How to read a image in java and turn it to byte array?[use Lena.bmp 512x512 as example] -

i want analysis pixel of lena.bmp (matlab) don't know how in java , store byte[]

i have read aritle:

how convert image byte array in java?

but when implement, find some pixel value did't exist. example , pixel range 0~255, can not find '120' on pixel of photo(lena.bmp).

there code

/*  * change template, choose tools | templates  * , open template in editor.  */ package image_io;  import java.awt.image.bufferedimage; import java.io.bytearrayoutputstream; import java.io.file; import java.io.ioexception; import javax.imageio.imageio;  /**  *  * @author user  */ public class readimage {     public static void main (string []args){         byte[] imageinbyte;         int [] kindofpixel = new int [256];         try{             bufferedimage originalimage = imageio.read(new file("c:\\users\\user\\desktop\\project\\lsb_implement\\lena.bmp"));             bytearrayoutputstream baos = new bytearrayoutputstream();             imageio.write( originalimage, "bmp", baos );             baos.flush();             imageinbyte = baos.tobytearray();             system.out.println(imageinbyte.length);             baos.close();              for(int i=0;i<imageinbyte.length;i++){                 kindofpixel[imageinbyte[i]+128]++;  //java byte defined -128~127  (+128)==> 0~256             }              for(int i=0;i<kindofpixel.length;i++){  //show pixel color value                 system.out.println(i+" , "+kindofpixel[i]);             }          }catch(ioexception e){             system.out.println(e.getmessage());         }      }     } 

i compare information histogram of lena.bmp , seems have different...

first on java byte

you aware signed java byte ranges -128 127. erred using 128 instead of calculating modulo 256:

that comes down to:

            kindofpixel[imageinbyte[i] & 0xff]++; 

to for:

• an unsigned byte 120: < 127 hence 120,
• an unsigned byte 130: >= 127 hence 130 - 256 = -126.

modulo 256 calculation in other direction goes:

byte signed = -126; int unsigned = signed < 0 ? signed + 256 : signed; int unsigned = signed & 0xff; // or 

the pixels

a bmp file not (only) linearized list of pixels. 1 better use int[] bufferedinage.getrgb:

int[] pixels = originalimage.getrgb(0, 0, width, height, null, 0, width); 

remains color representation

bmp knows many variants: rgb, indexed etcetera. indexed colors need lookup in palette.