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);
}

Or you could use the SVG convolution matrix

Does this make the sharpened image "visible" to any other code analysing the image later? Or is it just that what is visible on the screen becomes sharper?

The result should be baked into the imageData of the canvas, so any code that is reading the imageData after the image has been sharpened should get the sharpened image. Its not just a visual trick, its visible in code/data. Hope this helps.

@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);
  }