Why is WheelDelta = 120

mousewheelwinapi

I'm working with mouse events, specifically OnMouseWheel. Many code samples refer to distance the view changes (or zoom f.i. in 3D application) as Distance = Sign(WheelDelta)*Constant or Distance = WheelDelta / WHEEL_DELTA or something of that kind – assuming that WheelDelta is always a multiple of 120 (WHEEL_DELTA constant = 120).

I already found that in touch interfaces / tablets input may depend on scrolling length.

I was wondering why Microsoft has set default WheelDelta to 120, why not 100 or 10 or anything else? In what other cases wheel delta may be something different from 120?

Best Answer

The Qt Documentation elaborates a bit more on why it is actually 120:

QPoint QWheelEvent::angleDelta() const

Returns the distance that the wheel is rotated, in eighths of a degree. A positive value indicates that the wheel was rotated forwards away from the user; a negative value indicates that the wheel was rotated backwards toward the user.

Most mouse types work in steps of 15 degrees, in which case the delta value is a multiple of 120; i.e., 120 units * 1/8 = 15 degrees.

However, some mice have finer-resolution wheels and send delta values that are less than 120 units (less than 15 degrees). To support this possibility, you can either cumulatively add the delta values from events until the value of 120 is reached, then scroll the widget, or you can partially scroll the widget in response to each wheel event.

https://doc.qt.io/qt-5/qwheelevent.html#angleDelta

Related Solutions

Wpf – Mousewheel Delta value always 120

You should be able to handle the PreviewMouseWheel event. The delta for each event will be +/-120, but you will get one event for each "notch" of the wheel.

Why are “TranslateMessage” and “DispatchMessage” separate calls

The more traditional message loop looks like this:

while (GetMessage(&msg, 0, 0, 0)) 
{
    if (!TranslateAccelerator(hwndMain, haccel, &msg))
    {
        TranslateMessage(&msg); 
        DispatchMessage(&msg); 
    } 
}

It is a pretty big hint to what you'd want to do before dispatching the message: catch messages that ought to be intercepted and treated specially before the window sees them. Keyboard shortcuts are a classic example, they need to be detected no matter what window has the focus.

Any GUI class library exposes it with a virtual method named something like App.PreProcessMessage, a virtual function that can be overridden so your program can implement its own shortcuts and whatnot.

Best Answer

QPoint QWheelEvent::angleDelta() const

Related Solutions

Wpf – Mousewheel Delta value always 120

Why are “TranslateMessage” and “DispatchMessage” separate calls

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