function buildBlurKernel(r) {
            var radius = (r * 3.5) | 0;
            radius = radius < 1 ? 1 : radius < 248 ? radius : 248;

            if (blurRadius !== radius) {
              blurRadius = radius;
              blurKernelSize = (1 + blurRadius) << 1;
              blurKernel = new Int32Array(blurKernelSize);
              blurMult = new Array(blurKernelSize);
              for (var l = 0; l < blurKernelSize; l++) {
                blurMult[l] = new Int32Array(256);
              }

              var bk, bki;
              var bm, bmi;

              for (var i = 1, radiusi = radius - 1; i < radius; i++) {
                blurKernel[radius + i] = blurKernel[radiusi] = bki = radiusi * radiusi;
                bm = blurMult[radius + i];
                bmi = blurMult[radiusi--];
                for (var j = 0; j < 256; j++) {
                  bm[j] = bmi[j] = bki * j;
                }
              }
              bk = blurKernel[radius] = radius * radius;
              bm = blurMult[radius];

              for (var k = 0; k < 256; k++) {
                bm[k] = bk * k;
              }
            }
          }
increases the bright areas in an image
@private
@param  {Canvas} canvas