需要帮助在使用libjpeg读取JPEG文件 [英] Need help in reading JPEG file using libjpeg

问题描述

我按照libjpeg示例文件中的示例代码，但我无法读取图像数据。

I followed the example code in the libjpeg example file, however I was not able to read the image data.

我有以下结构，并且创建了此结构的实例。

I have the following struct, and I created an instance of this struct.

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

    ImageData *imageData;

下面是我的read_JPEG_file函数：

Below is my read_JPEG_file function:

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;
}

在此JPEG_READ_SCANLINES函数上面的步骤6。我在该行上有一个EXC_BAD_ACCESS信号。

It always fails on this JPEG_READ_SCANLINES function, in the step 6 above. I got an "EXC_BAD_ACCESS" signal on that line.

有没有人有任何想法，或有一些工作示例阅读.jpg文件libjpeg，你可以在这里分享？我已经检查了我的imageData->像素的大小，并将其与jpeg文件本身的大小，它具有相同的大小。这个变量的内存也动态分配，所以我知道这不是一个内存问题。

Does anyone have any idea, or have some working examples on reading .jpg file with libjpeg that you can share here? I have checked the size of my imageData->pixels, and compared it with the size of the jpeg file itself, and it has the same size. The memory for this variable has also dynamically allocated, so I know that it was not a memory problem.

任何想法？

推荐答案

jpeg_read_scanlines函数接收指针数组（不是像素的直接指针，如imageData-> pixels）。所以我们应该先创建一个JSAMPARRAY：

jpeg_read_scanlines function receives an array of pointers (not the direct pointer of pixels as imageData->pixels). So we should create a JSAMPARRAY first:

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

在你的代码中，你创建了一个cinfo.mem-> alloc_sarray你永远不会使用它。最后一步是传递buffer作为jpeg_read_scanlines的参数：

In your code you've created a "buffer" with "cinfo.mem->alloc_sarray" but you never use it. The final step is to pass the "buffer" as argument of jpeg_read_scanlines:

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

看到我们正在使用imageData-> pixels + counter只是imageData->像素在你的代码。这样，我们在整个imageData-> pixels内存块中逐行写入。

See that we're using "imageData->pixels+counter", not just "imageData->pixels" as in your code. In this way we write each row after another in the whole "imageData->pixels" memory chunk.

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