实现橡皮擦在Android绘图应用程序 - 黑色的踪迹，然后透明 [英] Implementing an eraser in an Android drawing app - black trail and then transparent

问题描述

我有一个Android的应用程序绘制和我目前正在为一个真正的橡皮擦添加到它。之前，我刚刚用白色油漆的橡皮擦，但不会做了，因为现在我让背景颜色和图像。我做到这一点，我有透明画布下面的示意图。

I have an drawing app for Android and I am currently trying to add a real eraser to it. Before, I had just used white paint for an eraser, but that won't do anymore since now I allow background colors and images. I do this by having an image view underneath my transparent canvas.

这是我现在面临的问题是，每当我让我的橡皮擦，它吸引了坚实的黑色踪迹，而我有我的手指下来，但一旦我放开去透明。请参阅下面的屏幕截图：

The problem that I am facing is that whenever I enable my eraser, it draws a solid black trail while I have my finger down, but once I release it goes to transparent. See the screen shots below:

这是它的外观，而我的手指在屏幕上 - 一个纯黑色的线索

This is how it looks while my finger is on the screen - a solid black trail

这是什么样子的，一旦我删除了我的手指从屏幕上

This is what it looks like once I remove my finger from the screen

所以，好像我越来越近，但我无法找到设置的正确组合，以避免黑色的踪迹，而我的手指触摸，而擦除。这里有一些相关的code片段：

So, it seems like I am getting close, but I can't find the right combination of settings to avoid the black trail while my finger is touching while erasing. Here are some relevant code snippets:

的OnDraw

@Override
protected void onDraw(Canvas canvas) {
    canvas.drawColor(Color.TRANSPARENT);
    canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
    canvas.drawPath(mPath, mPaint);
    canvas.drawPath(mPreviewPath, mPaint);
}

的onTouchEvent

@Override
public boolean onTouchEvent(MotionEvent event) {
    float currentX = event.getX();
    float currentY = event.getY();

    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            touchStart(currentX, currentY);
            invalidate();
            break;
        case MotionEvent.ACTION_MOVE:
            touchMove(currentX, currentY);
            invalidate();
            break;
        case MotionEvent.ACTION_UP:
            touchUp(currentX, currentY);
            invalidate();
            break;
    }
    return true;
}

当前尝试橡皮擦设置

public void startEraser() {
    mPaint.setAlpha(0);
    mColor = Color.TRANSPARENT;
    mPaint.setColor(Color.TRANSPARENT);
    mPaint.setStrokeWidth(mBrushSize);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setMaskFilter(null);
    mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
    mPaint.setAntiAlias(true);
}

有大约橡皮擦几个其他职位，但他们大多只是说使用 PorterDuff.Mode.CLEAR ， setMakFilter（空）并应工作。就我而言，事实并非如此。不管我怎么努力，我先升后得到的黑色踪迹期望的结果我释放之后。

There are several other posts about erasers, but most of them just say to use PorterDuff.Mode.CLEAR, setMakFilter(null) and that that should work. In my case, it doesn't. No matter what I try, I get the black trail first and then the desired result only after I release.

我可以在必要时提供更多的code。

I can provide more code if necessary.

推荐答案

我可以建议你阅读<一个官方样片href="https://android.googlesource.com/platform/development/+/master/samples/ApiDemos/src/com/example/android/apis/graphics/FingerPaint.java">FingerPaint.java
它完全匹配您正试图在这里实现什么。

I could suggest you to read the official sample of FingerPaint.java
It exactly matches what you are trying to achieve here.

不显示，当你删除的内容，一起来看看在的OnDraw（）方法和 eraserMode 变量：

To not show the trail when you erase content, take a look at the onDraw() method and the eraserMode variable:

@Override
protected void onDraw(Canvas canvas) {
    canvas.drawColor(0xFFAAAAAA);
    canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
    if (!eraserMode) {
        canvas.drawPath(mPath, mPaint);
    }
}

boolean eraserMode = false;

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    eraserMode = false;
    mPaint.setXfermode(null);
    mPaint.setAlpha(0xFF);
    switch (item.getItemId()) {
        /*...*/
        case ERASE_MENU_ID:
            // Add this line
            eraserMode = true;
            mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
            return true;
        /*...*/
    }
    return super.onOptionsItemSelected(item);
}

这篇关于实现橡皮擦在Android绘图应用程序 - 黑色的踪迹，然后透明的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！