在使用 libjpeg 读取 JPEG 文件时需要帮助

Question

我遵循了 libjpeg 示例文件中的示例代码，但是我无法读取图像数据。

我有以下结构，我创建了这个结构的一个实例。

 struct ImageData {
        unsigned char *pixels;
        long  width;
        long height;
    };

    ImageData *imageData;

下面是我的 read_JPEG_file 函数：

int read_JPEG_file (char * filename)
{
    struct jpeg_decompress_struct cinfo;
    struct my_error_mgr jerr;

    /* More stuff */
    FILE * infile;      /* source file */
    JSAMPARRAY buffer;      /* Output row buffer */
    int row_stride;     /* physical row width in output buffer */

    if ((infile = fopen(filename, "rb")) == NULL) {
        fprintf(stderr, "can't open %s\n", filename);
        return 0;
    }

    /* Step 1: allocate and initialize JPEG decompression object */

    /* We set up the normal JPEG error routines, then override error_exit. */
    cinfo.err = jpeg_std_error(&jerr.pub);
    jerr.pub.error_exit = my_error_exit;
    /* Establish the setjmp return context for my_error_exit to use. */
    if (setjmp(jerr.setjmp_buffer)) {

        jpeg_destroy_decompress(&cinfo);
        fclose(infile);
        return 0;
    }
    /* Now we can initialize the JPEG decompression object. */
    jpeg_create_decompress(&cinfo);

    /* Step 2: specify data source (eg, a file) */

    jpeg_stdio_src(&cinfo, infile);

    /* Step 3: read file parameters with jpeg_read_header() */

    (void) jpeg_read_header(&cinfo, TRUE);
    /* Step 4: set parameters for decompression */

    /* In this example, we don't need to change any of the defaults set by
     * jpeg_read_header(), so we do nothing here.
     */

    /* Step 5: Start decompressor */

    (void) jpeg_start_decompress(&cinfo);


    row_stride = cinfo.output_width * cinfo.output_components;
    /* Make a one-row-high sample array that will go away when done with image */
    buffer = (*cinfo.mem->alloc_sarray) ((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);

    imageData = new ImageData;
    imageData->width = cinfo.output_width;
    imageData->height = cinfo.output_height;

    imageData->pixels = new unsigned char [cinfo.output_width * cinfo.output_height * cinfo.output_components];
    long counter = 0;

   //step 6, read the image line by line
    while (cinfo.output_scanline < cinfo.output_height) {
        //IT ALWAYS crash ON THIS JPEG_READ_SCANLINES FUNCTION CALL BELOW
        (void) jpeg_read_scanlines(&cinfo, (JSAMPARRAY)(imageData->pixels), 1);
        counter +=row_stride;

    }
       /* Step 7: Finish decompression */

    (void) jpeg_finish_decompress(&cinfo);
    /* Step 8: Release JPEG decompression object */

    /* This is an important step since it will release a good deal of memory. */
    jpeg_destroy_decompress(&cinfo);

    fclose(infile);
    /* And we're done! */
    return 1;
}

在上面的第 6 步中，这个 JPEG_READ_SCANLINES 函数总是失败。我在那条线上收到了“EXC_BAD_ACCESS”信号。

有没有人有任何想法，或者有一些关于使用 libjpeg 读取 .jpg 文件的工作示例，您可以在这里分享吗？我检查了我的 imageData->pixels 的大小，并将其与 jpeg 文件本身的大小进行了比较，它具有相同的大小。这个变量的内存也是动态分配的，所以我知道这不是内存问题。

有什么想法吗？

Answer 1

这是一个读取 jpeg 图像的示例：

/***************************************************
    To read a jpg image file and download
    it as a texture map for openGL
    Derived from Tom Lane's example.c
    -- Obtain & install jpeg stuff from web 
    (jpeglib.h, jerror.h jmore.h, jconfig.h,jpeg.lib)
****************************************************/
#include <jpeglib.h>    
#include <jerror.h>
//================================
GLuint LoadJPEG(char* FileName)
//================================
{
  unsigned long x, y;
  unsigned int texture_id;
  unsigned long data_size;     // length of the file
  int channels;               //  3 =>RGB   4 =>RGBA 
  unsigned int type;  
  unsigned char * rowptr[1];    // pointer to an array
  unsigned char * jdata;        // data for the image
  struct jpeg_decompress_struct info; //for our jpeg info
  struct jpeg_error_mgr err;          //the error handler

  FILE* file = fopen(FileName, "rb");  //open the file

  info.err = jpeg_std_error(& err);     
  jpeg_create_decompress(& info);   //fills info structure

  //if the jpeg file doesn't load
  if(!file) {
     fprintf(stderr, "Error reading JPEG file %s!", FileName);
     return 0;
  }

  jpeg_stdio_src(&info, file);    
  jpeg_read_header(&info, TRUE);   // read jpeg file header

  jpeg_start_decompress(&info);    // decompress the file

  //set width and height
  x = info.output_width;
  y = info.output_height;
  channels = info.num_components;
  type = GL_RGB;
  if(channels == 4) type = GL_RGBA;

  data_size = x * y * 3;

  //--------------------------------------------
  // read scanlines one at a time & put bytes 
  //    in jdata[] array. Assumes an RGB image
  //--------------------------------------------
  jdata = (unsigned char *)malloc(data_size);
  while (info.output_scanline < info.output_height) // loop
  {
    // Enable jpeg_read_scanlines() to fill our jdata array
    rowptr[0] = (unsigned char *)jdata +  // secret to method
            3* info.output_width * info.output_scanline; 

    jpeg_read_scanlines(&info, rowptr, 1);
  }
  //---------------------------------------------------

  jpeg_finish_decompress(&info);   //finish decompressing

  //----- create OpenGL tex map (omit if not needed) --------
  glGenTextures(1,&texture_id);
  glBindTexture(GL_TEXTURE_2D, texture_id);
  gluBuild2DMipmaps(GL_TEXTURE_2D,3,x,y,GL_RGB,GL_UNSIGNED_BYTE,jdata);

  jpeg_destroy_decompress(&info);
  fclose(file);                    //close the file
  free(jdata);

  return texture_id;    // for OpenGL tex maps
}

Answer 2

jpeg_read_scanlines 函数接收一个指针数组（不是像 imageData->pixels 那样的像素直接指针）。所以我们应该先创建一个 JSAMPARRAY：

int buffer_height = 1;
JSAMPARRAY buffer = (JSAMPARRAY)malloc(sizeof(JSAMPROW) * buffer_height);
buffer[0] = (JSAMPROW)malloc(sizeof(JSAMPLE) * row_stride);

在您的代码中，您使用“cinfo.mem->alloc_sarray”创建了一个“缓冲区”，但您从未使用它。最后一步是将“缓冲区”作为 jpeg_read_scanlines 的参数传递：

while (cinfo.output_scanline < cinfo.output_height) {
  jpeg_read_scanlines(&cinfo, buffer, 1);
  memcpy(imageData->pixels+counter, buffer[0], row_stride);
  counter += row_stride;
}

看到我们使用的是“imageData->pixels+counter”，而不仅仅是代码中的“imageData->pixels”。通过这种方式，我们在整个“imageData->pixels”内存块中逐行写入。

Answer 3

就像 dacap 所说，它期待一个 JSAMPARRAY。话虽如此，如果您愿意，您可以直接写入您的 imageData->pixels 数组。你只需要这样做：

// Allocate imageData->pixels to be the correct size, start decompress and all
// that jazz, like you did in your code. Skip allocating buffer though.
// ...

JSAMPROW output_data;
unsigned int scanline_len = cinfo.output_width * cinfo.output_components;

unsigned int scanline_count = 0;
while (cinfo.output_scanline < cinfo.output_height)
{
    output_data = (imageData->pixels + (scanline_count * scanline_len));
    jpeg_read_scanlines(&cinfo, &output_data, 1);
    scanline_count++;
}

然后您可以完全跳过分配缓冲区。使用 memcpy 效果很好，但如果不需要，为什么还要额外复制呢？

Answer 4

这是由 Jim 整理的代码版本。它还使用CImg 作为输出格式，这使得提取 RGB 值的方式更加明确。

CImg<unsigned char> *ReadJpegIntoCImg( const char *path )
{
    FILE *file = fopen( path, "rb" );
    if ( file == NULL )
    {
        return NULL;
    }
    
    struct jpeg_decompress_struct info; //for our jpeg info
    struct jpeg_error_mgr err; //the error handler
    
    info.err = jpeg_std_error( &err );     
    jpeg_create_decompress( &info ); //fills info structure
    
    jpeg_stdio_src( &info, file );    
    jpeg_read_header( &info, true );
    
    jpeg_start_decompress( &info );
    
    unsigned int w = info.output_width;
    unsigned int h = info.output_height;
    unsigned int numChannels = info.num_components; // 3 = RGB, 4 = RGBA
    unsigned long dataSize = w * h * numChannels;
    
    // read RGB(A) scanlines one at a time into jdata[]
    unsigned char *data = (unsigned char *)malloc( dataSize );
    unsigned char* rowptr;
    while ( info.output_scanline < h )
    {
        rowptr = data + info.output_scanline * w * numChannels;
        jpeg_read_scanlines( &info, &rowptr, 1 );
    }
    
    jpeg_finish_decompress( &info );
    jpeg_destroy_decompress( &info );   
    
    fclose( file );
    
    // this code block is only if you happen to want output in CImg format, but is illustrative
    CImg<unsigned char> *image = new CImg<unsigned char>( w, h, 1, numChannels );
    for ( int x = 0 ; x < w ; x++ )
    {
        for ( int y = 0 ; y < h ; y++ )
        {
            for ( int c = 0 ; c < numChannels ; c++ )
            {
                *image->data( x, y, 0, c ) = data[ y * w * numChannels + x * numChannels + c ];
            }
        }
    }
    
    free( data );
    
    return image;
}

另外，对于那些可能遇到与我相同的问题的人来说，在使用 C++ 中的 libjpeg 时，拥有一个包含以下内容的 jpeglib.h 版本很重要

#ifdef __cplusplus
extern "C" {
#endif

我使用的文件版本没有该文件，并且出现链接错误。