p17|
   *       sB   |         |         |         |
   *            | p18  p19| p20  p21| p22  p23|
   *            +---------+---------+---------+
   *            | p24  p25| p26  p27| p28  p29|
   *       sC   |         |         |         |
   *            | p30  p31| p32  p33| p34  p35|
   *            +---------+---------+---------+
   *
   * Samples s0A-s2C were created by averaging the original pixel component
   * values centered at positions p0-p35 above.  To approximate one of those
   * original pixel component values, we proportionally blend the sample
   * containing the pixel center with the nearest neighboring samples in each
   * row, column, and diagonal.
   *
   * An upsampled pixel component value is computed by first blending the sample
   * containing the pixel center with the nearest neighboring samples in the
   * same column, in the ratio 3:1, and then blending each column sum with the
   * nearest neighboring column sum, in the ratio 3:1.  For example:
   *     p14(upsampled) = 3/4 * (3/4 * s1B + 1/4 * s1A) +
   *                      1/4 * (3/4 * s0B + 1/4 * s0A)
   *                    = 9/16 * s1B + 3/16 * s1A + 3/16 * s0B + 1/16 * s0A
   * When computing the first and last pixel component values in the row, there
   * is no horizontally adjacent sample to blend, so:
   *     p12(upsampled) = 3/4 * s0B + 1/4 * s0A
   *     p23(upsampled) = 3/4 * s2B + 1/4 * s2C
   * When computing the first and last pixel component values in the column,
   * there is no vertically adjacent sample to blend, so:
   *     p2(upsampled) = 3/4 * s1A + 1/4 * s0A
   *     p33(upsampled) = 3/4 * s1C + 1/4 * s2C
   * When computing the corner pixel component values, there is no adjacent
   * sample to blend, so:
   *     p0(upsampled) = s0A
   *     p35(upsampled) = s2C
   */
  
  void jsimd_h2v2_fancy_upsample_neon(int max_v_samp_factor,
                                      JDIMENSION downsampled_width,
                                      JSAMPARRAY input_data,
                                      JSAMPARRAY *output_data_ptr)
  {
    JSAMPARRAY output_data = *output_data_ptr;
    JSAMPROW inptr0, inptr1, inptr2, outptr0, outptr1;
    int inrow, outrow;
    unsigned colctr;
    /* Set up constants. */
    const uint16x8_t seven_u16 = vdupq_n_u16(7);
    const uint8x8_t three_u8 = vdup_n_u8(3);
    const uint16x8_t three_u16 = vdupq_n_u16(3);
  
    inrow = outrow = 0;
    while (outrow < max_v_samp_factor) {
      inptr0 = input_data[inrow - 1];
      inptr1 = input_data[inrow];
      inptr2 = input_data[inrow + 1];
      /* Suffixes 0 and 1 denote the upper and lower rows of output pixels,
       * respectively.
       */
      outptr0 = output_data[outrow++];
      outptr1 = output_data[outrow++];
  
      /* First pixel component value in this row of the original image */
      int s0colsum0 = GETJSAMPLE(*inptr1) * 3 + GETJSAMPLE(*inptr0);
      *outptr0 = (JSAMPLE)((s0colsum0 * 4 + 8) >> 4);
      int s0colsum1 = GETJSAMPLE(*inptr1) * 3 + GETJSAMPLE(*inptr2);
      *outptr1 = (JSAMPLE)((s0colsum1 * 4 + 8) >> 4);
  
      /* Step 1: Blend samples vertically in columns s0 and s1.
       * Leave the divide by 4 until the end, when it can be done for both
       * dimensions at once, right-shifting by 4.
       */
  
      /* Load and compute s0colsum0 and s0colsum1. */
      uint8x16_t s0A = vld1q_u8(inptr0);
      uint8x16_t s0B = vld1q_u8(inptr1);
      uint8x16_t s0C = vld1q_u8(inptr2);
      /* Multiplication makes vectors twice as wide.  '_l' and '_h' suffixes
       * denote low half and high half respectively.
       */
      uint16x8_t s0colsum0_l = vmlal_u8(vmovl_u8(vget_low_u8(s0A)),
                                        vget_low_u8(s0B), three_u8);
      uint16x8_t s0colsum0_h = vmlal_u8(vmovl_u8(vget_high_u8(s0A)),
                                        vget_high_u8(s0B), three_u8);
      uint16x8_t s0colsum1_l = vmlal_u8(vmovl_u8(vget_low_u8(s0C)),
                                        vget_low_u8(s0B), three_u8);
      uint16x8_t s0colsum1_h = vmlal_u8(vmovl_u8(vget_high_u8(s0C)),
                                        vget_high_u8(s0B), three_u8);
      /* Load and compute s1colsum0 and s1colsum1. */
      uint8x16_t s1A = vld1q_u8(inptr0 + 1);
      uint8x16_t s1B = vld1q_u8(inptr1 + 1);
      uint8x16_t s1C = vld1q_u8(inptr2 + 1);
      uint16x8_t s1colsum0_l = vmlal_u8(vmovl_u8(vget_low_u8(s1A)),
                                        vget_low_u8(s1B), three_u8);
      uint16x8_t s1colsum0_h = vmlal_u8(vmovl_u8(vget_high_u8(s1A)),
                                        vget_high_u8(s1B), three_u8);
      uint16x8_t s1colsum1_l = vmlal_u8(vmovl_u8(vget_low_u8(s1C)),
                                        vget_low_u8(s1B), three_u8);
      uint16x8_t s1colsum1_h = vmlal_u8(vmovl_u8(vget_high_u8(s1C)),
                                        vget_high_u8(s1B), three_u8);
  
      /* Step 2: Blend the already-blended columns. */
  
      uint16x8_t output0_p1_l = vmlaq_u16(s1colsum0_l, s0colsum0_l, three_u16);
      uint16x8_t output0_p1_h = vmlaq_u16(s1colsum0_h, s0colsum0_h, three_u16);
      uint16x8_t output0_p2_l = vmlaq_u16(s0colsum0_l, s1colsum0_l, three_u16);
      uint16x8_t output0_p2_h = vmlaq_u16(s0colsum0_h, s1colsum0_h, three_u16);
      uint16x8_t output1_p1_l = vmlaq_u16(s1colsum1_l, s0colsum1_l, three_u16);
      uint16x8_t output1_p1_h = vmlaq_u16(s1colsum1_h, s0colsum1_h, three_u16);
      uint16x8_t output1_p2_l = vmlaq_u16(s0colsum1_l, s1colsum1_l, three_u16);
      uint16x8_t output1_p2_h = vmlaq_u16(s0colsum1_h, s1colsum1_h, three_u16);
      /* Add ordered dithering bias to odd pixel values. */
      output0_p1_l = vaddq_u16(output0_p1_l, seven_u16);
      output0_p1_h = vaddq_u16(output0_p1_h, seven_u16);
      output1_p1_l = vaddq_u16(output1_p1_l, seven_u16);
      output1_p1_h = vaddq_u16(output1_p1_h, seven_u16);
      /* Right-shift by 4 (divide by 16), narrow to 8-bit, and combine. */
      uint8x16x2_t output_pixels0 = { {
        vcombine_u8(vshrn_n_u16(output0_p1_l, 4), vshrn_n_u16(output0_p1_h, 4)),
        vcombine_u8(vrshrn_n_u16(output0_p2_l, 4), vrshrn_n_u16(output0_p2_h, 4))
      } };
      uint8x16x2_t output_pixels1 = { {
        vcombine_u8(vshrn_n_u16(output1_p1_l, 4), vshrn_n_u16(output1_p1_h, 4)),
        vcombine_u8(vrshrn_n_u16(output1_p2_l, 4), vrshrn_n_u16(output1_p2_h, 4))
      } };
  
      /* Store pixel component values to memory.
       * The minimum size of the output buffer for each row is 64 bytes => no
       * need to worry about buffer overflow here.  See "Creation of 2-D sample
       * arrays" in jmemmgr.c for more details.
       */
      vst2q_u8(outptr0 + 1, output_pixels0);
      vst2q_u8(outptr1 + 1, output_pixels1);
  
      /* The first pixel of the image shifted our loads and stores by one byte.
       * We have to re-align on a 32-byte boundary at some point before the end
       * of the row (we do it now on the 32/33 pixel boundary) to stay within the
       * bounds of the sample buffers without having to resort to a slow scalar
       * tail case for the last (downsampled_width % 16) samples.  See "Creation
       * of 2-D sample arrays" in jmemmgr.c for more details.
       */
      for (colctr = 16; colctr < downsampled_width; colctr += 16) {
        /* Step 1: Blend samples vertically in columns s0 and s1. */
  
        /* Load and compute s0colsum0 and s0colsum1. */
        s0A = vld1q_u8(inptr0 + colctr - 1);
        s0B = vld1q_u8(inptr1 + colctr - 1);
        s0C = vld1q_u8(inptr2 + colctr - 1);
        s0colsum0_l = vmlal_u8(vmovl_u8(vget_low_u8(s0A)), vget_low_u8(s0B),
                               three_u8);
        s0colsum0_h = vmlal_u8(vmovl_u8(vget_high_u8(s0A)), vget_high_u8(s0B),
                               three_u8);
        s0colsum1_l = vmlal_u8(vmovl_u8(vget_low_u8(s0C)), vget_low_u8(s0B),
                               three_u8);
        s0colsum1_h = vmlal_u8(vmovl_u8(vget_high_u8(s0C)), vget_high_u8(s0B),
                               three_u8);
        /* Load and compute s1colsum0 and s1colsum1. */
        s1A = vld1q_u8(inptr0 + colctr);
        s1B = vld1q_u8(inptr1 + colctr);
        s1C = vld1q_u8(inptr2 + colctr);
        s1colsum0_l = vmlal_u8(vmovl_u8(vget_low_u8(s1A)), vget_low_u8(s1B),
                               three_u8);
        s1colsum0_h = vmlal_u8(vmovl_u8(vget_high_u8(s1A)), vget_high_u8(s1B),
                               three_u8);
        s1colsum1_l = vmlal_u8(vmovl_u8(vget_low_u8(s1C)), vget_low_u8(s1B),
                               three_u8);
        s1colsum1_h = vmlal_u8(vmovl_u8(vget_high_u8(s1C)), vget_high_u8(s1B),
                               three_u8);
  
        /* Step 2: Blend the already-blended columns. */
  
        output0_p1_l = vmlaq_u16(s1colsum0_l, s0colsum0_l, three_u16);
        output0_p1_h = vmlaq_u16(s1colsum0_h, s0colsum0_h, three_u16);
        output0_p2_l = vmlaq_u16(s0colsum0_l, s1colsum0_l, three_u16);
        output0_p2_h = vmlaq_u16(s0colsum0_h, s1colsum0_h, three_u16);
        output1_p1_l = vmlaq_u16(s1colsum1_l, s0colsum1_l, three_u16);
        output1_p1_h = vmlaq_u16(s1colsum1_h, s0colsum1_h, three_u16);
        output1_p2_l = vmlaq_u16(s0colsum1_l, s1colsum1_l, three_u16);
        output1_p2_h = vmlaq_u16(s0colsum1_h, s1colsum1_h, three_u16);
        /* Add ordered dithering bias to odd pixel values. */
        output0_p1_l = vaddq_u16(output0_p1_l, seven_u16);
        output0_p1_h = vaddq_u16(output0_p1_h, seven_u16);
        output1_p1_l = vaddq_u16(output1_p1_l, seven_u16);
        output1_p1_h = vaddq_u16(output1_p1_h, seven_u16);
        /* Right-shift by 4 (divide by 16), narrow to 8-bit, and combine. */
        output_pixels0.val[0] = vcombine_u8(vshrn_n_u16(output0_p1_l, 4),
                                            vshrn_n_u16(output0_p1_h, 4));
        output_pixels0.val[1] = vcombine_u8(vrshrn_n_u16(output0_p2_l, 4),
                                            vrshrn_n_u16(output0_p2_h, 4));
        output_pixels1.val[0] = vcombine_u8(vshrn_n_u16(output1_p1_l, 4),
                                            vshrn_n_u16(output1_p1_h, 4));
        output_pixels1.val[1] = vcombine_u8(vrshrn_n_u16(output1_p2_l, 4),
                                            vrshrn_n_u16(output1_p2_h, 4));
        /* Store pixel component values to memory. */
        vst2q_u8(outptr0 + 2 * colctr - 1, output_pixels0);
        vst2q_u8(outptr1 + 2 * colctr - 1, output_pixels1);
      }
  
      /* Last pixel component value in this row of the original image */
      int s1colsum0 = GETJSAMPLE(inptr1[downsampled_width - 1]) * 3 +
                      GETJSAMPLE(inptr0[downsampled_width - 1]);
      outptr0[2 * downsampled_width - 1] = (JSAMPLE)((s1colsum0 * 4 + 7) >> 4);
      int s1colsum1 = GETJSAMPLE(inptr1[downsampled_width - 1]) * 3 +
                      GETJSAMPLE(inptr2[downsampled_width - 1]);
      outptr1[2 * downsampled_width - 1] = (JSAMPLE)((s1colsum1 * 4 + 7) >> 4);
      inrow++;
    }
  }
  
  
  /* The diagram below shows a column of samples produced by h1v2 downsampling
   * (or by losslessly rotating or transposing an h2v1-downsampled image.)
   *
   *            +---------+
   *            |   p0    |
   *     sA     |         |
   *            |   p1    |
   *            +---------+
   *            |   p2    |
   *     sB     |         |
   *            |   p3    |
   *            +---------+
   *            |   p4    |
   *     sC     |         |
   *            |   p5    |
   *            +---------+
   *
   * Samples sA-sC were created by averaging the original pixel component values
   * centered at positions p0-p5 above.  To approximate those original pixel
   * component values, we proportionally blend the adjacent samples in each
   * column.
   *
   * An upsampled pixel component value is computed by blending the sample
   * containing the pixel center with the nearest neighboring sample, in the
   * ratio 3:1.  For example:
   *     p1(upsampled) = 3/4 * sA + 1/4 * sB
   *     p2(upsampled) = 3/4 * sB + 1/4 * sA
   * When computing the first and last pixel component values in the column,
   * there is no adjacent sample to blend, so:
   *     p0(upsampled) = sA
   *     p5(upsampled) = sC
   */
  
  void jsimd_h1v2_fancy_upsample_neon(int max_v_samp_factor,
                                      JDIMENSION downsampled_width,
                                      JSAMPARRAY input_data,
                                      JSAMPARRAY *output_data_ptr)
  {
    JSAMPARRAY output_data = *output_data_ptr;
    JSAMPROW inptr0, inptr1, inptr2, outptr0, outptr1;
    int inrow, outrow;
    unsigned colctr;
    /* Set up constants. */
    const uint16x8_t one_u16 = vdupq_n_u16(1);
    const uint8x8_t three_u8 = vdup_n_u8(3);
  
    inrow = outrow = 0;
    while (outrow < max_v_samp_factor) {
      inptr0 = input_data[inrow - 1];
      inptr1 = input_data[inrow];
      inptr2 = input_data[inrow + 1];
      /* Suffixes 0 and 1 denote the upper and lower rows of output pixels,
       * respectively.
       */
      outptr0 = output_data[outrow++];
      outptr1 = output_data[outrow++];
      inrow++;
  
      /* The size of the input and output buffers is always a multiple of 32
       * bytes => no need to worry about buffer overflow when reading/writing
       * memory.  See "Creation of 2-D sample arrays" in jmemmgr.c for more
       * details.
       */
      for (colctr = 0; colctr < downsampled_width; colctr += 16) {
        /* Load samples. */
        uint8x16_t sA = vld1q_u8(inptr0 + colctr);
        uint8x16_t sB = vld1q_u8(inptr1 + colctr);
        uint8x16_t sC = vld1q_u8(inptr2 + colctr);
        /* Blend samples vertically. */
        uint16x8_t colsum0_l = vmlal_u8(vmovl_u8(vget_low_u8(sA)),
                                        vget_low_u8(sB), three_u8);
        uint16x8_t colsum0_h = vmlal_u8(vmovl_u8(vget_high_u8(sA)),
                                        vget_high_u8(sB), three_u8);
        uint16x8_t colsum1_l = vmlal_u8(vmovl_u8(vget_low_u8(sC)),
                                        vget_low_u8(sB), three_u8);
        uint16x8_t colsum1_h = vmlal_u8(vmovl_u8(vget_high_u8(sC)),
                                        vget_high_u8(sB), three_u8);
        /* Add ordered dithering bias to pixel values in even output rows. */
        colsum0_l = vaddq_u16(colsum0_l, one_u16);
        colsum0_h = vaddq_u16(colsum0_h, one_u16);
        /* Right-shift by 2 (divide by 4), narrow to 8-bit, and combine. */
        uint8x16_t output_pixels0 = vcombine_u8(vshrn_n_u16(colsum0_l, 2),
                                                vshrn_n_u16(colsum0_h, 2));
        uint8x16_t output_pixels1 = vcombine_u8(vrshrn_n_u16(colsum1_l, 2),
                                                vrshrn_n_u16(colsum1_h, 2));
        /* Store pixel component values to memory. */
        vst1q_u8(outptr0 + colctr, output_pixels0);
        vst1q_u8(outptr1 + colctr, output_pixels1);
      }
    }
  }
  
  
  /* The diagram below shows a row of samples produced by h2v1 downsampling.
   *
   *                s0        s1
   *            +---------+---------+
   *            |         |         |
   *            | p0   p1 | p2   p3 |
   *            |         |         |
   *            +---------+---------+
   *
   * Samples s0 and s1 were created by averaging the original pixel component
   * values centered at positions p0-p3 above.  To approximate those original
   * pixel component values, we duplicate the samples horizontally:
   *     p0(upsampled) = p1(upsampled) = s0
   *     p2(upsampled) = p3(upsampled) = s1
   */
  
  void jsimd_h2v1_upsample_neon(int max_v_samp_factor, JDIMENSION output_width,
                                JSAMPARRAY input_data,
                                JSAMPARRAY *output_data_ptr)
  {
    JSAMPARRAY output_data = *output_data_ptr;
    JSAMPROW inptr, outptr;
    int inrow;
    unsigned colctr;
  
    for (inrow = 0; inrow < max_v_samp_factor; inrow++) {
      inptr = input_data[inrow];
      outptr = output_data[inrow];
      for (colctr = 0; 2 * colctr < output_width; colctr += 16) {
        uint8x16_t samples = vld1q_u8(inptr + colctr);
        /* Duplicate the samples.  The store operation below interleaves them so
         * that adjacent pixel component values take on the same sample value,
         * per above.
         */
        uint8x16x2_t output_pixels = { { samples, samples } };
        /* Store pixel component values to memory.
         * Due to the way sample buffers are allocated, we don't need to worry
         * about tail cases when output_width is not a multiple of 32.  See
         * "Creation of 2-D sample arrays" in jmemmgr.c for details.
         */
        vst2q_u8(outptr + 2 * colctr, output_pixels);
      }
    }
  }
  
  
  /* The diagram below shows an array of samples produced by h2v2 downsampling.
   *
   *                s0        s1
   *            +---------+---------+
   *            | p0   p1 | p2   p3 |
   *       sA   |         |         |
   *            | p4   p5 | p6   p7 |
   *            +---------+---------+
   *            | p8   p9 | p10  p11|
   *       sB   |         |         |
   *            | p12  p13| p14  p15|
   *            +---------+---------+
   *
   * Samples s0A-s1B were created by averaging the original pixel component
   * values centered at positions p0-p15 above.  To approximate those original
   * pixel component values, we duplicate the samples both horizontally and
   * vertically:
   *     p0(upsampled) = p1(upsampled) = p4(upsampled) = p5(upsampled) = s0A
   *     p2(upsampled) = p3(upsampled) = p6(upsampled) = p7(upsampled) = s1A
   *     p8(upsampled) = p9(upsampled) = p12(upsampled) = p13(upsampled) = s0B
   *     p10(upsampled) = p11(upsampled) = p14(upsampled) = p15(upsampled) = s1B
   */
  
  void jsimd_h2v2_upsample_neon(int max_v_samp_factor, JDIMENSION output_width,
                                JSAMPARRAY input_data,
                                JSAMPARRAY *output_data_ptr)
  {
    JSAMPARRAY output_data = *output_data_ptr;
    JSAMPROW inptr, outptr0, outptr1;
    int inrow, outrow;
    unsigned colctr;
  
    for (inrow = 0, outrow = 0; outrow < max_v_samp_factor; inrow++) {
      inptr = input_data[inrow];
      outptr0 = output_data[outrow++];
      outptr1 = output_data[outrow++];
  
      for (colctr = 0; 2 * colctr < output_width; colctr += 16) {
        uint8x16_t samples = vld1q_u8(inptr + colctr);
        /* Duplicate the samples.  The store operation below interleaves them so
         * that adjacent pixel component values take on the same sample value,
         * per above.
         */
        uint8x16x2_t output_pixels = { { samples, samples } };
        /* Store pixel component values for both output rows to memory.
         * Due to the way sample buffers are allocated, we don't need to worry
         * about tail cases when output_width is not a multiple of 32.  See
         * "Creation of 2-D sample arrays" in jmemmgr.c for details.
         */
        vst2q_u8(outptr0 + 2 * colctr, output_pixels);
        vst2q_u8(outptr1 + 2 * colctr, output_pixels);
      }
    }
  }
  

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