android - 使用枢轴点缩放 Canvas 后，x 和 y 坐标错误

Question

我正在尝试在应该关注轴心点的 Canvas 上实现缩放。缩放效果很好，但之后用户应该能够在 Canvas 上选择元素。问题是，我的翻译值似乎不正确，因为它们的偏移量与我不缩放到轴心点的值不同(没有轴心点缩放和拖动效果很好)。我使用了 this example 中的一些代码.

相关代码为:

class DragView extends View {

private static float MIN_ZOOM = 0.2f;
private static float MAX_ZOOM = 2f;

// These constants specify the mode that we're in
private static int NONE = 0;
private int mode = NONE;
private static int DRAG = 1;
private static int ZOOM = 2;
public ArrayList<ProcessElement> elements;

// Visualization
private boolean checkDisplay = false;
private float displayWidth;
private float displayHeight;
// These two variables keep track of the X and Y coordinate of the finger when it first
// touches the screen
private float startX = 0f;
private float startY = 0f;
// These two variables keep track of the amount we need to translate the canvas along the X
//and the Y coordinate
// Also the offset from initial 0,0
private float translateX = 0f;
private float translateY = 0f;

private float lastGestureX = 0;
private float lastGestureY = 0;

private float scaleFactor = 1.f;
private ScaleGestureDetector detector;
...

private void sharedConstructor() {
    elements = new ArrayList<ProcessElement>();
    flowElements = new ArrayList<ProcessFlow>();
    detector = new ScaleGestureDetector(getContext(), new ScaleListener());
}

/**
 * checked once to get the measured screen height/width
 * @param hasWindowFocus
 */
@Override
public void onWindowFocusChanged(boolean hasWindowFocus) {
    super.onWindowFocusChanged(hasWindowFocus);
    if (!checkDisplay) {
        displayHeight = getMeasuredHeight();
        displayWidth = getMeasuredWidth();
        checkDisplay = true;
    }
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    ProcessBaseElement lastElement = null;

    switch (event.getAction() & MotionEvent.ACTION_MASK) {
        case MotionEvent.ACTION_DOWN:
            mode = DRAG;

            // Check if an Element has been touched.
            // Need to use the absolute Position that's why we take the offset into consideration
            touchedElement = isElementTouched(((translateX * -1) + event.getX()) / scaleFactor, (translateY * -1 + event.getY()) / scaleFactor);


                //We assign the current X and Y coordinate of the finger to startX and startY minus the previously translated
                //amount for each coordinates This works even when we are translating the first time because the initial
                //values for these two variables is zero.
                startX = event.getX() - translateX;
                startY = event.getY() - translateY;
            }
            // if an element has been touched -> no need to take offset into consideration, because there's no dragging possible
            else {
                startX = event.getX();
                startY = event.getY();
            }

            break;

        case MotionEvent.ACTION_MOVE:
            if (mode != ZOOM) {
                if (touchedElement == null) {
                    translateX = event.getX() - startX;
                    translateY = event.getY() - startY;
                } else {
                    startX = event.getX();
                    startY = event.getY();
                }
            }

            if(detector.isInProgress()) {
                lastGestureX = detector.getFocusX();
                lastGestureY = detector.getFocusY();
            }

            break;

        case MotionEvent.ACTION_UP:
            mode = NONE;

            break;
        case MotionEvent.ACTION_POINTER_DOWN:
            mode = ZOOM;

            break;
        case MotionEvent.ACTION_POINTER_UP:
            break;
    }

    detector.onTouchEvent(event);
    invalidate();

    return true;
}

private ProcessBaseElement isElementTouched(float x, float y) {
    for (int i = elements.size() - 1; i >= 0; i--) {
        if (elements.get(i).isTouched(x, y))
            return elements.get(i);
    }
    return null;
}

@Override
public void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    canvas.save();

    if(detector.isInProgress()) {
        canvas.scale(scaleFactor,scaleFactor,detector.getFocusX(),detector.getFocusY());
    } else
        canvas.scale(scaleFactor, scaleFactor,lastGestureX,lastGestureY);     // zoom

//        canvas.scale(scaleFactor,scaleFactor);

    //We need to divide by the scale factor here, otherwise we end up with excessive panning based on our zoom level
    //because the translation amount also gets scaled according to how much we've zoomed into the canvas.
    canvas.translate(translateX / scaleFactor, translateY / scaleFactor);

    drawContent(canvas);

    canvas.restore();
}

/**
 * scales the canvas
 */
private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
    @Override
    public boolean onScale(ScaleGestureDetector detector) {
        scaleFactor *= detector.getScaleFactor();
        scaleFactor = Math.max(MIN_ZOOM, Math.min(scaleFactor, MAX_ZOOM));
        return true;
    }
}
}

元素以它们在 Canvas 上的绝对位置保存(考虑到拖动)。我怀疑我没有考虑从枢轴点到 translateX 和 translateY 的新偏移量，但我不知道应该在哪里以及如何做.任何帮助将不胜感激。

Answer 1

好的，所以您基本上是在围绕某个轴心点 {Px, Py} 缩放 View 后，找出某个屏幕 X/Y 坐标对应的位置。

那么，让我们试着分解一下。

为了便于讨论，我们假设 Px & Py = 0，并且 s = 2。这意味着 View 在 View 的左上角周围缩放了 2 倍。

在这种情况下，屏幕坐标 {0, 0} 对应于 View 中的 {0, 0}，因为该点是唯一未更改的点。一般来说，如果屏幕坐标等于轴心点，则没有变化。

如果用户点击其他点会发生什么，比方说 {2, 3}？在这种情况下，曾经是 {2, 3} 的位置现在已经从枢轴点(即 {0, 0})移动了 2 倍，因此对应的位置是 {4, 6}。

当轴心点为 {0, 0} 时，这一切都很容易，但如果不是，会发生什么？

好吧，让我们看另一种情况 - 轴心点现在位于 View 的右下角(宽度 = w，高度 = h - {w, h})。同样，如果用户点击相同的位置，那么相应的位置也是 {w, h}，但是假设用户点击了其他位置，例如 {w - 2, h - 3}？同样的逻辑也出现在这里:翻译后的位置是 {w - 4, h - 6}。

总而言之，我们要做的是将屏幕坐标转换为平移坐标。我们需要对收到的 X/Y 坐标执行与对缩放 View 中的每个像素执行的操作相同的操作。

第 1 步 - 我们要根据枢轴点平移 X/Y 位置:

X = X - Px
Y = Y - Py

第 2 步 - 然后我们缩放 X 和 Y:

X = X * s
Y = Y * s

第 3 步 - 然后我们翻译回去:

X = X + Px
Y = Y + Py

如果我们将此应用于我给出的最后一个示例(我将仅针对 X 进行演示):

Original value: X = w - 2, Px = w
Step 1: X <-- X - Px = w - 2 - w = -2
Step 2: X <-- X * s = -2 * 2 = -4
Step 3: X <-- X + Px = -4 + w = w - 4

一旦您将此应用于您收到的任何在缩放之前相关的 X/Y，该点将被平移，以便它相对于缩放状态。

希望这对您有所帮助。