ImageViewer - limit zoom by the window/screen size (#2993)

- add zoom field, which is the same across different screen/window sizes

- scale is not the base state now, it is derived from the current zoom and the current screen/window size. it now represents the end scale of the image

- drag amount is still independent of scale/zoom (if we drag by 5 pixels, the image moves by 5 pixels)

- offset is still limited by the area and the current scale

* ImageViewer - limit zoom by the window/screen size

- add zoom field, which is the same across different screen/window sizes

- scale is not the base state now, it is derived from the current zoom and the current screen/window size. it now represents the end scale of the image

- drag amount is still independent of scale/zoom (if we drag by 5 pixels, the image moves by 5 pixels)

- offset is still limited by the area and the current scale

examples/imageviewer/shared/src/commonMain/kotlin/example/imageviewer/model/ScalableState.kt

@@ -1,32 +1,56 @@
 package example.imageviewer.model
 
+import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.geometry.Size
 import androidx.compose.ui.geometry.isSpecified
+import kotlin.math.max
 
 /**
  * Encapsulate all transformations about showing some target (an image, relative to its center)
  * scaled and shifted in some area (a window, relative to its center)
  */
 class ScalableState {
-    val scaleLimits = 0.2f..10f
+    var zoomLimits = 1.0f..5f
+
+    private var offset by mutableStateOf(Offset.Zero)
 
     /**
-     * Offset of the camera before scaling (an offset in pixels in the area coordinate system)
+     * Zoom of the target relative to the area size. 1.0 - the target completely fits the area.
      */
-    var offset by mutableStateOf(Offset.Zero)
-        private set
-    var scale by mutableStateOf(1f)
+    var zoom by mutableStateOf(1f)
         private set
 
-    private var areaSize: Size = Size.Unspecified
-    private var targetSize: Size = Size.Zero
+    private var areaSize: Size by mutableStateOf(Size.Unspecified)
+    private var targetSize: Size by mutableStateOf(Size.Zero)
+
+    /**
+     * A transformation that should be applied to render the target in the area.
+     *   offset - in pixels in the area coordinate system, should be applied before scaling
+     *   scale - scale of the target in the area
+     */
+    val transformation: Transformation by derivedStateOf {
+        Transformation(
+            offset = offset,
+            scale = zoomToScale(zoom)
+        )
+    }
+
+    /**
+     * The calculated base scale for 100% zoom. Calculated so that the target fits the area.
+     */
+    private val scaleFor100PercentZoom by derivedStateOf {
+        if (targetSize.isSpecified && areaSize.isSpecified) {
+            max(areaSize.width / targetSize.width, areaSize.height / targetSize.height)
+        } else {
+            1.0f
+        }
+    }
 
-    private var offsetXLimits = Float.NEGATIVE_INFINITY..Float.POSITIVE_INFINITY
-    private var offsetYLimits = Float.NEGATIVE_INFINITY..Float.POSITIVE_INFINITY
+    private fun zoomToScale(zoom: Float) = zoom * scaleFor100PercentZoom
 
     /**
      * Limit the target center position, so:
@@ -46,22 +70,20 @@ class ScalableState {
 
     private fun applyLimits() {
         if (targetSize.isSpecified && areaSize.isSpecified) {
-            offsetXLimits = centerLimits(targetSize.width * scale, areaSize.width)
-            offsetYLimits = centerLimits(targetSize.height * scale, areaSize.height)
+            val offsetXLimits = centerLimits(targetSize.width * transformation.scale, areaSize.width)
+            val offsetYLimits = centerLimits(targetSize.height * transformation.scale, areaSize.height)
+
             offset = Offset(
                 offset.x.coerceIn(offsetXLimits),
                 offset.y.coerceIn(offsetYLimits),
             )
-        } else {
-            offsetXLimits = Float.NEGATIVE_INFINITY..Float.POSITIVE_INFINITY
-            offsetYLimits = Float.NEGATIVE_INFINITY..Float.POSITIVE_INFINITY
         }
     }
 
-    private fun centerLimits(imageSize: Float, areaSize: Float): ClosedFloatingPointRange<Float> {
+    private fun centerLimits(targetSize: Float, areaSize: Float): ClosedFloatingPointRange<Float> {
         val areaCenter = areaSize / 2
-        val imageCenter = imageSize / 2
-        val extra = (imageCenter - areaCenter).coerceAtLeast(0f)
+        val targetCenter = targetSize / 2
+        val extra = (targetCenter - areaCenter).coerceAtLeast(0f)
         return -extra / 2..extra / 2
     }
 
@@ -75,17 +97,37 @@ class ScalableState {
      * After we apply the new scale, the camera should be focused on the same point in
      * the target coordinate system.
      */
-    fun addScale(scaleMultiplier: Float, focus: Offset = Offset.Zero) {
-        setScale(scale * scaleMultiplier, focus)
+    fun addZoom(zoomMultiplier: Float, focus: Offset = Offset.Zero) {
+        setZoom(zoom * zoomMultiplier, focus)
     }
 
-    fun setScale(scale: Float, focus: Offset = Offset.Zero) {
-        val newScale = scale.coerceIn(scaleLimits)
-        val focusInTargetSystem = (focus - offset) / this.scale
-        // calculate newOffset from this equation:
-        // focusInTargetSystem = (focus - newOffset) / newScale
-        offset = focus - focusInTargetSystem * newScale
-        this.scale = newScale
+    /**
+     * @param focus on which point the camera is focused in the area coordinate system.
+     * After we apply the new scale, the camera should be focused on the same point in
+     * the target coordinate system.
+     */
+    fun setZoom(zoom: Float, focus: Offset = Offset.Zero) {
+        val newZoom = zoom.coerceIn(zoomLimits)
+        val newOffset = Transformation.offsetOf(
+            point = transformation.pointOf(focus),
+            transformedPoint = focus,
+            scale = zoomToScale(newZoom)
+        )
+        this.offset = newOffset
+        this.zoom = newZoom
         applyLimits()
     }
+
+    data class Transformation(
+        val offset: Offset,
+        val scale: Float,
+    ) {
+        fun pointOf(transformedPoint: Offset) = (transformedPoint - offset) / scale
+
+        companion object {
+            // is derived from the equation `point = (transformedPoint - offset) / scale`
+            fun offsetOf(point: Offset, transformedPoint: Offset, scale: Float) =
+                transformedPoint - point * scale
+        }
+    }
 }

examples/imageviewer/shared/src/commonMain/kotlin/example/imageviewer/view/ScalableImage.common.kt

@@ -18,9 +18,18 @@ import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.ui.platform.LocalDensity
 import example.imageviewer.model.ScalableState
 import example.imageviewer.utils.onPointerEvent
-import kotlin.math.min
 import kotlin.math.pow
 
+/**
+ * Initial zoom of the image. 1.0f means the image fully fits the window.
+ */
+private const val INITIAL_ZOOM = 1.0f
+
+/**
+ * This zoom means that the image isn't significantly zoomed for the user yet.
+ */
+private const val SLIGHTLY_INCREASED_ZOOM = 1.5f
+
 @Composable
 internal fun ScalableImage(scalableState: ScalableState, image: ImageBitmap, modifier: Modifier = Modifier) {
     BoxWithConstraints {
@@ -29,24 +38,14 @@ internal fun ScalableImage(scalableState: ScalableState, image: ImageBitmap, mod
         val imageCenter = Offset(image.width / 2f, image.height / 2f)
         val areaCenter = Offset(areaSize.width / 2f, areaSize.height / 2f)
 
-        if (areaSize.width > 0 && areaSize.height > 0) {
-            DisposableEffect(Unit) {
-                scalableState.setScale(
-                    min(areaSize.width / imageSize.width, areaSize.height / imageSize.height),
-                    Offset.Zero,
-                )
-                onDispose { }
-            }
-        }
-
         Box(
             modifier
                 .drawWithContent {
                     drawIntoCanvas {
                         it.withSave {
                             it.translate(areaCenter.x, areaCenter.y)
-                            it.translate(scalableState.offset.x, scalableState.offset.y)
-                            it.scale(scalableState.scale, scalableState.scale)
+                            it.translate(scalableState.transformation.offset.x, scalableState.transformation.offset.y)
+                            it.scale(scalableState.transformation.scale, scalableState.transformation.scale)
                             it.translate(-imageCenter.x, -imageCenter.y)
                             drawImage(image)
                         }
@@ -55,22 +54,24 @@ internal fun ScalableImage(scalableState: ScalableState, image: ImageBitmap, mod
                 .pointerInput(Unit) {
                     detectTransformGestures { centroid, pan, zoom, _ ->
                         scalableState.addPan(pan)
-                        scalableState.addScale(zoom, centroid - areaCenter)
+                        scalableState.addZoom(zoom, centroid - areaCenter)
                     }
                 }
                 .onPointerEvent(PointerEventType.Scroll) {
                     val centroid = it.changes[0].position
                     val delta = it.changes[0].scrollDelta
                     val zoom = 1.2f.pow(-delta.y)
-                    scalableState.addScale(zoom, centroid - areaCenter)
+                    scalableState.addZoom(zoom, centroid - areaCenter)
                 }
                 .pointerInput(Unit) {
                     detectTapGestures(onDoubleTap = { position ->
-                        scalableState.setScale(
-                            if (scalableState.scale > 2.0) {
-                                scalableState.scaleLimits.start
+                        // If a user zoomed significantly, the zoom should be the restored on double tap,
+                        // otherwise the zoom should be increased
+                        scalableState.setZoom(
+                            if (scalableState.zoom > SLIGHTLY_INCREASED_ZOOM) {
+                                INITIAL_ZOOM
                             } else {
-                                scalableState.scaleLimits.endInclusive
+                                scalableState.zoomLimits.endInclusive
                             },
                             position - areaCenter
                         )

examples/imageviewer/shared/src/desktopMain/kotlin/example/imageviewer/view/ZoomControllerView.desktop.kt

@@ -14,9 +14,9 @@ import example.imageviewer.model.ScalableState
 internal actual fun ZoomControllerView(modifier: Modifier, scalableState: ScalableState) {
     Slider(
         modifier = modifier.fillMaxWidth(0.5f).padding(12.dp),
-        value = scalableState.scale,
-        valueRange = scalableState.scaleLimits.start..scalableState.scaleLimits.endInclusive,
-        onValueChange = { scalableState.setScale(it) },
+        value = scalableState.zoom,
+        valueRange = scalableState.zoomLimits.start..scalableState.zoomLimits.endInclusive,
+        onValueChange = { scalableState.setZoom(it) },
         colors = SliderDefaults.colors(thumbColor = Color.White, activeTrackColor = Color.White)
     )
 }