从 png 文件中获取文本字段

Question

到目前为止似乎没有任何效果。我通过pnginfo 看到了以下消息：

concept_Sjet_dream6.png...
  Image Width: 200 Image Length: 240
  Bitdepth (Bits/Sample): 8
  Channels (Samples/Pixel): 2
  Pixel depth (Pixel Depth): 16
  Colour Type (Photometric Interpretation): GRAYSCALE with alpha channel 
  Image filter: Single row per byte filter 
  Interlacing: No interlacing 
  Compression Scheme: Deflate method 8, 32k window
  Resolution: 11811, 11811 (pixels per meter)
  FillOrder: msb-to-lsb
  Byte Order: Network (Big Endian)
  Number of text strings: 1 of 9
    Comment (xTXt deflate compressed): The comment

但缺少剩余的文本字符串。我也在stackoverflow 中尝试了其他解决方案，但它们也不起作用。 pngchunks 提供了这些信息：

Chunk: Data Length 13 (max 2147483647), Type 1380206665 [IHDR]
  Critical, public, PNG 1.2 compliant, unsafe to copy
  IHDR Width: 200
  IHDR Height: 240
  IHDR Bitdepth: 8
  IHDR Colortype: 4
  IHDR Compression: 0
  IHDR Filter: 0
  IHDR Interlace: 0
  IHDR Compression algorithm is Deflate
  IHDR Filter method is type zero (None, Sub, Up, Average, Paeth)
  IHDR Interlacing is disabled
  Chunk CRC: -277290027
Chunk: Data Length 1 (max 2147483647), Type 1111970419 [sRGB]
  Ancillary, public, PNG 1.2 compliant, unsafe to copy
  ... Unknown chunk type
  Chunk CRC: -1362223895
Chunk: Data Length 2 (max 2147483647), Type 1145523042 [bKGD]
  Ancillary, public, PNG 1.2 compliant, unsafe to copy
  ... Unknown chunk type
  Chunk CRC: -2020619073
Chunk: Data Length 9 (max 2147483647), Type 1935231088 [pHYs]
  Ancillary, public, PNG 1.2 compliant, safe to copy
  ... Unknown chunk type
  Chunk CRC: 2024095606
Chunk: Data Length 7 (max 2147483647), Type 1162692980 [tIME]
  Ancillary, public, PNG 1.2 compliant, unsafe to copy
  ... Unknown chunk type
  Chunk CRC: 292503155
Chunk: Data Length 19 (max 2147483647), Type 1951942004 [tEXt]
  Ancillary, public, PNG 1.2 compliant, safe to copy
  ... Unknown chunk type
  Chunk CRC: -528748773
Chunk: Data Length 8192 (max 2147483647), Type 1413563465 [IDAT]
  Critical, public, PNG 1.2 compliant, unsafe to copy
  IDAT contains image data
  Chunk CRC: -309524018
Chunk: Data Length 8192 (max 2147483647), Type 1413563465 [IDAT]
  Critical, public, PNG 1.2 compliant, unsafe to copy
  IDAT contains image data
  Chunk CRC: -1646200198
Chunk: Data Length 2301 (max 2147483647), Type 1413563465 [IDAT]
  Critical, public, PNG 1.2 compliant, unsafe to copy
  IDAT contains image data
  Chunk CRC: -810299134
Chunk: Data Length 0 (max 2147483647), Type 1145980233 [IEND]
  Critical, public, PNG 1.2 compliant, unsafe to copy
  IEND contains no data
  Chunk CRC: -1371381630

示例图片：

我很困惑。谢谢大家。

PD：显然每张图片都会发生这种情况，这不是特殊情况，而是常见情况。

Answer 1

这是一种从“有效”png 文件中提取文本信息的简单 Java 方法。国际字符可能无法在控制台上很好地显示。

import java.io.*;
import java.util.zip.InflaterInputStream;

public class PNGExtractText
{  
  /** PNG signature constant */
  public static final long SIGNATURE = 0x89504E470D0A1A0AL;
  /** PNG Chunk type constants, 4 Critical chunks */
  /** Image header */
  private static final int IHDR = 0x49484452;   // "IHDR"
  /** Image trailer */
  private static final int IEND = 0x49454E44;   // "IEND"
  /** Palette */
  /** Textual data */
  private static final int tEXt = 0x74455874;   // "tEXt"
  /** Compressed textual data */
  private static final int zTXt = 0x7A545874;   // "zTXt"
  /** International textual data */
  private static final int iTXt = 0x69545874;   // "iTXt"
  /** Background color */

  public static void showText(InputStream is) throws Exception
  {
      //Local variables for reading chunks
      int data_len = 0;
      int chunk_type = 0;
      byte[] buf=null;

      long signature = readLong(is);

      if (signature != SIGNATURE)
      {
          System.out.println("--- NOT A PNG IMAGE ---");
          return;
      }

        /** Read header */
        /** We are expecting IHDR */
      if ((readInt(is)!=13)||(readInt(is) != IHDR))
      {
          System.out.println("--- NOT A PNG IMAGE ---");
          return;
      }

      buf = new byte[13+4];//13 plus 4 bytes CRC
      is.read(buf,0,17);

      while (true)
      {
          data_len = readInt(is);
          chunk_type = readInt(is);
          //System.out.println("chunk type: 0x"+Integer.toHexString(chunk_type));

          if (chunk_type == IEND)
          {
             System.out.println("IEND found");
             int crc = readInt(is);
             break;
          }

          switch (chunk_type)
          {
               case zTXt:
               {  
                   System.out.println("zTXt chunk:");
                   buf = new byte[data_len];
                   is.read(buf);
                   int keyword_len = 0;
                   while(buf[keyword_len]!=0) keyword_len++;
                   System.out.print(new String(buf,0,keyword_len,"UTF-8")+": ");
                   InflaterInputStream ii = new InflaterInputStream(new ByteArrayInputStream(buf,keyword_len+2, data_len-keyword_len-2));
                   InputStreamReader ir = new InputStreamReader(ii,"UTF-8");
                   BufferedReader br = new BufferedReader(ir);                       
                   String read = null;
                   while((read=br.readLine()) != null) {
                      System.out.println(read);
                   }
                   System.out.println("**********************");
                   is.skip(4);
                   break;
               }

               case tEXt:
               {
                   System.out.println("tEXt chunk:");
                   buf = new byte[data_len];
                   is.read(buf);
                   int keyword_len = 0;
                   while(buf[keyword_len]!=0) keyword_len++;
                   System.out.print(new String(buf,0,keyword_len,"UTF-8")+": ");
                   System.out.println(new String(buf,keyword_len+1,data_len-keyword_len-1,"UTF-8"));
                   System.out.println("**********************");
                   is.skip(4);
                   break;
               }

               case iTXt:
               {
                  // System.setOut(new PrintStream(new File("TextChunk.txt"),"UTF-8"));
                  /**
                   * Keyword:             1-79 bytes (character string)
                   * Null separator:      1 byte
                   * Compression flag:    1 byte
                   * Compression method:  1 byte
                   * Language tag:        0 or more bytes (character string)
                   * Null separator:      1 byte
                   * Translated keyword:  0 or more bytes
                   * Null separator:      1 byte
                   * Text:                0 or more bytes
                   */
                   System.out.println("iTXt chunk:");
                   buf = new byte[data_len];
                   is.read(buf);
                   int keyword_len = 0;
                   int trans_keyword_len = 0;
                   int lang_flg_len = 0;
                   boolean compr = false;
                   while(buf[keyword_len]!=0) keyword_len++;
                   System.out.print(new String(buf,0,keyword_len,"UTF-8"));
                   if(buf[++keyword_len]==1) compr = true;
                   keyword_len++;//Skip the compresssion method byte.
                   while(buf[++keyword_len]!=0) lang_flg_len++;
                   //////////////////////
                   System.out.print("(");
                   if(lang_flg_len>0)
                       System.out.print(new String(buf,keyword_len-lang_flg_len, lang_flg_len, "UTF-8"));
                   while(buf[++keyword_len]!=0) trans_keyword_len++;
                   if(trans_keyword_len>0)
                       System.out.print(" "+new String(buf,keyword_len-trans_keyword_len, trans_keyword_len, "UTF-8"));
                   System.out.print("): ");
                   /////////////////////// End of key.
                   if(compr) //Compressed text
                   {
                       InflaterInputStream ii = new InflaterInputStream(new ByteArrayInputStream(buf,keyword_len+1, data_len-keyword_len-1));
                       InputStreamReader ir = new InputStreamReader(ii,"UTF-8");
                       BufferedReader br = new BufferedReader(ir);                       
                       String read = null;
                       while((read=br.readLine()) != null) {
                          System.out.println(read);
                       }
                   }
                   else //Uncompressed text
                   {
                       System.out.println(new String(buf,keyword_len+1,data_len-keyword_len-1,"UTF-8"));                           
                   }
                   System.out.println("**********************");
                   is.skip(4);
                   break;
               }    

               default:
               {
                   buf = new byte[data_len+4];
                   is.read(buf,0, data_len+4);
                   break;
               }
          }
      }
      is.close();
 }

 private static int readInt(InputStream is) throws Exception
 {
     byte[] buf = new byte[4];
     is.read(buf,0,4);
     return (((buf[0]&0xff)<<24)|((buf[1]&0xff)<<16)|
                            ((buf[2]&0xff)<<8)|(buf[3]&0xff));
 }

 private static long readLong(InputStream is) throws Exception
 {
     byte[] buf = new byte[8];
     is.read(buf,0,8);
     return (((buf[0]&0xffL)<<56)|((buf[1]&0xffL)<<48)|
                            ((buf[2]&0xffL)<<40)|((buf[3]&0xffL)<<32)|((buf[4]&0xffL)<<24)|
                              ((buf[5]&0xffL)<<16)|((buf[6]&0xffL)<<8)|(buf[7]&0xffL));
 }

 public static void main(String args[]) throws Exception
 {
    FileInputStream fs = new FileInputStream(args[0]);
    showText(fs);       
 }
}

注意：：用法：java PNGExtractText image.png

这是我从官方 png 测试套件的测试图像 ctzn0g04.png 中得到的：

D:\tmp>java PNGExtractText ctzn0g04.png
tEXt chunk:
Title: PngSuite
**********************
tEXt chunk:
Author: Willem A.J. van Schaik
(willem@schaik.com)
**********************
zTXt chunk:
Copyright: Copyright Willem van Schaik, Singapore 1995-96
**********************
zTXt chunk:
Description: A compilation of a set of images created to test the
various color-types of the PNG format. Included are
black&white, color, paletted, with alpha channel, with
transparency formats. All bit-depths allowed according
to the spec are present.
**********************
zTXt chunk:
Software: Created on a NeXTstation color using "pnmtopng".
**********************
zTXt chunk:
Disclaimer: Freeware.
**********************
IEND found

编辑：刚刚找到您的链接并尝试使用图片，得到了这个：

D:\tmp>java PNGExtractText concept_Sjet_dream6.png
tEXt chunk:
Comment: The comment
**********************
IEND found

上面的例子已经成为Java图像库的一部分，可以在https://github.com/dragon66/icafe找到