Use logarithmic scale for volume slider.

Fixes #1222

changes/changelog.d/1222.enhancement

@@ -0,0 +1 @@
+Logarithmic scale for volume slider (#1222)

front/src/audio/volume.js

@@ -0,0 +1,32 @@
+
+// Provides functions to convert between linear and logarithmic volume scales.
+// The logarithmic volume from the UI is converted to a linear volume with a
+// logarithmic function like exp(b*x)/a.
+// Compare https://www.dr-lex.be/info-stuff/volumecontrols.html for how the
+// values for a and b got derived.
+
+const PARAM_A = 1000
+const PARAM_B = Math.log(1000)  // ~ 6.908
+
+function toLinearVolumeScale(v) {
+    // Or as approximation:
+    // return Math.pow(v, 4)
+    if (v == 0.0) {
+        return 0.0
+    }
+
+    return Math.min(Math.exp(PARAM_B * v) / PARAM_A, 1.0)
+}
+
+function toLogarithmicVolumeScale(v) {
+    // Or as approximation:
+    // return Math.exp(Math.log(v) / 4)
+    if (v == 0.0) {
+        return 0.0
+    }
+
+    return Math.log(v * PARAM_A) / PARAM_B
+}
+
+exports.toLinearVolumeScale = toLinearVolumeScale
+exports.toLogarithmicVolumeScale = toLogarithmicVolumeScale

front/src/components/audio/VolumeControl.vue

@@ -29,7 +29,7 @@
       <input
         id="volume-slider"
         type="range"
-        step="0.05"
+        step="0.02"
         min="0"
         max="1"
         v-model="sliderVolume" />
@@ -38,6 +38,7 @@
 </template>
 <script>
 import { mapState, mapGetters, mapActions } from "vuex"
+import { toLinearVolumeScale, toLogarithmicVolumeScale } from '@/audio/volume'
 
 export default {
   data () {
@@ -49,10 +50,10 @@ export default {
   computed: {
     sliderVolume: {
       get () {
-        return this.$store.state.player.volume
+        return toLogarithmicVolumeScale(this.$store.state.player.volume)
       },
       set (v) {
-        this.$store.commit("player/volume", v)
+        this.$store.commit("player/volume", toLinearVolumeScale(v))
       }
     },
     labels () {

front/tests/unit/specs/audio/volume.spec.js

@@ -0,0 +1,34 @@
+import { expect } from 'chai'
+
+import { toLinearVolumeScale, toLogarithmicVolumeScale } from '@/audio/volume'
+
+describe('store/auth', () => {
+    describe('toLinearVolumeScale', () => {
+        describe('it should return real 0', () => {
+            expect(toLinearVolumeScale(0.0)).to.equal(0.0)
+        })
+
+        describe('it should have logarithmic scale', () => {
+            expect(2 * toLinearVolumeScale(0.5)).to.be.closeTo(toLinearVolumeScale(0.6), 0.001)
+        })
+
+        describe('it should return full volume', () => {
+            expect(toLogarithmicVolumeScale(1.0)).to.be.closeTo(1.0, 0.001)
+        })
+    })
+
+    describe('toLogarithmicVolumeScale', () => {
+        describe('it should return real 0', () => {
+            expect(toLogarithmicVolumeScale(0.0)).to.equal(0.0)
+        })
+
+        describe('it should have logarithmic scale', () => {
+            expect(toLogarithmicVolumeScale(0.6)).to.be.closeTo(0.9261, 0.001)
+            expect(toLogarithmicVolumeScale(0.7)).to.be.closeTo(0.9483, 0.001)
+        })
+
+        describe('it should return full volume', () => {
+            expect(toLogarithmicVolumeScale(1.0)).to.be.closeTo(1.0, 0.001)
+        })
+    })
+})