使用Matrix.setPolyToPoly在位图上选择4个点的区域 [英] Select an area on bitmap with 4 points using Matrix.setPolyToPoly

问题描述

我正在Android上使用位图，并且使用4点在位图上选择区域时遇到了问题.并非所有的4分集都对我有用.在某些情况下，结果只是空白位图，而不是裁剪后的位图(如图片中所示)，并且logcat中没有任何错误(甚至是内存错误). 这是我用来进行转换的基本代码.

I am playing with bitmap on Android and I am facing an issue when selecting an area on the bitmap using the 4 points. Not all the sets of 4 points work for me. In some cases, the result is just a blank bitmap instead of the cropped bitmap (like in the picture) and there is not any error in logcat(even memory error). Here is the basic code I used to do the transformation.

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.widget.ImageView;

public class CropImageActivity extends Activity {
private ImageView mCroppedImageView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.crop);
    setupViews();
    doCropping();
}

private void doCropping() {
    Bitmap srcBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.sample);

    //target size
    int bitmapWidth = 400;
    int bitmapHeight = 400;

    Bitmap bitmap = Bitmap.createBitmap(bitmapWidth, bitmapHeight, Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(bitmap);

    //This is one of bad quadangle.
    points[0] = 0;    //top-left.x
    points[1] = 0;    //top-left.y
    points[2] = 230;  //top-right.x
    points[3] = 100;  //top-right.y
    points[4] = 350;  //bottom-right.x
    points[5] = 350;  //bottom-right.y
    points[6] = 0;    //bottom-left.x
    points[7] = 350;  //bottom-left.y

    float[] src = new float[]{
            points[0], points[1],
            points[2], points[3],
            points[4], points[5],
            points[6], points[7]
    };
    float[] dsc = new float[]{
            0, 0,
            bitmapWidth, 0,
            bitmapWidth, bitmapHeight,
            0, bitmapHeight
    };

    Matrix matrix = new Matrix();
    boolean transformResult = matrix.setPolyToPoly(src, 0, dsc, 0, 4);

    canvas.drawBitmap(srcBitmap, matrix, null);
    mCroppedImageView.setImageBitmap(bitmap);
}

private void setupViews() {
    mCroppedImageView = (ImageView) findViewById(R.id.croppedImageView);
}
}

那么，这4个点的坐标会影响画布绘制或矩阵变换吗?感谢您的帮助.

So, does the 4 points coordinates affect the canvas drawing or the matrix transformation? Any help is appreciated.

谢谢

推荐答案

最后，我使用OpenCV解决了我的问题.感谢您在这个问题中的回答！似乎Matrix.setPolyToPoly并不能在所有情况下都起作用.

Finally, I solve my issue using OpenCV. Thanks for the answer in this question! It seems that Matrix.setPolyToPoly does not work for all the cases.

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