instantreality 1.0

X3DOM mouse events further improved

September 2nd, 2012

We have further improved picking to cope with several problems that came along with the original approach (which was mentioned in a very old post). Therefore, picking now supports 64k different objects, a higher precision pick position and the normal at the picked position (both in world space) for all mouse events.

We still use a single-pass render-buffer-based approach, but instead of rendering the normalized world position into an FBO’s 8-bit RGB channel and the (internal) Shape ID into the (also 8-bit) alpha channel, we now render just the distance of the picked object position to the camera position into the RG channel (encoded as 16-bit value in the shader) and the Shape ID into the texture’s BA channel (also encoded as 16-bit).

Having the distance d between both positions provides enough information to calculate the full 3D position, since the x and y components (along with z later on) can be obtained by computing the view ray through the the picked pixel position (x,y).

var line = viewarea.calcViewRay(x, y);
var pickPos = line.pos.add(line.dir.multiply(d));

And instead of just reading back a single (8-bit) RGBA value at the picked pixel position (x,y), we now read back a small 2×2 window, so that we can also directly compute the object’s normal by taking the cross product of the (decoded) world space position above (x,y-1) and to the right (x+1,y).

This way, the corresponding UI Event object now not only provides the picked world position (worldX, worldY, and worldZ), but also normalX, normalY, and normalZ. A little demo can be found here.

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