如何在android Canvas上绘制实心三角形
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How to draw filled triangle on android Canvas
提问by Egis
I have class MyView that extends View class. MyView should draw filled triangle. I drew a triangle but I cannot get it filled. This is my onDraw() method:
我有扩展 View 类的 MyView 类。MyView 应该绘制实心三角形。我画了一个三角形，但我不能把它填满。这是我的 onDraw() 方法：
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
Paint paint = new Paint();
paint.setColor(android.graphics.Color.BLACK);
canvas.drawPaint(paint);
paint.setStrokeWidth(4);
paint.setColor(android.graphics.Color.RED);
paint.setStyle(Paint.Style.FILL_AND_STROKE);
paint.setAntiAlias(true);
Point a = new Point(0, 0);
Point b = new Point(0, 100);
Point c = new Point(87, 50);
Path path = new Path();
path.setFillType(FillType.EVEN_ODD);
path.moveTo(a.x, a.y);
path.lineTo(b.x, b.y);
path.moveTo(b.x, b.y);
path.lineTo(c.x, c.y);
path.moveTo(c.x, c.y);
path.lineTo(a.x, a.y);
path.close();
canvas.drawPath(path, paint);
}
This is what I get as a result:
这就是我得到的结果：
回答by Egis
I've found the answer
我找到了答案
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
Paint paint = new Paint();
paint.setColor(android.graphics.Color.BLACK);
canvas.drawPaint(paint);
paint.setStrokeWidth(4);
paint.setColor(android.graphics.Color.RED);
paint.setStyle(Paint.Style.FILL_AND_STROKE);
paint.setAntiAlias(true);
Point a = new Point(0, 0);
Point b = new Point(0, 100);
Point c = new Point(87, 50);
Path path = new Path();
path.setFillType(FillType.EVEN_ODD);
path.lineTo(b.x, b.y);
path.lineTo(c.x, c.y);
path.lineTo(a.x, a.y);
path.close();
canvas.drawPath(path, paint);
}
回答by Joe Maher
This answer provides a bit of clarity on where the numbers given in the answer by @Egis come from. (this will draw an upside down equilateral triangle and is written in kotlin)
这个答案对@Egis 的答案中给出的数字的来源提供了一些清晰的说明。（这将绘制一个倒置的等边三角形并用 kotlin 编写）
class TriangleView(context: Context?, attrs: AttributeSet?) : View(context, attrs) {
val paint = Paint()
val path = Path()
override fun onDraw(canvas: Canvas?) {
super.onDraw(canvas)
canvas ?: return
canvas.drawPath(configurePath(canvas.width.toFloat(), path), configurePaint(paint))
}
fun getHeight(width: Double): Float {
return Math.sqrt((Math.pow(width, 2.0) - Math.pow((width / 2), 2.0))).toFloat()
}
fun configurePaint(paint: Paint): Paint {
paint.color = android.graphics.Color.WHITE
paint.isAntiAlias = true
return paint
}
fun configurePath(width: Float, path: Path): Path {
path.lineTo((width / 2f), getHeight(width.toDouble()))
path.lineTo(width, 0F)
path.lineTo(0f, 0f)
return path
}
}
The get height function is Pythagoras' Theoremand will always find the height of an equilateral triangle to be ~87% of its side length
获取高度函数是毕达哥拉斯定理，并且总是会发现等边三角形的高度是其边长的 87%
Gistcan be found here, it contains code for the other direction
要点可以在这里找到，它包含另一个方向的代码
回答by Jarvis
I recently created a small demo app drawing various shapes, including triangles, rectangles and spirals. Here is the code for drawing the triangles. For full context, reference the project. In this app, the user can draw a triangle as a single drawing or a composition of two triangles forming into one as the user scrolls. The increasing triangle growth is accomplished through the principles of Triangle Similarity, with each smaller triangle similar to the large triangle in the end.
我最近创建了一个小型演示应用程序，可以绘制各种形状，包括三角形、矩形和螺旋形。这是绘制三角形的代码。有关完整上下文，请参考该项目。在此应用程序中，用户可以将一个三角形绘制为单个图形，也可以在用户滚动时将两个三角形组合成一个。三角形的增长是通过三角形相似的原则来实现的，每个小三角形最终都与大三角形相似。
Project: https://github.com/jdgreene2008/android_custom_views
项目：https: //github.com/jdgreene2008/android_custom_views
源代码摘录：https: //github.com/jdgreene2008/android_custom_views/blob/master/CustomUiComponents/app/src/main/java/com/jarvis/dragdropresearch/views/FlashShapeView.java
private void drawTriangleShape(Canvas canvas, RectF bounds,
TriangleInterpolator triangleInterpolator, Paint paint) {
paint.setStyle(Paint.Style.FILL);
float baseInterpolation = triangleInterpolator
.getInterpolatedValues()[TriangleInterpolator.INTERPOLATION_VALUES_BASE];
float altitudeInterpolation = triangleInterpolator
.getInterpolatedValues()[TriangleInterpolator.INTERPOLATION_VALUES_ALTITUDE];
// *** Construct the Left Triangle ** //
// Bottom left vertex
float bottomLeftX = bounds.left;
float bottomLeftY = bounds.bottom;
// Bottom right corner
float bottomRightX = bottomLeftX + baseInterpolation;
float bottomRightY = bounds.bottom;
//Top Vertex
float topX = bottomRightX;
float topY = bottomRightY - altitudeInterpolation;
Path leftTriangle = new Path();
leftTriangle.lineTo(bottomLeftX, bottomLeftY);
leftTriangle.lineTo(bottomRightX, bottomRightY);
leftTriangle.lineTo(topX, topY);
leftTriangle.lineTo(bottomLeftX, bottomLeftY);
canvas.drawPath(leftTriangle, paint);
if (triangleInterpolator.isSymmetric()) {
// *** Construct the Right Triangle ** //
bottomLeftX = bounds.right - baseInterpolation;
bottomLeftY = bounds.bottom;
bottomRightX = bounds.right;
bottomRightY = bounds.bottom;
topX = bottomLeftX;
topY = bottomRightY - altitudeInterpolation;
Path rightTriangle = new Path();
rightTriangle.lineTo(bottomLeftX, bottomLeftY);
rightTriangle.lineTo(bottomRightX, bottomRightY);
rightTriangle.lineTo(topX, topY);
rightTriangle.lineTo(bottomLeftX, bottomLeftY);
canvas.drawPath(rightTriangle, paint);
}
}
回答by Yogendra
回答by Bismeet Singh
I would like to point out that you should never initiialize an object from onDraw() as it gets called multiple times and leads to performance problems.
我想指出的是，永远不要从 onDraw() 初始化对象，因为它会被多次调用并导致性能问题。
