Matrix的前乘和后乘不太明白？
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Assessment
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Newbie friendliness
- 25/100
- Issue type
- Documentation
- Clarity
- Needs clarification
- Activity status
- Stale
- Tech stack
- android, java
- Domain
- computer-graphics, mobile
Research direction
Start with the two snippets and the Android Matrix calls shown: postRotate, postTranslate, preTranslate, and PathMeasure.getMatrix. Compare their transformation order and explain why the two approaches use different multiplication directions. Done means a clear explanation grounded in the displayed examples; no file or test is identified in the issue.
Written by the indexing model from the issue text.
Description
measure.getPosTan(measure.getLength() * currentValue, pos, tan); // 获取当前位置的坐标以及趋势
mMatrix.reset(); // 重置Matrix
float degrees = (float) (Math.atan2(tan[1], tan[0]) * 180.0 / Math.PI); // 计算图片旋转角度
mMatrix.postRotate(degrees, mBitmap.getWidth() / 2, mBitmap.getHeight() / 2); // 旋转图片
mMatrix.postTranslate(pos[0] - mBitmap.getWidth() / 2, pos[1] - mBitmap.getHeight() / 2); // 将图片绘制中心调整到与当前点重合
这是第一种方案，
// 获取当前位置的坐标以及趋势的矩阵
measure.getMatrix(measure.getLength() * currentValue, mMatrix, PathMeasure.TANGENT_MATRIX_FLAG | PathMeasure.POSITION_MATRIX_FLAG);
mMatrix.preTranslate(-mBitmap.getWidth() / 2, -mBitmap.getHeight() / 2); // <-- 将图片绘制中心调整到与当前点重合(注意:此处是前乘pre)
这是第二种方案，为什么第一种是用的后乘，第二种用的是前乘，具体看了那个matrix的原理还是不太明白？可以具体讲讲吗？
- Dominant language
- Java
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