如何获得任意矩阵的缩放值? [英] How to get zoom value of arbitrary matrix?

问题描述

图像按矩阵缩放:

Matrix matrix = new Matrix();
matrix.postScale(...);
matrix.postTranslate(...);
matrix.postRotate(...);
...

我希望缩放后的图像不会小于原始图像的一半，因此总缩放比例应不小于0.5.

I hope the zoomed image won't be less than the half of original, so the total zoom should not less that 0.5.

但是该怎么做呢?我试图获取要检查的矩阵的第一个值:

But how to do that? I tried to get the first value of matrix to check:

float pendingZoom = 0.6f;

float[] values = new float[9];
Matrix.getValues(values);
float scalex = values[Matrix.MSCALE_X];

然后:

if(scalex<0.5) {
    pendingZoom = pendingZoom * (0.5f / scalex);
}

不幸的是，有时它不起作用.如果图像已旋转，则scalex可能为负，pendingZoom也将为负.

unfortunately, it doesn't work sometimes. If the image has been rotated, the scalex may be negative, the pendingZoom will be negative too.

如何正确执行此操作?

更新

我刚刚发现values[Matrix.MSCALE_X]不是可实现的缩放值.我用它来计算矩形的新宽度，这是不正确的.

I just found the values[Matrix.MSCALE_X] is not a realiable zoom value. I use it to calculate the new width of a rect, it's not correct.

相反，我尝试通过矩阵映射两个点，然后计算两个距离:

Instead, I tried to map two points by the matrix, and calculate the two distances:

PointF newP1 = mapPoint(matrix, new PointF(0, 0));
PointF newP2 = mapPoint(matrix, new PointF(width, 0));
float scale = calcDistance(newP1, newP2) / width;

我现在可以获得正确的scale值.但是我不确定这是否是最佳解决方案.

I can get correct scale value now. But I'm not sure if it's best solution.

推荐答案

    float scalex = values[Matrix.MSCALE_X];
    float skewy = values[Matrix.MSKEW_Y];
    float scale = (float) Math.sqrt(scalex * scalex + skewy * skewy);

此解决方案看起来更简单，但我认为您的解决方案要清晰得多，并且几乎与该解决方案一样快.

This solution looks simpler but I think that your's is much more clear and almost as fast as this one.

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