Take a partial screenshot?

I need to take a partial screenshot, lets say from position 10,10 with 200 pixel width and 100 pixels height.

I tried to read from the canvas (this._iRuntime._canvas), but its just black.

The snapshotfunction only delivers a full screen shot, already as encoded url.

In any other html5 framework that works, but not in Construct3.

Thats what i Do:

screenshot(this._iRuntime._canvas, 10, 10, 200, 100);

function screenshot (_canvas, _x, _y, _sx, _sy)

{

var SaveCan = document.createElement('canvas');

SaveCan.setAttribute('width', _sx);

SaveCan.setAttribute('height', _sy);

var SaveCtx = SaveCan.getContext('2d');

SaveCtx.fillStyle = "rgb(100,100,200)"; //Make sure it is not black

SaveCtx.clearRect(0,0,256,220);

SaveCtx.drawImage(_canvas,_x,_y,_sx,_sy,0,0,_sx,_sy);

var canvasData = SaveCan.toDataURL("image/jpg");

-> POST to a php script

}

Any help welcome