读取.bmp映像，并从映像的第10个字节中减去10，然后用Java重新创建映像

Question

I am creating an application which will read image byte/pixel/data from an .bmp image and store it in an byte/char/int/etc array.

Now, from this array, I want to subtract 10 (in decimal) from the data stored in the 10th index of an array.

I am able to successfully store the image information in the array created. But when I try to write the array information back to .bmp image, the image created is not viewable.

This is the piece of code which I tried to do so.

In this code, I am not subtracting 10 from the 10th index of an array .

public class Test1 {

    public static void main(String[] args) throws IOException{
        File inputFile = new File("d://test.bmp");
        FileReader inputStream = new FileReader("d://test.bmp");
        FileOutputStream outputStream = new FileOutputStream("d://test1.bmp");
        /*
         * Create byte array large enough to hold the content of the file.
         * Use File.length to determine size of the file in bytes.
         */
        char fileContent[] = new char[(int)inputFile.length()];


        for(int i = 0; i < (int)inputFile.length(); i++){
            fileContent[i] = (char) inputStream.read();
        }

        for(int i = 0; i < (int)inputFile.length(); i++){
            outputStream.write(fileContent[i]);
        }
    }
}

Answer 1

Instead of char[], use byte[]

Here's a modified version if your code which works:

public class Test {

public static void main(String[] args) throws IOException {
    File inputFile = new File("someinputfile.bmp");
    FileOutputStream outputStream = new FileOutputStream("outputfile.bmp");
    /*
     * Create byte array large enough to hold the content of the file.
     * Use File.length to determine size of the file in bytes.
     */
    byte fileContent[] = new byte[(int)inputFile.length()];


    new FileInputStream(inputFile).read(fileContent);

    for(int i = 0; i < (int)inputFile.length(); i++){
        outputStream.write(fileContent[i]);
    }

    outputStream.close();
 }
}

Answer 2

To make your existing code work you should replace the FileReader with a FileInputStream. According to the FileReader javadoc:

FileReader is meant for reading streams of characters. For reading streams of raw bytes, consider using a FileInputStream.

Modifying your sample as below

  public static void main(String[] args) throws IOException
 {
File inputFile = new File("d://test.bmp");
FileInputStream inputStream = new FileInputStream("d://test.bmp");
FileOutputStream outputStream = new FileOutputStream("d://test1.bmp");
/*
 * Create byte array large enough to hold the content of the file.
 * Use File.length to determine size of the file in bytes.
 */
byte fileContent[] = new byte[(int)inputFile.length()];

for(int i = 0; i < (int)inputFile.length(); i++){
    fileContent[i] =  (byte) inputStream.read();
}

inputStream.close();
for(int i = 0; i < (int)inputFile.length(); i++){
    outputStream.write(fileContent[i]);
}

outputStream.flush();
outputStream.close();

}

This work for me to create a copy of the original image.

Though as mentioned in the comments above this is probably not the correct approach for what you are trying to achieve.

Answer 3

Other have pointed you at errors in your code (using char instead of byte mostly), however, even if you fix that, you probably will end up with a non-loadable image if you change the value of the 10th byte in the file.

This is because, a .bmp image file starts with an header containing information about the file (color depth, dimensions, ... see BMP file format ) before any actual image data. Specifically, the 10th byte is part of a 4 byte integer storing the offset of the actual image data (pixel array). So subtracting 10 from this value will probably make the offset pointing at the wrong point in the file, and your image loader doing bound checking will probably consider this invalid.

What you really want to do is load the image as an image and manipulate the pixel values directly. Something like that:

BufferedImage originalImage = ImageIO.read(new File("d://test.bmp"));
int rgb = originalImage.getRGB(10, 0);
originalImage.setRGB(rgb >= 10 ? rgb - 10 : 0);
ImageIO.write(originalImage, "bmp", new File("d://test1.bmp"));