Javascript function for sharpening images.

javascript-canvas-sharpen.js

function sharpen(ctx, w, h, mix) {
    var x, sx, sy, r, g, b, a, dstOff, srcOff, wt, cx, cy, scy, scx,
        weights = [0, -1, 0, -1, 5, -1, 0, -1, 0],
        katet = Math.round(Math.sqrt(weights.length)),
        half = (katet * 0.5) | 0,
        dstData = ctx.createImageData(w, h),
        dstBuff = dstData.data,
        srcBuff = ctx.getImageData(0, 0, w, h).data,
        y = h;

    while (y--) {
        x = w;
        while (x--) {
            sy = y;
            sx = x;
            dstOff = (y * w + x) * 4;
            r = 0;
            g = 0;
            b = 0;
            a = 0;

            for (cy = 0; cy < katet; cy++) {
                for (cx = 0; cx < katet; cx++) {
                    scy = sy + cy - half;
                    scx = sx + cx - half;

                    if (scy >= 0 && scy < h && scx >= 0 && scx < w) {
                        srcOff = (scy * w + scx) * 4;
                        wt = weights[cy * katet + cx];

                        r += srcBuff[srcOff] * wt;
                        g += srcBuff[srcOff + 1] * wt;
                        b += srcBuff[srcOff + 2] * wt;
                        a += srcBuff[srcOff + 3] * wt;
                    }
                }
            }

            dstBuff[dstOff] = r * mix + srcBuff[dstOff] * (1 - mix);
            dstBuff[dstOff + 1] = g * mix + srcBuff[dstOff + 1] * (1 - mix);
            dstBuff[dstOff + 2] = b * mix + srcBuff[dstOff + 2] * (1 - mix);
            dstBuff[dstOff + 3] = srcBuff[dstOff + 3];
        }
    }

    ctx.putImageData(dstData, 0, 0);
}

@forumwt This version prevents the changing the color of the edges:

  export const sharpen = (ctx, w, h, mix) => {
    var x, sx, sy, r, g, b, a, dstOff, srcOff, wt, cx, cy, scy, scx,
      weights = [0, -1, 0, -1, 5, -1, 0, -1, 0],
      katet = Math.round(Math.sqrt(weights.length)),
      half = (katet * 0.5) | 0,
      dstData = ctx.createImageData(w, h),
      dstBuff = dstData.data,
      srcBuff = ctx.getImageData(0, 0, w, h).data,
      y = h;
    while (y--) {
      x = w;
      while (x--) {
        sy = y;
        sx = x;
        dstOff = (y * w + x) * 4;
        r = 0;
        g = 0;
        b = 0;
        a = 0;
        if(x>0 && y>0 && x<w-1 && y<h-1) {
          for (cy = 0; cy < katet; cy++) {
            for (cx = 0; cx < katet; cx++) {
              scy = sy + cy - half;
              scx = sx + cx - half;
  
              if (scy >= 0 && scy < h && scx >= 0 && scx < w) {
                srcOff = (scy * w + scx) * 4;
                wt = weights[cy * katet + cx];
  
                r += srcBuff[srcOff] * wt;
                g += srcBuff[srcOff + 1] * wt;
                b += srcBuff[srcOff + 2] * wt;
                a += srcBuff[srcOff + 3] * wt;
              }
            }
          }
  
          dstBuff[dstOff] = r * mix + srcBuff[dstOff] * (1 - mix);
          dstBuff[dstOff + 1] = g * mix + srcBuff[dstOff + 1] * (1 - mix);
          dstBuff[dstOff + 2] = b * mix + srcBuff[dstOff + 2] * (1 - mix);
          dstBuff[dstOff + 3] = srcBuff[dstOff + 3];
        } else {
          dstBuff[dstOff] = srcBuff[dstOff];
          dstBuff[dstOff + 1] = srcBuff[dstOff + 1];
          dstBuff[dstOff + 2] = srcBuff[dstOff + 2];
          dstBuff[dstOff + 3] = srcBuff[dstOff + 3];
        }
      }
    }
  
      ctx.putImageData(dstData, 0, 0);
  }