如何用alpha绘画？

Question

我想在Canvas上绘制图形,使颜色成为可加.例如,我想生成这个:

但相反,我得到了这个:

请注意,半白色,半黑色背景是有意的,只是为了看看alpha如何与两个背景相互作用.我很乐意在任何一个背景下工作.这是我的代码:

public class VennColorsActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        class VennView extends View {
            public VennView(Context context) {
                super(context);
            }

            @Override
            protected void onDraw(Canvas canvas) {
                super.onDraw(canvas);
                int alpha = 60, val = 255;
                int ar = Color.argb(alpha, val, 0, 0);
                int ag = Color.argb(alpha, 0, val, 0);
                int ab = Color.argb(alpha, 0, 0, val);
                float w = canvas.getWidth();
                float h = canvas.getHeight();
                float cx = w / 2f;
                float cy = h / 2;
                float r = w / 5;
                float tx = (float) (r * Math.cos(30 * Math.PI / 180));
                float ty = (float) (r * Math.sin(30 * Math.PI / 180));
                float expand = 1.5f;
                Paint paint = new Paint();
                paint.setColor(Color.WHITE);
                canvas.drawRect(new Rect(0, 0, (int) w, (int) (h / 2)), paint);
                PorterDuff.Mode mode = android.graphics.PorterDuff.Mode.ADD;
                paint = new Paint(Paint.ANTI_ALIAS_FLAG);
                paint.setColorFilter(new PorterDuffColorFilter(ar, mode));
                paint.setColor(ar);
                canvas.drawCircle(cx, cy - r, expand * r, paint);
                paint.setColorFilter(new PorterDuffColorFilter(ag, mode));
                paint.setColor(ag);
                canvas.drawCircle(cx - tx, cy + ty, expand * r, paint);
                paint.setColorFilter(new PorterDuffColorFilter(ab, mode));
                paint.setColor(ab);
                canvas.drawCircle(cx + tx, cy + ty, expand * r, paint);
            }
        }
        this.setContentView(new VennView(this));
    }
}

有人可以帮我理解如何在Android图形中添加添加颜色吗？

Answer 1

你走在正确的轨道上.您的代码中有3个主要问题:

• 您需要设置xfer模式iso颜色过滤器
• 使用临时位图渲染图像
• Alpha应该是0xFF才能获得您想要的结果

这是我使用xfer模式得到的.我正在做的是 - 将所有内容绘制到临时位图中,然后将整个位图渲染到主画布.

你问为什么需要临时位图？好问题!如果您在主画布上绘制所有内容,您的颜色将与主画布背景颜色混合,因此所有颜色都会混乱.透明的临时位图有助于保持您的颜色远离UI的其他部分

请确保您没有分配任何内容onDraw()- 您将很快以这种方式耗尽内存.还要确保在不再需要时回收了临时位图.

package com.example.stack2;

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.PorterDuff.Mode;
import android.graphics.PorterDuffXfermode;
import android.os.Bundle;
import android.view.View;

public class YouAreWelcome extends Activity {

    Bitmap tempBmp = Bitmap.createBitmap(1, 1, Config.ARGB_8888);
    Canvas c = new Canvas();
    Paint paint = new Paint();

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        class VennView extends View {
            public VennView(Context context) {
                super(context);

            }

            protected void onDraw(Canvas canvas) {
                super.onDraw(canvas);
                if(tempBmp.isRecycled() || tempBmp.getWidth()!=canvas.getWidth() || tempBmp.getHeight()!=canvas.getHeight())
                {
                    tempBmp.recycle();
                    tempBmp = Bitmap.createBitmap(canvas.getWidth(), canvas.getHeight(), Config.ARGB_8888);
                    c.setBitmap(tempBmp);
                }

                    //clear previous drawings
                c.drawColor(Color.TRANSPARENT, Mode.CLEAR);

                int alpha = 255, val = 255;
                int ar = Color.argb(alpha, val, 0, 0);
                int ag = Color.argb(alpha, 0, val, 0);
                int ab = Color.argb(alpha, 0, 0, val);
                float w = canvas.getWidth();
                float h = canvas.getHeight();
                float cx = w / 2f;
                float cy = h / 2;
                float r = w / 5;
                float tx = (float) (r * Math.cos(30 * Math.PI / 180));
                float ty = (float) (r * Math.sin(30 * Math.PI / 180));
                float expand = 1.5f;
                paint.setAntiAlias(true);
                paint.setXfermode(new PorterDuffXfermode(Mode.ADD));
                paint.setColor(ar);
                c.drawCircle(cx, cy - r, expand * r, paint);
                paint.setColor(ag);
                c.drawCircle(cx - tx, cy + ty, expand * r, paint);
                paint.setColor(ab);
                c.drawCircle(cx + tx, cy + ty, expand * r, paint);
                canvas.drawBitmap(tempBmp, 0, 0, null);
            }
        }
        this.setContentView(new VennView(this));
    }
}