Android下正确获取屏幕触摸点下的图片像素值


有的时候我们可能需要获取屏幕上触碰点下的图片像素值，比如我们在ImageView控件中显示了一张图片，当我们点击图片的某位置时想要获取这个点击位置的像素，最先想到的是使用Bitmap.getPixel(x, y)函数，但是需要注意的是，这个函数的坐标是以原始图片为基准的。通常我们获取屏幕触碰点的坐标是以屏幕为基准的，这就需要对坐标进行转换，尤其是当图片有进行拉伸的时候，如果不进行坐标转换，获取的图片像素是不准确的。

由于本人十分的懒惰，实在不想自己写一篇文章来阐述如何转换坐标，不过我偶然发现了这篇文章正好满足我想说明的问题，就直接粘贴过来了。

Get touched pixel color of scaled ImageView

To get touched pixel color of a ImageView, you can implement OnTouchListener of the ImageView, get touched position by calling event.getX() and event.getY(), and get the pixel color by calling bitmap.getPixel(x, y)...IF THE BITMAP IN YOUR ImageView
IS DISPLAY AT 1:1, layout_width and layout_height ="wrap_content".

If your bitmap is scaled, such as layout_width and layout_height ="match_parent". It will not work! It's a example demonstrate how to invert the touched position to get pixel from scaled ImageView.

package com.example.androidtouchedpixel;

import android.os.Bundle;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.Matrix;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.widget.ImageView;
import android.widget.TextView;

public class MainActivity extends Activity
{
TextView touchedXY, invertedXY, imgSize, colorRGB;
ImageView imgSource1, imgSource2;

@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

touchedXY = (TextView)findViewById(R.id.xy);
invertedXY = (TextView)findViewById(R.id.invertedxy);
imgSize = (TextView)findViewById(R.id.size);
colorRGB = (TextView)findViewById(R.id.colorrgb);
imgSource1 = (ImageView)findViewById(R.id.source1);
imgSource2 = (ImageView)findViewById(R.id.source2);

imgSource1.setOnTouchListener(imgSourceOnTouchListener);
imgSource2.setOnTouchListener(imgSourceOnTouchListener);

}

OnTouchListener imgSourceOnTouchListener = new OnTouchListener()
{

@Override
public boolean onTouch(View view, MotionEvent event)
{
float eventX = event.getX();
float eventY = event.getY();
float[] eventXY = new float[] {eventX, eventY};

Matrix invertMatrix = new Matrix();
((ImageView)view).getImageMatrix().invert(invertMatrix);

invertMatrix.mapPoints(eventXY);
int x = Integer.valueOf((int)eventXY[0]);
int y = Integer.valueOf((int)eventXY[1]);

touchedXY.setText(
"touched position: "
+ String.valueOf(eventX) + " / "
+ String.valueOf(eventY));

invertedXY.setText(
"touched position: "
+ String.valueOf(x) + " / "
+ String.valueOf(y));

Drawable imgDrawable = ((ImageView)view).getDrawable();
Bitmap bitmap = ((BitmapDrawable)imgDrawable).getBitmap();

imgSize.setText(
"drawable size: "
+ String.valueOf(bitmap.getWidth()) + " / "
+ String.valueOf(bitmap.getHeight()));

//Limit x, y range within bitmap
if(x < 0)
{
x = 0;
}
else if(x > bitmap.getWidth()-1)
{
x = bitmap.getWidth()-1;
}

if(y < 0)
{
y = 0;
}
else if(y > bitmap.getHeight()-1)
{
y = bitmap.getHeight()-1;
}

int touchedRGB = bitmap.getPixel(x, y);

colorRGB.setText("touched color: " + "#" + Integer.toHexString(touchedRGB));
colorRGB.setTextColor(touchedRGB);

return true;
}
};
}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/hello_world"
tools:context=".MainActivity" />
<TextView
android:id="@+id/xy"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="touched position: "/>
<TextView
android:id="@+id/invertedxy"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="inverted touched position: "/>
<TextView
android:id="@+id/size"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="drawable size: "/>
<TextView
android:id="@+id/colorrgb"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="touched color: "/>
<ImageView
android:id="@+id/source1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/ic_launcher"/>
<ImageView
android:id="@+id/source2"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:src="@drawable/ic_launcher"/>

</LinearLayout>