Test/node_modules/pica/lib/mm_resize/convolve.js

// Resize convolvers, pure JS implementation
//
'use strict';


// Precision of fixed FP values
//var FIXED_FRAC_BITS = 14;


function clampTo8(i) { return i < 0 ? 0 : (i > 255 ? 255 : i); }


// Convolve image in horizontal directions and transpose output. In theory,
// transpose allow:
//
// - use the same convolver for both passes (this fails due different
//   types of input array and temporary buffer)
// - making vertical pass by horisonltal lines inprove CPU cache use.
//
// But in real life this doesn't work :)
//
function convolveHorizontally(src, dest, srcW, srcH, destW, filters) {

  var r, g, b, a;
  var filterPtr, filterShift, filterSize;
  var srcPtr, srcY, destX, filterVal;
  var srcOffset = 0, destOffset = 0;

  // For each row
  for (srcY = 0; srcY < srcH; srcY++) {
    filterPtr  = 0;

    // Apply precomputed filters to each destination row point
    for (destX = 0; destX < destW; destX++) {
      // Get the filter that determines the current output pixel.
      filterShift = filters[filterPtr++];
      filterSize  = filters[filterPtr++];

      srcPtr = (srcOffset + (filterShift * 4))|0;

      r = g = b = a = 0;

      // Apply the filter to the row to get the destination pixel r, g, b, a
      for (; filterSize > 0; filterSize--) {
        filterVal = filters[filterPtr++];

        // Use reverse order to workaround deopts in old v8 (node v.10)
        // Big thanks to @mraleph (Vyacheslav Egorov) for the tip.
        a = (a + filterVal * src[srcPtr + 3])|0;
        b = (b + filterVal * src[srcPtr + 2])|0;
        g = (g + filterVal * src[srcPtr + 1])|0;
        r = (r + filterVal * src[srcPtr])|0;
        srcPtr = (srcPtr + 4)|0;
      }

      // Bring this value back in range. All of the filter scaling factors
      // are in fixed point with FIXED_FRAC_BITS bits of fractional part.
      //
      // (!) Add 1/2 of value before clamping to get proper rounding. In other
      // case brightness loss will be noticeable if you resize image with white
      // border and place it on white background.
      //
      dest[destOffset + 3] = clampTo8((a + (1 << 13)) >> 14/*FIXED_FRAC_BITS*/);
      dest[destOffset + 2] = clampTo8((b + (1 << 13)) >> 14/*FIXED_FRAC_BITS*/);
      dest[destOffset + 1] = clampTo8((g + (1 << 13)) >> 14/*FIXED_FRAC_BITS*/);
      dest[destOffset]     = clampTo8((r + (1 << 13)) >> 14/*FIXED_FRAC_BITS*/);
      destOffset = (destOffset + srcH * 4)|0;
    }

    destOffset = ((srcY + 1) * 4)|0;
    srcOffset  = ((srcY + 1) * srcW * 4)|0;
  }
}

// Technically, convolvers are the same. But input array and temporary
// buffer can be of different type (especially, in old browsers). So,
// keep code in separate functions to avoid deoptimizations & speed loss.

function convolveVertically(src, dest, srcW, srcH, destW, filters) {

  var r, g, b, a;
  var filterPtr, filterShift, filterSize;
  var srcPtr, srcY, destX, filterVal;
  var srcOffset = 0, destOffset = 0;

  // For each row
  for (srcY = 0; srcY < srcH; srcY++) {
    filterPtr  = 0;

    // Apply precomputed filters to each destination row point
    for (destX = 0; destX < destW; destX++) {
      // Get the filter that determines the current output pixel.
      filterShift = filters[filterPtr++];
      filterSize  = filters[filterPtr++];

      srcPtr = (srcOffset + (filterShift * 4))|0;

      r = g = b = a = 0;

      // Apply the filter to the row to get the destination pixel r, g, b, a
      for (; filterSize > 0; filterSize--) {
        filterVal = filters[filterPtr++];

        // Use reverse order to workaround deopts in old v8 (node v.10)
        // Big thanks to @mraleph (Vyacheslav Egorov) for the tip.
        a = (a + filterVal * src[srcPtr + 3])|0;
        b = (b + filterVal * src[srcPtr + 2])|0;
        g = (g + filterVal * src[srcPtr + 1])|0;
        r = (r + filterVal * src[srcPtr])|0;
        srcPtr = (srcPtr + 4)|0;
      }

      // Bring this value back in range. All of the filter scaling factors
      // are in fixed point with FIXED_FRAC_BITS bits of fractional part.
      //
      // (!) Add 1/2 of value before clamping to get proper rounding. In other
      // case brightness loss will be noticeable if you resize image with white
      // border and place it on white background.
      //
      dest[destOffset + 3] = clampTo8((a + (1 << 13)) >> 14/*FIXED_FRAC_BITS*/);
      dest[destOffset + 2] = clampTo8((b + (1 << 13)) >> 14/*FIXED_FRAC_BITS*/);
      dest[destOffset + 1] = clampTo8((g + (1 << 13)) >> 14/*FIXED_FRAC_BITS*/);
      dest[destOffset]     = clampTo8((r + (1 << 13)) >> 14/*FIXED_FRAC_BITS*/);
      destOffset = (destOffset + srcH * 4)|0;
    }

    destOffset = ((srcY + 1) * 4)|0;
    srcOffset  = ((srcY + 1) * srcW * 4)|0;
  }
}


module.exports = {
  convolveHorizontally,
  convolveVertically
};