从图像获取像素值时出错

Question

am working with an android project , in my project , i want to store the pixel values of an image into an array, i used getPixels() function for it and store it in an array named as pixels, but when i tried to print it in a TextView , am getting some negetive values sometinh like -1623534 and so on . Why it is like that. ?

Here is my code :-

Bitmap result = BitmapFactory.decodeFile(filePath);
        TextView resultText=(TextView)findViewById(R.id.txtResult);
        try
        {
            int pich=(int)result.getHeight();
            int picw=(int)result.getWidth();

            int[] pixels = new int[pich*picw];
            result.getPixels(pixels, 0, picw, 0, 0, picw, pich);


            //To convert  into String

            StringBuffer buff = new StringBuffer();
            for (int i = 0; i <100; i++) 
              {
                   // getting values from array
                 buff.append(pixels[i]).append("  ");
              }


             //To save the binary in newString

             String newString=new String(buff.toString());

            resultText.setText(newString);

And i found in some other post like

            int R, G, B;

            for (int y = 0; y < pich; y++)
            {
                for (int x = 0; x < picw; x++)
                {
                    int index = y * picw + x;
                    R = (pixels[index] >> 16) & 0xff;     //bitwise shifting
                    G = (pixels[index] >> 8) & 0xff;
                    B = pixels[index] & 0xff;

                    //R,G.B - Red, Green, Blue
                    //to restore the values after RGB modification, use 
                    //next statement
                    pixels[index] = 0xff000000 | (R << 16) |  (G << 8) | B;
                }
            }

So i modified my code as :-

Bitmap result = BitmapFactory.decodeFile(filePath);
        TextView resultText=(TextView)findViewById(R.id.txtResult);
        try
        {
            int pich=(int)result.getHeight();
            int picw=(int)result.getWidth();

            int[] pixels = new int[pich*picw];
            result.getPixels(pixels, 0, picw, 0, 0, picw, pich);

            int R, G, B;

            for (int y = 0; y < pich; y++)
            {
                for (int x = 0; x < picw; x++)
                {
                    int index = y * picw + x;
                    R = (pixels[index] >> 16) & 0xff;     //bitwise shifting
                    G = (pixels[index] >> 8) & 0xff;
                    B = pixels[index] & 0xff;

                    //R,G.B - Red, Green, Blue
                    //to restore the values after RGB modification, use 
                    //next statement
                    pixels[index] = 0xff000000 | (R << 16) | (G << 8) | B;
                }
            }

            //To convert  into String

            StringBuffer buff = new StringBuffer();
            for (int i = 0; i <100; i++) 
            {
                   // getting values from array
                 buff.append(pixels[i]).append("  ");
            }


             //To save the binary in newString

            String newString=new String(buff.toString());

            resultText.setText(newString);

Is it correct ? Am getting the negetive values even after the modification , pleasehelp me , Thanks in advance

Answer 1

The reason why you are getting negative values is the following: Each pixle of an image contains of 4 values(red, green, blue, alpha). Each value has 8 bit (one byte). All 4 together have exactly 32 bit, which is the size of an integer value. But when you print a (signed-)integer, the first bit is interpreted as sign-flag, so you can get negative values, if this bit is set to 1 (happens when the first channel is >= 128).

To get the RGB values from a pixel I normally use this:

int pixel = bmp.getPixel(x, y);
int red = Color.red(pixel);
int green = Color.green(pixel);
int blue = Color.blue(pixel);