在BufferedImage中使用setRGB（）方法

Question

I can sucessfully read and write values to an image file which accurately shows an image created.

I simply read the values using getRGB(), and then bit shift them into red, green and blue arrays respectively. Then I simply set them back into another BufferedImage object using the setRGB() method.

Now, I am trying to alter the pixel values, say the very first pixel of the red array. I then print out the first 5 pixels of the red array and the first value is changed as expected before invoking the setRGB() method, but when I read in that image again the first value is now back to its original value?

Does anybody know that using the setRGB() only changes the values in memory, but does not actually write that altered values?

EDITS - THIS IS A SAMPLE REPRESENTATION OF MY CODE (this works perfectly due to getting back an image)

//READ IN IMAGE
BufferedImage imgBuf =null;
imgBuf = ImageIO.read(new File("test.jpg"));

int w = imgBuf.getWidth();
int h = imgBuf.getHeight();
int[] RGBarray = imgBuf.getRGB(0,0,w,h,null,0,w);

//BIT SHIFT VALUES INTO ARRAY
for(int row=0; row<h; row++)
      {
         for(int col=0; col<w; col++)
         {
            alphaPixels[row][col] = ((RGBarray[g]>>24)&0xff);
            redPixels[row][col] = ((RGBarray[g]>>16)&0xff);
            greenPixels[row][col] = ((RGBarray[g]>>8)&0xff);
            bluePixels[row][col] = (RGBarray[g]&0xff);
            g++;
         }
      }

//BIT SHIFT VALUES BACK TO INTEGERS
for(int row=0; row<h; row++)
{
    for(int col=0; col<w; col++)
    {
        int rgb = (alphaPixels[row][col] & 0xff) << 24 | (redPixels[row][col] & 0xff) << 16 | (greenPixels[row][col] & 0xff) << 8 | (bluePixels[row][col] & 0xff);
        imgBuf.setRGB(col, row, rgb);
     }
}

//WRITE IMAGE BACK OUT
ImageIO.write(imgBuf, "jpeg", new File("new-test.jpg"));

Answer 1

Write where? If you change the RGB value of the BufferedImage's raster, then yes the memory value is written to and changed. If you mean does it change it to disk? No, not unless you write the image yourself to disk, often with ImageIO.write(...) . Changes to the memory representation of disk data will not mathemagically change the disk representation on it's own; instead you have to explicitly do this with your code. I think that you may be missing this last important step.

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Edit
You state in comment:

Currently I can write to an image created on disk with a new name. So if that works, then surely changing a few values should be the same effect? (Using setRBG() )

I'm still not clear on this. Say for instance:

• If you have an image on disk, say imageA.jpg,
• and say you read this into a BufferedImage via ImageIO.read(...) , say into the bufferedImageA variable,
• and then you change the data raster via setRGB(...)
• and then write your changed BufferedImage to disk with ImageIO.write(...) , say to a new file, imageB.jpg,
• Then if you read in imageB.jpg, it should show the changes made.
• But if you re-read in the unchanged imageA.jpg file, it will remain unchanged.