[SOLVED] Logarithmic slider - Slider class override freezes the Component

Hello !

I’m only on my second day of JUCE (and C++ for that matter), so the issue might be very stupid, but please bear with me.

I want a a logarithmic Slider. I originally tried with a skew of 1/e, but it wasn’t right, so I went through a lot of posts to see how I could implement it.

I saw Jules said to override proportionOfLengthToValue and valueToProportionOfLength to achieve that.
So far, I’ve created a subclass of Slider, called, LogSlider, which implements all this in the following fashion:

    x: proportion           (0) |-----------|(1)
    y: value         (minValue) |-----------|(maxValue)

    To get a logarithmic scale, we have to solve the following: 
    y = a * exp(b * x)
    (x1;y1) = (0;minValue)
    (x2;y2) = (1;maxValue)

    B and A have to be computed at instantiation and every time minValue or maxValue are changed.

    b = (log(y2/y1)) / (x2 - x1)
    a = y1 / (exp(b * x1)) OR a = y2 / (exp(b * x2))
    So in our case, 
    b = log(maxValue/minValue)
    a = minValue

void LogSlider::updateLogCoefficients()
    a = getMinimum();
    b = log(getMaximum() / getMinimum());

// TODO: optimize with exp / log approximations
double LogSlider::proportionOfLengthToValue(double proportion)
    double value = a * exp(b * proportion);
    return value;
double LogSlider::valueToProportionOfLength(double value)
    double proportion = log(value / a) / b;
    return proportion;

void LogSlider::setRange(double newMinimum, double newMaximum, double newInterval)
    Slider::setRange(newMinimum, newMaximum, newInterval);

The issue I’m having now is the sliders have no “thumbs”.
I think the paint() method is broken because of the subclass, somehow.

How could I either fix the paint() method, or implement the scaling without overriding the Slider class ?

Thank you for your time.

EDIT: I just realised I also need to change LogSlider::setRange() to prevent non-strictly-positive values for newMinimum and newMaximum. Possible culprit right there.

EDIT2: It was that stupid. minimumValue was 0, and froze the component. I now have a working prototype. I’ll have to see how I can correctly prevent those non-strictly-positive values.

1 Like

I just added a rework of the text function so that the values were also in line with the log scale. It needs tweaking but im happy with it for my purposes. Keeping the min value to 0.0001.

String LogSlider::getTextFromValue(double value) //like so
	String s = Decibels::toString(Decibels::gainToDecibels(value));
	return s;