How to detect shape on a transparent canvas?

What you need to do:

• Get the buffer
• Get a 32-bits reference of that buffer (If your other pixels are transparent then you can use a Uint32Array buffer to iterate).
• Scan 0 - width to find x1 edge
• Scan width - 0 to find x2 edge
• Scan 0 - height to find y1 edge
• Scan height - 0 to find y2 edge

These scans can be combined but for simplicity I'll show each step separately.

Online demo of this can be found here.

Result:

When image is loaded draw it in (if the image is small then the rest of this example would be waste as you would know the coordinates when drawing it - assuming here the image you draw is large with a small image inside it)

(note: this is a non-optimized version for the sake of simplicity)

ctx.drawImage(this, 0, 0, w, h);

var idata = ctx.getImageData(0, 0, w, h),      // get image data for canvas
    buffer = idata.data,                       // get buffer (unnes. step)
    buffer32 = new Uint32Array(buffer.buffer), // get a 32-bit representation
    x, y,                                      // iterators
    x1 = w, y1 = h, x2 = 0, y2 = 0;            // min/max values

Then scan each edge. For left edge you scan from 0 to width for each line (non optimized):

// get left edge
for(y = 0; y < h; y++) {                       // line by line
    for(x = 0; x < w; x++) {                   // 0 to width
        if (buffer32[x + y * w] > 0) {         // non-transparent pixel?
            if (x < x1) x1 = x;                // if less than current min update
        }
    }
}

For the right edge you just reverse x iterator:

// get right edge
for(y = 0; y < h; y++) {                       // line by line
    for(x = w; x >= 0; x--) {                  // from width to 0
        if (buffer32[x + y * w] > 0) {
            if (x > x2) x2 = x;
        }
    }
}

And the same is for top and bottom edges just that the iterators are reversed:

// get top edge
for(x = 0; x < w; x++) {
    for(y = 0; y < h; y++) {
        if (buffer32[x + y * w] > 0) {
            if (y < y1) y1 = y;
        }
    }
}

// get bottom edge
for(x = 0; x < w; x++) {
    for(y = h; y >= 0; y--) {
        if (buffer32[x + y * w] > 0) {
            if (y > y2) y2 = y;
        }
    }
}

The resulting region is then:

ctx.strokeRect(x1, y1, x2-x1, y2-y1);

There are various optimizations you could implement but they depend entirely on the scenario such as if you know approximate placement then you don't have to iterate all lines/columns.

You could do a brute force guess of he placement by skipping x number of pixels and when you found a non-transparent pixel you could make a max search area based on that and so forth, but that is out of scope here.

Hope this helps!