如何在 Java 中读取 PGM 图像?

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时间:2020-10-30 02:46:54  来源:igfitidea点击:

How to read PGM images in Java?

javajavax.imageiopgm

提问by Magsol

I feel like I'm missing something simple here (as usual).

我觉得我在这里遗漏了一些简单的东西(像往常一样)。

I'm trying to read PGM images using Java. Matlab does it just fine - outputting the image pixels (for example, a small 32x32 image) in Matlab gives me something like this:

我正在尝试使用 Java 读取 PGM 图像。Matlab 做得很好 - 在 Matlab 中输出图像像素(例如,一个小的 32x32 图像)给了我这样的东西:

1 0 11 49 94 118 118 106 95 88 85 96 124 143 142 133

My Java reader, however, outputs this:

然而,我的 Java 阅读器输出如下:

1 0 11 49 94 118 118 106 95 88 85 96 124 65533 65533 65533 

It seems like pixel values above 127 are filled in with 65533, though it does get some random values incorrect, and even assigns almost the entire bottom row to the value of -1.

似乎 127 以上的像素值填充了 65533,尽管它确实得到了一些不正确的随机值,甚至将几乎整个底行分配给值 -1。

Here's the code I'm using:

这是我正在使用的代码:

filePath = 'imagepath.pgm';
FileInputStream fileInputStream = new FileInputStream(filePath);
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(fileInputStream));

// read the header information ...

int [][] data2D = new int [picWidth] [picHeight];

for (int row = 0; row < picHeight; row++) {
  for (int col = 0; col < picWidth; col++) {
    data2D[row][col] = bufferedReader.read();
    System.out.print(data2D[row][col] + " ");
  }
  System.out.println();
}

fileInputStream.close();

Any ideas would be greatly appreciated.

任何想法将不胜感激。

EditHere are the unsigned PGM values:

编辑这里是未签名的 PGM 值:

     1     0    11    49    94   118   118   106    95    88    85    96   124   143   142   133
    30    26    29    57    96   122   125   114   102    94    91   101   127   146   145   136
    96    85    70    75   101   128   136   126   111   106   106   112   131   149   153   147
   163   147   114    93    99   120   132   123   110   113   124   129   137   154   166   168
   215   195   149   105    88    99   114   111   106   123   148   158   160   174   191   197
   245   224   173   115    81    82   100   109   117   144   179   194   194   205   222   230
   235   217   170   115    78    78   113   117   100    83    80   212   214   226   244   253
   178   167   135    93    68    78   123   129   106    77    69   202   204   222   244   255
   114   110    92    64    54    81   107   105    83    59    56   182   184   201   222   231
    79    80    71    52    55    97    67    55    41    33    42   184   179   181   185   183
    62    66    65    52    63   115    29    16    12    17    30   209   197   174   150   132
    40    47    52    44    55   109   171   196   188   186   208   229   218   179   136   107
    31    38    44    37    43    89   145   167   158   159   191   223   219   179   133   105
    48    52    56    51    57    91   128   133   117   120   157   196   200   168   128   105
    64    67    70    73    87   114   127   107    79    81   118   159   173   154   123   104
    63    67    73    83   107   132   129    91    54    54    88   130   153   146   123   106

The header looks like this:

标题如下所示:

P5
# MatLab PGMWRITE file, saved 27-Jun-2002
16 16
255

Edit #2

编辑 #2

Here's the full output to the proof of concept code below:

以下是概念验证代码的完整输出:

Skipping unknow token: ""
Skipping unknow token: "1^vvj_XU`|???"
Skipping unknow token: ""
Skipping unknow token: "9`z}rf^[e???`UFKe??~ojjp??????r]cx?{nq|???????iXcroj{????????sQRdmu?????????sNNqudSP?????]DN{?jME?????rn\@6QkiS;8?????OPG47aC7)!*?????>BA4?s"
Skipping unknow token: ""
Skipping unknow token: ""
Skipping unknow token: "????(/4,7m???????k"
Skipping unknow token: "&,%+Y????????i04839[??ux????i@CFIWrkOQv???{h?CISk??[66X???{j"
Exception in thread "main" java.util.NoSuchElementException
    at java.util.Scanner.throwFor(Scanner.java:838)
    at java.util.Scanner.next(Scanner.java:1347)
    at Test.main(Test.java:49)

Line 49 referred to in the thrown exception is:

抛出的异常中引用的第 49 行是:

System.out.println(String.format("Skipping unknow token: \"%s\"", scan.next()));

The problem, I'm sure, has something to do with the fact that these image files consist of both ASCII text/numbers as well as binary image data. But if Java has no problem reading PNGs, why the lack of support for PGMs?

我敢肯定,这个问题与这些图像文件由 ASCII 文本/数字以及二进制图像数据组成的事实有关。但是如果 Java 读取 PNG 没有问题,为什么缺少对 PGM 的支持?

Edit 3

编辑 3

Ok, I found an implementation that works...unfortunately, it's deprecated:

好的,我找到了一个有效的实现......不幸的是,它已被弃用:

  filePath = "imagepath.pgm"
  FileInputStream fileInputStream = new FileInputStream(filePath);
  DataInputStream dis = new DataInputStream(fileInputStream);
  StreamTokenizer streamTokenizer = new StreamTokenizer(dis);

  // read header text using StreamTokenizer.nextToken()

  data2D = new int [picWidth] [picHeight];
  for (int row = 0; row < picHeight; row++) {
    for (int col = 0; col < picWidth; col++) {
      data2D[row][col] = dis.readUnsignedByte();
      System.out.print(data2D[row][col] + " ");
    }
    System.out.println();
  }

According to the Java documentation, the StreamTokenizer(InputStream)constructor is deprecated, because the DataInputStream.readLine()method does not correctly convert raw bytes to characters. However, it seems to work in this specific case on the header, and obviously works for the ensuing binary image data.

根据 Java 文档,StreamTokenizer(InputStream)不推荐使用构造函数,因为该DataInputStream.readLine()方法没有正确地将原始字节转换为字符。但是,它似乎适用于标题的这种特定情况,并且显然适用于随后的二进制图像数据。

Unfortunately, it's still deprecated, and it seems that by intermixing a BufferedReaderas the documentation suggests only results in EOFExceptions after reading the header and attempting to use the DataInputStreamto read the raw bytes. Still looking for a solution...

不幸的是,它仍然被弃用,并且似乎通过混合 aBufferedReader作为文档建议仅EOFException在读取标头并尝试使用DataInputStream读取原始字节后产生s 。仍在寻找解决方案...

回答by Giuseppe Cardone

The problem with your code is that you are using the wrong class to read raw data from the file. As the BufferedReaderdocumentation says:

您的代码的问题在于您使用错误的类从文件中读取原始数据。正如BufferedReader文档所说:

public int read() throws IOException

Reads a single character.

Returns: The character read, as an integer in the range 0 to 65535 (0x00-0xffff), or -1 if the end of the stream has been reached

public int read() throws IOException

读取单个字符。

返回: 读取的字符,为 0 到 65535 (0x00-0xffff) 范围内的整数,如果已到达流末尾,则返回 -1

So each call to the read()method of BufferedReaderactually consumes one or two bytes (based on character encoding) from the input stream, which is not what you want. This also explains why you get a lot of -1: the stream ended much earlier than you thought.

所以每次调用read()方法BufferedReader实际上从输入流中消耗一两个字节(基于字符编码),这不是你想要的。这也解释了为什么您会得到很多 -1:流结束的时间比您想象的要早得多。

Since PGM contains values as ASCII decimal, it is easy to parse using the Scannerclass.

由于 PGM 包含 ASCII 十进制值,因此使用Scanner类很容易解析。

Here's an almost untestedcode that shows how to read a PGM image assuming that:

这是一个几乎未经测试的代码,展示了如何读取 PGM 图像,假设:

  • it contains a single comment after the magic number (i.e. it does not have lines that start with a # except the second one)
  • the PGM file is exactly 4 lines long.
  • 它在幻数之后包含一个注释(即,除了第二个之外,它没有以 # 开头的行)
  • PGM 文件正好有 4 行长。

Here's the code:

这是代码:

String filePath = "image.pgm";
fileInputStream = new FileInputStream(filePath);
Scanner scan = new Scanner(fileInputStream);
// Discard the magic number
scan.nextLine();
// Discard the comment line
scan.nextLine();
// Read pic width, height and max value
int picWidth = scan.nextInt();
int picHeight = scan.nextInt();
int maxvalue = scan.nextInt();

fileInputStream.close();

 // Now parse the file as binary data
 fileInputStream = new FileInputStream(filePath);
 DataInputStream dis = new DataInputStream(fileInputStream);

 // look for 4 lines (i.e.: the header) and discard them
 int numnewlines = 4;
 while (numnewlines > 0) {
     char c;
     do {
         c = (char)(dis.readUnsignedByte());
     } while (c != '\n');
     numnewlines--;
 }

 // read the image data
 int[][] data2D = new int[picHeight][picWidth];
 for (int row = 0; row < picHeight; row++) {
     for (int col = 0; col < picWidth; col++) {
         data2D[row][col] = dis.readUnsignedByte();
         System.out.print(data2D[row][col] + " ");
     }
     System.out.println();
 }

Need to implement: support for comment lines, values for each element should be divided by maxvalue, error checking for malformed files, exception handling. I tested it on a PGM file using UNIX end-of-lines, but it should work on Windows too.

需要实现:支持注释行,每个元素的值应该被除以maxvalue,对格式错误的文件进行错误检查,异常处理。我使用 UNIX 行尾在 PGM 文件上对其进行了测试,但它也应该适用于 Windows。

Let me stress that this is not a robust nor complete implementation of a PGM parser. This code is intended just as proof of concept that maybe accomplishes just enough for your needs.

让我强调一下,这不是 PGM 解析器的健壮或完整实现。此代码旨在作为概念证明,可能足以满足您的需求。

If you really need a robust PGM parser, you may use the tools provided by Netpbm.

如果你真的需要一个强大的 PGM 解析器,你可以使用Netpbm提供的工具。