我一直在用 Android 的 Canvas 做一些工作，特别是试图确定它的 getClipBounds 结果是如何确定的。我了解 Canvas 内部保留了一个转换 Matrix，它在我调用 translate、scale 等时更新，但尝试复制 Matrix 的结果让我感到困惑。

@Override
public void onDraw(Canvas canvas) {
    Rect clipBounds;
    RectF viewport;

    canvas.save();
    canvas.concat(translationMatrix);
    //viewportMatrix.preConcat(canvas.getMatrix());
    viewportMatrix.set(canvas.getMatrix());

    clipBounds = canvas.getClipBounds();
    viewport = GetViewport();

    Log.d("clipBounds", clipBounds.toString() + " (" + clipBounds.width() + ", " + clipBounds.height() + ")");
    Log.d("viewport", viewport.toString() + " (" + viewport.width() + ", " + viewport.height() + ")");

    //drawing is done here

    canvas.restore();
}

//viewport code modeled after http://stackoverflow.com/a/17142856/2245528
private RectF GetViewport() {
    RectF viewport = new RectF();

    viewportMatrix.mapRect(viewport, originalViewport);

    return viewport;
}

private void Translate(float x, float y) {
    translationMatrix.postTranslate(x, y);

    invalidate();
}

private void Scale(float scaleFactor, PointF focusPoint) {
    if (focusPoint == null) {
        translationMatrix.postScale(scaleFactor, scaleFactor);
    }
    //keep the focus point in focus if possible
    else {
        translationMatrix.postScale(scaleFactor, scaleFactor, focusPoint.x, focusPoint.y);
    }

    invalidate();
}

private final Matrix translationMatrix = new Matrix();
private final RectF originalViewport = new RectF();
private final Matrix viewportMatrix = new Matrix();

originalViewport 设置为 0, 0, canvas.getWidth(), canvas.getHeight()。 Translate 和 Scale 是从手势事件处理程序调用的，它们工作正常。

让我感到困惑的部分是viewportMatrix。做不做似乎并不重要

viewportMatrix.set(canvas.getMatrix());

或

viewportMatrix.preConcat(canvas.getMatrix());

甚至一次性调用

viewportMatrix.set(canvas.getMatrix());

在开头，然后是对两个矩阵的并排 Translate/Scale 调用。我什至尝试过完全忽略 Canvas 的内置 Matrix 并将 GetViewport 重写为

//viewport code modeled after http://stackoverflow.com/a/17142856/2245528
private RectF GetViewport() {
    RectF viewport = new RectF();

    translationMatrix.mapRect(viewport, originalViewport);

    return viewport;
}

我似乎永远无法匹配 getClipBounds()，而且差异相当严重:

使用viewportMatrix.set(canvas.getMatrix):

clipBounds: Rect(-97, -97 - 602, 452) (699, 549)
viewport: RectF(97.04178, 97.06036, 797.04175, 647.06036) (700.0, 550.0)

使用viewportMatrix.preConcat(canvas.getMatrix):

clipBounds: Rect(-97, -96 - 602, 453) (699, 549)
viewport: RectF(2708.9663, 2722.2754, 3408.9663, 3272.2754) (700.0, 550.0)

使用translationMatrix.mapRect:

clipBounds: Rect(-96, -96 - 603, 453) (699, 549)
viewport: RectF(96.73213, 96.85794, 796.7321, 646.8579) (700.0, 550.0)

一次性调用 viewportMatrix.preConcat(canvas.getMatrix()) 然后并排 Translate/Scale 调用:

clipBounds: Rect(-96, -97 - 603, 452) (699, 549)
viewport: RectF(96.57738, 97.78168, 796.5774, 647.7817) (700.0, 550.0)

一次性调用 viewportMatrix.set(canvas.getMatrix()) 然后并排 Translate/Scale 调用:

clipBounds: Rect(-96, -96 - 603, 453) (699, 549)
viewport: RectF(96.40051, 96.88153, 796.4005, 646.88153) (700.0, 550.0)

我什至无法检查 the Canvas source ，因为所有 Matrix 代码都消失在未显示其代码的私有(private) native 调用中。

为什么我的 GetViewport 调用如此严重，getClipBounds 的幕后发生了什么？

最佳答案

我通读了this answer on GameDev SE ，它使用矩阵求逆在屏幕和世界坐标系之间交换:

To go from screen to world space simply use Vector2.Transform. This is commonly used to get the location of the mouse in the world for object picking.

Vector2.Transform(mouseLocation, Matrix.Invert(Camera.TransformMatrix));

To go from world to screen space simply do the opposite.

Vector2.Transform(mouseLocation, Camera.TransformMatrix);

受这种方法的启发，我尝试像这样反转变换矩阵:

private RectF GetViewport() {
    RectF viewport = new RectF();
    Matrix viewportMatrix = new Matrix();

    translationMatrix.invert(viewportMatrix);

    viewportMatrix.mapRect(viewport, originalViewport);

    return viewport;
}

此视口(viewport)与 getClipBounds 返回的结果正确匹配。

This matrix tutorial解释了我从 transform 调用的结果中注意到但没有应用到我的矩阵的内容:

In a computer program the camera doesn’t move at all and in actuality, the world is just moving in the opposite direction and orientation of how you would want the camera to move in reality.

In order to understand this correctly, we must think in terms of two different things:

1. The Camera Transformation Matrix: The transformation that places the camera in the correct position and orientation in world space (this is the transformation that you would apply to a 3D model of the camera if you wanted to represent it in the scene).
2. The View Matrix: This matrix will transform vertices from world-space to view-space. This matrix is the inverse of the camera’s transformation matrix described above.

关于java - Canvas 如何确定其裁剪范围？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/36683187/