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Thread: Reading pgm file

  1. #1
    Member
    Join Date
    Jan 2012
    Location
    Hyderabad, Andhra Pradesh, India
    Posts
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    Default Reading pgm file

    I need to read a pgm image file. The first few lines contain information like height, width, magic number and maxval. The remaining lines contain bytes which represent the image. Each of those bytes represent a nunber between 0 and Maxval. Lines starting with # may be comments.
    Here is the file: https://www.dropbox.com/s/nknndksfsx51mtl/brain_001.pgm
    Here is my code:
    import java.io.FileInputStream;
    import java.io.FileNotFoundException;
    import java.io.IOException;
     
    public class Problem1 {
     
        public static void main(String[] args) throws FileNotFoundException, IOException {
            FileInputStream f = new FileInputStream("C:\\Users\\srgf\\study related\\data structures\\Lab\\brain_001.pgm");
            boolean wasComment = false;
            byte b;
            while ((b = (byte) f.read()) != -1) {
                if (b == '\n') {
                    System.out.println("<newLine>");
                    wasComment = false;
                    continue;
                }
                if (b == '#') {
                    System.out.println("#");
                    wasComment = true;
                    continue;
                }
                if (wasComment) {
                    continue;
                }
                if (Character.isWhitespace(b)) {
                    System.out.print("<Whitespace>");
                    continue;
                }
                System.out.print(b + " ");
            }
    }
    }

    The problem is that in the output, I am getting a lot of the values as negative. See a part of the output below (towards the end) :
                     80 53 <newLine>
                     50 53 56 <Whitespace>50 53 56 <newLine>
                     50 53 53 <newLine>
                     0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0                                                       0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 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0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 5 8 14 19 24 <Whitespace>33 37 39 41 44 45 46 50 53 54 52 51 49 48 49 49 50 48 43 40 36 <Whitespace>26 23 19 15 <Whitespace>7 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 <Whitespace>17 23 <Whitespace>33 41 48 51 51 52 55 57 58 60 63 67 69 73 75 74 71 68 68 68 70 72 70 67 63 59 55 52 50 48 41 #
                     #
                     #
                     <newLine>
                     15 19 24 #
                     <newLine>
                     5 2 2 3 3 6 <Whitespace><Whitespace><Whitespace>15 19 21 21 22 23 17 8 5 6 8 8 8 <Whitespace><newLine>
    <Whitespace>18 27 44 62 79 98 112 -123 -92 -83 -98 -106 -104 -118 118 102 86 75 62 50 39 <Whitespace>19 8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 15 26 41 57 72 85 96 111 121 -116 -94 -90 -98 -109 -108 -105 126 94 71 45 27 19 <Whitespace>6 6 <Whitespace><newLine>
                     <Whitespace><Whitespace>14 15 17 17 19 21 24 26


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Reading pgm file

    problem is that in the output, I am getting a lot of the values as negative.
    What is the value of Maxval? byte variables are signed so the max (positive) value a byte can hold is 127 (0x7F) If the high order bit of the byte is a 1, the value of the byte will be negative.
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: Reading pgm file

    Quote Originally Posted by Norm View Post
    What is the value of Maxval? byte variables are signed so the max (positive) value a byte can hold is 127 (0x7F) If the high order bit of the byte is a 1, the value of the byte will be negative.
    I have posted a clearer version of this question here: Reading a pgm file in Java - Stack Overflow
    The maxVal came out to be 255.


    so the max (positive) value a byte can hold is 127

    The question mentions that each gray value can be a number between 0 and maxVal (in this case, 256). Each gray value is represented in bure binary by 1 byte.
    I have modified the program to convert the signed byte into an unsigned byte using the following expression:
    byte mybyte = -104;
    int mynewint = mybyte & 0xFF;
    Here is the new code:
     
    import java.io.DataInputStream;
    import java.io.EOFException;
    import java.io.FileInputStream;
    import java.io.FileNotFoundException;
    import java.io.IOException;
    import java.util.Scanner;
     
    public class P1St {
     
        public static void main(String args[]) throws Exception {
            FileInputStream f = new FileInputStream("C:\\Users\\srgf\\study related\\data structures\\Lab\\brain_001.pgm");
            DataInputStream d = new DataInputStream(f);
            d.readLine();//first line contains P5
            String line = d.readLine();//second line contains height and width
            Scanner s = new Scanner(line);
            int width = s.nextInt();
            int height = s.nextInt();
            line = d.readLine();//third line contains maxVal
            s = new Scanner(line);
            int maxVal = s.nextInt();
            int[][] im = new int[height][width];
            for (int i = 0; i < 258; i++) {
                for (int j = 0; j < 258; j++) {
                    im[i][j] = -1;
                }
            }
            int count = 0;
            byte b;
            try {
                while (true) {
                    b = (byte) (d.readUnsignedByte());
                    int newB=(int) (b & 0xFF);
                    if (b == '\n') {
                    } else if (b == '#') {
                        d.readLine();
                    } else if (Character.isWhitespace(b)) {
                    } else {
                        im[count / width][count % width] = newB;
                        count++;
                    }
                }
            } catch (EOFException e) {
            }
            System.out.println("Height=" + height);
            System.out.println("Width=" + height);
            System.out.println("Maxval=" +maxVal);
            System.out.println("Required elemnts=" + (height * width));
            System.out.println("Obtained elemnets=" + count);
            for(int i=0;i<im.length;i++){
                for(int j=0;j<im[i].length;j++)
                    System.out.printf("%4d",im[i][j]);
                System.out.println();
            }
        }
    }
    Now, I don't get any negative numbers, but still the array is not getting filled to the end.
    I'm not sure but I think that the problem is with whitespaces, new lines and #. How should I interpret them, as signed or unsigned?
    b = (byte) (d.readUnsignedByte());
                    if (b == '\n') {

    or
    b = (byte) (d.readUnsignedByte());
                  int newInt= mybyte & 0xFF;
                    if (b == '\n') {

  4. #4
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    Default Re: Reading pgm file

    the array is not getting filled to the end.
    Are any of the values of the bytes the same as the newline('\n') character's value?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Reading pgm file

    Adding the following code
    while (true) {
                    b = (byte) (d.readUnsignedByte());
     
                    if (b == '\n') {
                        System.out.println(" new line");
                    }
    I see that there are 53 new line characters.

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    Default Re: Reading pgm file

    Why are there newline characters mixed in with the bytes that can contain any value from 0 to 255?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Reading pgm file

    This is an assignment given to us. The question states that the file may contain whitespace characters along with the image data. The file is an actual pgm file. I opened it with a pgm file viewer and it was displayed correctly. It is the picture of a human brain.

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    Default Re: Reading pgm file

    How can you tell difference between a byte with image data and a byte with a whitespace,
    If the image data values can be from 0 to 255?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Reading pgm file

    I understood what you are asking but even I'm not sure of it. The same thing with whitespace should be true for '#' also. When I opened the file using notepad, I could find some '#' characters. So, the file probably contains whitespace also.

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    Default Re: Reading pgm file


  11. #11
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    Default Re: Reading pgm file

    If the data can include any byte values, I don't see how you can tell when a byte is data or when a byte is whitespace.
    Are you sure the data bytes have values from 0 to 255?
    Or is the data ASCII characters?
    If you don't understand my answer, don't ignore it, ask a question.

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