如何从 PNG 中删除 Gamma 信息

Question

我正在尝试生成没有伽马信息的图像，以便 IE8 可以正确显示它们。 使用了以下代码，但结果是扭曲的图像，看起来与原始图像完全不同。

 ///PNG
  PNGEncodeParam params= PNGEncodeParam.getDefaultEncodeParam(outImage);
  params.unsetGamma();
  params.setChromaticity(DEFAULT_CHROMA);
  params.setSRGBIntent(PNGEncodeParam.INTENT_ABSOLUTE);
  ImageEncoder  encoder= ImageCodec.createImageEncoder("PNG", response.getOutputStream(), params);
  encoder.encode(outImage);
  response.getOutputStream().close();

这是原始图像和由上述代码产生的扭曲图像。

谢谢！

Answer 1

我在几个地方看到同样的问题，但似乎没有答案，所以我在这里提供我的。 我不知道 Java imageio 是否保存伽马。 鉴于 gamma 是系统相关的，imageio 不太可能处理它。 有一件事是肯定的：imageio 在读取 png 时会忽略 gamma。

PNG 是一种基于块的图像格式。 Gamma 是 14 个辅助块之一，它负责创建图像的计算机系统的差异，使它们在不同的系统上看起来或多或少同样“明亮”。 每个中继线以数据长度和中继线标识符开头，后跟 4 个字节的 CRC 校验和。 数据长度不包括数据长度属性本身和中继标识符。 gAMA 块由十六进制0x67414D41标识。

这是从 png 图像中删除 gAMA 的原始方法：我们假设输入流是有效的 PNG 格式。 首先读取 8 个字节，即 png 标识符0x89504e470d0a1a0aL 。 然后读取另外 25 个字节，其中包括图像标题。 我们总共从文件顶部读取了 33 个字节。 现在将它们保存到另一个扩展名为 png 的临时文件中。 现在涉及到一个 while 循环。 我们逐个读取块：如果它不是 IEND 并且不是 gAMA 块，我们将其复制到输出临时文件。 如果它是 gAMA 主干，我们跳过它，直到我们到达 IEND 这应该是最后一个块，然后我们将它复制到临时文件。 完毕。 以下是整个测试代码，用于展示如何完成（仅用于演示目的，未优化）：

import java.io.*;

public class RemoveGamma
{  
     /** 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 data */
     private static final int IDAT = 0x49444154;   // "IDAT"
    /** Image trailer */
     private static final int IEND = 0x49454E44;   // "IEND"
     /** Palette */
     private static final int PLTE = 0x504C5445;   // "PLTE"
     /** 14 Ancillary chunks */
     /** Transparency */
     private static final int tRNS = 0x74524E53;   // "tRNs"
    /** Image gamma */
     private static final int gAMA = 0x67414D41;   // "gAMA"
     /** Primary chromaticities */
     private static final int cHRM = 0x6348524D;   // "cHRM"
     /** Standard RGB color space */
     private static final int sRGB = 0x73524742;   // "sRGB"
     /** Embedded ICC profile */
     private static final int iCCP = 0x69434350;   // "iCCP"
     /** 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 */
     private static final int bKGD = 0x624B4744;   // "bKGD"
     /** Physical pixel dimensions */
     private static final int pHYs = 0x70485973;   // "pHYs"
     /** Significant bits */
     private static final int sBIT = 0x73424954;   // "sBIT"
     /** Suggested palette */
     private static final int sPLT = 0x73504C54;   // "sPLT"
     /** Palette histogram */
     private static final int hIST = 0x68495354;   // "hIST"
     /** Image last-modification time */
     private static final int tIME = 0x74494D45;   // "tIME"

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

          DataOutputStream ds = new DataOutputStream(new FileOutputStream("temp.png")); 

          long signature = readLong(is);

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

          ds.writeLong(SIGNATURE);

          //*******************************
          //Chuncks follow, start with IHDR
          //*******************************
          /** Chunk layout
              Each chunk consists of four parts:

              Length
                 A 4-byte unsigned integer giving the number of bytes in the chunk's data field.
                 The length counts only the data field, not itself, the chunk type code, or the CRC.
                 Zero is a valid length. Although encoders and decoders should treat the length as unsigned, 
                 its value must not exceed 2^31-1 bytes.

              Chunk Type
                 A 4-byte chunk type code. For convenience in description and in examining PNG files, 
                 type codes are restricted to consist of uppercase and lowercase ASCII letters 
                 (A-Z and a-z, or 65-90 and 97-122 decimal). However, encoders and decoders must treat 
                 the codes as fixed binary values, not character strings. For example, it would not be
                 correct to represent the type code IDAT by the EBCDIC equivalents of those letters. 
                 Additional naming conventions for chunk types are discussed in the next section.

              Chunk Data
                 The data bytes appropriate to the chunk type, if any. This field can be of zero length.

              CRC
                 A 4-byte CRC (Cyclic Redundancy Check) calculated on the preceding bytes in the chunk,
                 including the chunk type code and chunk data fields, but not including the length field. 
                 The CRC is always present, even for chunks containing no data. See CRC algorithm. 
            */

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

            ds.writeInt(13);//We expect length to be 13 bytes
            ds.writeInt(IHDR);

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

            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");
                   ds.writeInt(data_len);
                   ds.writeInt(IEND);
                   int crc = readInt(is);
                   ds.writeInt(crc);
                   break;
                }

                switch (chunk_type)
                {
                   case gAMA://or any non-significant chunk you want to remove
                   {
                       System.out.println("gamma found");
                       is.skip(data_len+4);
                       break;
                   }
                   default:
                   {
                       buf = new byte[data_len+4];
                       is.read(buf,0, data_len+4);
                       ds.writeInt(data_len);
                       ds.writeInt(chunk_type);
                       ds.write(buf);
                       break;
                   }
                }
            }
            is.close();
            ds.close();
     }

     private 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 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]);
        RemoveGamma rg = new RemoveGamma();
        rg.remove(fs);       
     }
}

由于输入是 Java InputStream，我们可以使用某种编码器将图像编码为 PNG 并将其写入 ByteArrayOutputStream 稍后将作为 ByteArrayInputSteam 提供给上述测试类，伽马信息（如果有）将被移除。 结果如下：

左边是加了gAMA的原图，右边是去掉了gAMA的同一张图。

图片来源： http : //r6.ca/cs488/kosh.png

编辑：这是删除任何辅助块的代码的修订版本。

import java.io.*;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;

public class PNGChunkRemover
{  
     /** PNG signature constant */
     private static final long SIGNATURE = 0x89504E470D0A1A0AL;
     /** PNG Chunk type constants, 4 Critical chunks */
     /** Image header */
     private static final int IHDR = 0x49484452;   // "IHDR"
     /** Image data */
     private static final int IDAT = 0x49444154;   // "IDAT"
    /** Image trailer */
     private static final int IEND = 0x49454E44;   // "IEND"
     /** Palette */
     private static final int PLTE = 0x504C5445;   // "PLTE"

     //Ancillary chunks keys
     private static String[] KEYS = { "TRNS", "GAMA","CHRM","SRGB","ICCP","TEXT","ZTXT",
                                      "ITXT","BKGD","PHYS","SBIT","SPLT","HIST","TIME"};

     private static int[]  VALUES = {0x74524E53,0x67414D41,0x6348524D,0x73524742,0x69434350,0x74455874,0x7A545874,
                                     0x69545874,0x624B4744,0x70485973,0x73424954,0x73504C54,0x68495354,0x74494D45};

     private static HashMap<String, Integer> TRUNK_TYPES = new HashMap<String, Integer>()
     {{ 
         for(int i=0;i<KEYS.length;i++)
           put(KEYS[i],VALUES[i]);
     }};

     private static HashMap<Integer, String> REVERSE_TRUNK_TYPES = new HashMap<Integer,String>()
     {{ 
         for(int i=0;i<KEYS.length;i++)
           put(VALUES[i],KEYS[i]);
     }};

     private static Set<Integer> REMOVABLE = new HashSet<Integer>();

     private static void remove(InputStream is, File dir, String fileName) throws Exception
     {
         //Local variables for reading chunks
          int data_len = 0;
          int chunk_type = 0;
          byte[] buf=null;

          DataOutputStream ds = new DataOutputStream(new FileOutputStream(new File(dir,fileName))); 

          long signature = readLong(is);

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

          ds.writeLong(SIGNATURE);

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

          ds.writeInt(13);//We expect length to be 13 bytes
          ds.writeInt(IHDR);

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

          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");
                   ds.writeInt(data_len);
                   ds.writeInt(IEND);
                   int crc = readInt(is);
                   ds.writeInt(crc);
                   break;
                }
                if(REMOVABLE.contains(chunk_type))
                {
                    System.out.println(REVERSE_TRUNK_TYPES.get(chunk_type)+"Chunk removed!");
                    is.skip(data_len+4);
                }
                else
                {
                    buf = new byte[data_len+4];
                    is.read(buf,0, data_len+4);
                    ds.writeInt(data_len);
                    ds.writeInt(chunk_type);
                    ds.write(buf);
                }
          }
          is.close();
          ds.close();
     }

     private static int readInt(InputStream is) throws Exception
     {
         byte[] buf = new byte[4];
         int bytes_read = is.read(buf,0,4);
         if(bytes_read<0) return IEND; 
         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];
         int bytes_read = is.read(buf,0,8);
         if(bytes_read<0) return IEND; 
         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
     {
        if(args.length>0)
        {
          File[] files = {new File(args[0])};
          File dir = new File(".");

          if(files[0].isDirectory())
          {
             dir = files[0];

             files = files[0].listFiles(new FileFilter(){
                public boolean accept(File file)
                {
                   if(file.getName().toLowerCase().endsWith("png")){
                      return true;
                   }
                   return false;
                }
             }
            );
          }     

          if(args.length>1)
          { 
             FileInputStream fs = null;

             if(args[1].equalsIgnoreCase("all")){
                REMOVABLE = REVERSE_TRUNK_TYPES.keySet();
             }
             else
             {
                String key = "";
                for (int i=1;i<args.length;i++)
                {
                    key = args[i].toUpperCase();
                    if(TRUNK_TYPES.containsKey(key))
                      REMOVABLE.add(TRUNK_TYPES.get(key));
                }
             }
             for(int i= files.length-1;i>=0;i--)
             {
                String outFileName = files[i].getName();
                outFileName = outFileName.substring(0,outFileName.lastIndexOf('.'))
                    +"_slim.png";
                System.out.println("<<"+files[i].getName());
                fs = new FileInputStream(files[i]);
                remove(fs, dir, outFileName);
                System.out.println(">>"+outFileName);   
                System.out.println("************************");
             }
          }
        }
     }
}

用法： java PNGChunkRemover filename.png all将删除任何预定义的 14 个辅助块。

java PNGChunkRemover filename.png gama time ...只会删除 png 文件之后指定的块。

注意：如果将文件夹名称指定为 PNGChunkRemover 的第一个参数，则将处理文件夹中的所有 png 文件。

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

Answer 2

您也可以使用（我的）PNGJ 库http://code.google.com/p/pngj/

例如

 PngReader pngr = FileHelper.createPngReader(new File(origFilename));
 PngWriter pngw = FileHelper.createPngWriter(new File(destFilename), pngr.imgInfo, false);
 pngw.copyChunksFirst(pngr, ChunkCopyBehaviour.COPY_ALL); // all chunks are queued
 PngChunkGAMA gama = (PngChunkGAMA) pngw.getChunkList().getQueuedById1(ChunkHelper.gAMA);
 if (gama != null) {
   System.out.println("removing gama chunk gamma=" + gama.getGamma());
   pngw.getChunkList().removeChunk(gama);
 } 
 for (int row = 0; row < pngr.imgInfo.rows; row++) {
   ImageLine l1 = pngr.readRow(row);
   pngw.writeRow(l1, row);
 }
 pngw.copyChunksLast(pngr, ChunkCopyBehaviour.COPY_ALL); // in case some new metadata has been read
 pngw.end();

包含在库示例中。

Answer 3

工具pngcrush可以删除伽马信息和其他不需要的块：

pngcrush -m 3 -rem gAMA -rem cHRM -rem iCCP -rem sRGB in.png out.png

它同时重新压缩 PNG，尝试不同的方法。 -m 3选项仅尝试第 3 种方法，这似乎是快速且相当有效的。 如果您想要最小的 png，请忽略它。