Convert colour images to grayscale images & get pixel data from them

Question

Is the code below correct to convert colour images to grayscale images in Java?
public void convertToGrayscale (String sourceName,String destName) throws Exception {
JPEGImageDecoder decoder=JPEGCodec.createJPEGDecoder(new FileInputStream(sourceName));
JPEGImageEncoder encoder=JPEGCodec.createJPEGEncoder(new FileOutputStream(destName));
BufferedImage sourceImg=decoder.decodeAsBufferedImage();
BufferedImageOp op =new ColorConvertOp(
          ColorSpace.getInstance(ColorSpace.CS_GRAY),null);
     BufferedImage destImg = op.filter(sourceImg,null);
encoder.encode(destImg);
decoder = null;
encoder = null;
When I get grayscale images from the code below, I would like to access the pixels of those images. So I tried to do:
byte[] dd=((DataBufferByte)mImage.getRaster().getDataBuffer()).getData();
BUT the data result array is not 0-255. Could anyone suggest how to obtain pixel data from grayscale images?
In case that my code shown is not correct or suitable, please give your advice. What I would like to do are in the steps as follows:
1 change 100*70 jpeg-images to 100*70 grayscale images.
2 create two dimensional array of pixel data (example [100][70]) from converted images. The number in the array should be between 0-255, right??? And 0 refers to black colour and 255 refers to white colour???
I am confused about grayscale images. Please help.
Thank you so much

Answer 1

I am not sure i understand what is the problem exactly.
Structure of DataBuffer is described by SampleModel used by same Raster
object. E.g. it might be 1 byte per xipex or 4 bytes per pixel.
In your example convertToGrayscale saves images to file as JPEG and
it seems you later read it back. It is possible what image you read back
is not greyscale but ARGB and data buffer has different format.
Technically, if you just need level of grey you may simply
call getRGB on your output image. Grey is uniform mix of R, G and B.
Also, instead of ColorConverOp you may dimply create output image of
grayscale type and draw your color input image with drawImage().
If none of these helps please try to provide more details.