Android的 - 如何划出一道弧线基于梯度 [英] Android - How to draw an arc based gradient

问题描述

我想创建一个弧（可变度数）逐渐从一种颜色变为另一种。从例如，从蓝色到红色：

I am trying to create an arc (variable number of degrees) that gradually goes from one color to another. From example from blue to red:

这是我的code：

SweepGradient shader = new SweepGradient(center.x, center.y, resources.getColor(R.color.startColor),resources.getColor(R.color.endColor));
Paint paint = new Paint()
paint.setStrokeWidth(1);
paint.setStrokeCap(Paint.Cap.FILL);
paint.setStyle(Paint.Style.FILL);
paint.setShader(shader);
canvas.drawArc(rectF, startAngle, sweepAngle, true, paint);

但结果是，整个弧绘有相同的颜色。

But the result is the entire arc is painted with the same color.

编辑：
经过实验，我发现了颜色s- $ P $垫是由圆弧的角度来确定。如果我画一个弧线，一个小角度，只显示第一种颜色。所述的角度时，更多的颜色绘制。如果角度小似乎没有梯度。
下面是一个例子。我画4弧 - 90，180，270和360：

After more experimenting, I found out that the color spread is determined by the angle of the arc. If I draw an arc with a small angle, only the first color is displayed. The larger the angle, more colors are drawn. If the angle is small it seems that there is no gradient.
Here is an example. I am drawing 4 arcs - 90, 180, 270 and 360:

RectF rect1 = new RectF(50, 50, 150, 150);
Paint paint1 = new Paint();
paint1.setStrokeWidth(1);
paint1.setStrokeCap(Paint.Cap.SQUARE);
paint1.setStyle(Paint.Style.FILL);

SweepGradient gradient1 = new SweepGradient(100, 100,
        Color.RED, Color.BLUE);
paint1.setShader(gradient1);

canvas.drawArc(rect1, 0, 90, true, paint1);

RectF rect2 = new RectF(200, 50, 300, 150);
Paint paint2 = new Paint();
paint2.setStrokeWidth(1);
paint2.setStrokeCap(Paint.Cap.SQUARE);
paint2.setStyle(Paint.Style.FILL);

SweepGradient gradient2 = new SweepGradient(250, 100,
        Color.RED, Color.BLUE);
paint2.setShader(gradient2);

canvas.drawArc(rect2, 0, 180, true, paint2);

RectF rect3 = new RectF(50, 200, 150, 300);
Paint paint3 = new Paint();
paint3.setStrokeWidth(1);
paint3.setStrokeCap(Paint.Cap.SQUARE);
paint3.setStyle(Paint.Style.FILL);

SweepGradient gradient3 = new SweepGradient(100, 250,
        Color.RED, Color.BLUE);
paint3.setShader(gradient3);

canvas.drawArc(rect3, 0, 270, true, paint3);

RectF rect4 = new RectF(200, 200, 300, 300);
Paint paint4 = new Paint();
paint4.setStrokeWidth(1);
paint4.setStrokeCap(Paint.Cap.SQUARE);
paint4.setStyle(Paint.Style.FILL);

SweepGradient gradient4 = new SweepGradient(250, 250,
        Color.RED, Color.BLUE);
paint4.setShader(gradient4);

canvas.drawArc(rect4, 0, 360, true, paint4);

这里是结果：

And here is the result:

这是令人惊讶的，因为我期望的RED是在圆弧的起点，蓝色在年底enything之间为s $ P $垫均匀，无论角度。
我试图手动空间使用位置的颜色参数，但结果是一样的：

This is surprising because I'd expect the RED to be at the start of the arc, the BLUE at the end and enything between to be spread evenly regardless of angle.
I tried to space the colors manually using the positions parameter but the results were the same:

int[] colors = {Color.RED, Color.BLUE};
float[] positions = {0,1};
SweepGradient gradient = new SweepGradient(100, 100, colors , positions);

不知道如何解决这个问题？

Any idea how to solve this?

推荐答案

对此的解决办法是将蓝色的位置。这是像这样做：

The solution for this is to set the position of BLUE. This is done like so:

int[] colors = {Color.RED, Color.BLUE};
float[] positions = {0,1};
SweepGradient gradient = new SweepGradient(100, 100, colors , positions);

这里的问题是，设置1的蓝色到位置时，这并不意味着它将被定位在拉弧的端部，而是在该圆的端部的圆弧是一部分。 为了解决这个，蓝色的位置应该考虑到的度数在电弧。所以，如果我画一个弧线，X度，位置将被设置像这样：

The problem here is that when setting the position of BLUE to '1' this does not mean that it will be positioned at the end of the drawn arc, but instead at the end of the circle of which the arc is part of. To solve this, BLUE position should take into account the number of degrees in the arc. So if I'm drawing an arc with X degrees, position will be set like so:

float[] positions = {0,Xf/360f};

所以，如果X为90，梯度将放置在蓝色的圆的0.25：

So if X is 90, the gradient will place BLUE at 0.25 of the circle:

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