我在互联网上找到以下代码，用于使用库libjpeg读取和写入JPEG文件。
我将函数v oid write_JPEG_file (char * filename, int quality)更改为以下内容：
void write_JPEG_vetor (JSAMPLE * image_data, int height, int width, int quality)
{
printf("%s\n","write_JPEG_vetor");
/* This struct contains the JPEG compression parameters and pointers to
* working space (which is allocated as needed by the JPEG library).
* It is possible to have several such structures, representing multiple
* compression/decompression processes, in existence at once. We refer
* to any one struct (and its associated working data) as a "JPEG object".
*/
struct jpeg_compress_struct cinfo;
/* This struct represents a JPEG error handler. It is declared separately
* because applications often want to supply a specialized error handler
* (see the second half of this file for an example). But here we just
* take the easy way out and use the standard error handler, which will
* print a message on stderr and call exit() if compression fails.
* Note that this struct must live as long as the main JPEG parameter
* struct, to avoid dangling-pointer problems.
*/
struct jpeg_error_mgr jerr;
/* More stuff */
FILE * outfile; /* target file */
JSAMPROW row_pointer[1]; /* pointer to JSAMPLE row[s] */
int row_stride; /* physical row width in image buffer */
printf ("%s\n","Step 1: allocate and initialize JPEG compression object */");
/* We have to set up the error handler first, in case the initialization
* step fails. (Unlikely, but it could happen if you are out of memory.)
* This routine fills in the contents of struct jerr, and returns jerr's
* address which we place into the link field in cinfo.
*/
cinfo.err = jpeg_std_error(&jerr);
/* Now we can initialize the JPEG compression object. */
jpeg_create_compress(&cinfo);
printf ("%s\n","/* Step 2: specify data destination (eg, a file) */");
/* Note: steps 2 and 3 can be done in either order. */
/* Here we use the library-supplied code to send compressed data to a
* stdio stream. You can also write your own code to do something else.
* VERY IMPORTANT: use "b" option to fopen() if you are on a machine that
* requires it in order to write binary files.
*/
char * filename = {"novo_arquivo.jpeg"};
if ((outfile = fopen(filename, "wb")) == NULL) {
fprintf(stderr, "can't open %s\n", filename);
exit(1);
}
jpeg_stdio_dest(&cinfo, outfile);
printf ("%s\n","/* Step 3: set parameters for compression */");
/* First we supply a description of the input image.
* Four fields of the cinfo struct must be filled in:
*/
cinfo.image_width = width; /* image width and height, in pixels */
cinfo.image_height = height;
cinfo.input_components = 3; /* # of color components per pixel */
cinfo.in_color_space = JCS_RGB; /* colorspace of input image */
/* Now use the library's routine to set default compression parameters.
* (You must set at least cinfo.in_color_space before calling this,
* since the defaults depend on the source color space.)
*/
jpeg_set_defaults(&cinfo);
/* Now you can set any non-default parameters you wish to.
* Here we just illustrate the use of quality (quantization table) scaling:
*/
jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);
printf ("%s\n","/* Step 4: Start compressor */");
/* TRUE ensures that we will write a complete interchange-JPEG file.
* Pass TRUE unless you are very sure of what you're doing.
*/
jpeg_start_compress(&cinfo, TRUE);
printf ("%s\n","/* Step 5: while (scan lines remain to be written) */");
/* jpeg_write_scanlines(...); */
/* Here we use the library's state variable cinfo.next_scanline as the
* loop counter, so that we don't have to keep track ourselves.
* To keep things simple, we pass one scanline per call; you can pass
* more if you wish, though.
*/
row_stride = width * 3; /* JSAMPLEs per row in image_buffer */
while (cinfo.next_scanline < cinfo.image_height) {
printf ("%s\n","Loop WHILE");
/* jpeg_write_scanlines expects an array of pointers to scanlines.
* Here the array is only one element long, but you could pass
* more than one scanline at a time if that's more convenient.
*/
row_pointer[0] = &image_data[cinfo.next_scanline * row_stride];
(void) jpeg_write_scanlines(&cinfo, row_pointer, row_stride);
}
printf ("%s\n","/* Step 6: Finish compression */");
jpeg_finish_compress(&cinfo);
/* After finish_compress, we can close the output file. */
fclose(outfile);
printf ("%s\n","/* Step 7: release JPEG compression object */");
/* This is an important step since it will release a good deal of memory. */
jpeg_destroy_compress(&cinfo);
/* And we're done! */
}
现在，当我运行程序时（在Linux环境中），我收到错误Segmentation Fault。有人能说出为什么会这样吗？我的主要嫌疑人是代码：
while (cinfo.next_scanline < cinfo.image_height) {
printf ("%s\n","Loop WHILE");
/* jpeg_write_scanlines expects an array of pointers to scanlines.
* Here the array is only one element long, but you could pass
* more than one scanline at a time if that's more convenient.
*/
row_pointer[0] = &image_data[cinfo.next_scanline * row_stride];
(void) jpeg_write_scanlines(&cinfo, row_pointer, row_stride);
}
但我对此并不确定，并且无法花费大量时间尝试，无法找到解决此问题的解决方案。
===更新=== 我在代码的这一部分中包含了以下调试代码：
while (cinfo.next_scanline < cinfo.image_height) {
printf ("%s\n","Loop WHILE");
/* jpeg_write_scanlines expects an array of pointers to scanlines.
* Here the array is only one element long, but you could pass
* more than one scanline at a time if that's more convenient.
*/
printf ("%s\n","parte 1.1");
row_pointer[0] = &image_data[cinfo.next_scanline * row_stride];
printf ("%s\n","parte 1.2");
printf ("%s\n","parte 2.1");
(void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
printf ("%s\n","parte 2.2");
}
这样运行程序时的输出是：
Loop WHILE
parte 1.1
parte 1.2
parte 2.1
===更新2 === 为了记录，在我的程序中，此函数正在接收此函数的返回值：
JSAMPLE * inverte_imagem()
{
int tamanho = image_height*image_width*image_colors;
int i;
JSAMPLE * vetor = malloc(sizeof(JSAMPLE)*(image_height*image_width*image_colors));
for( i=0; i<tamanho; i++)
vetor [i] = image_buffer [tamanho - (i+1)];
}
答案 0 :(得分：0)
这看起来不对：
(void) jpeg_write_scanlines(&cinfo, row_pointer, row_stride);
最后一个参数是要写入的行数，而不是行长度。你可能想要：
(void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
答案 1 :(得分：0)
好的，我解决了在函数inverte_imagem（）中调用write_JPEG_vector的问题。我不知道为什么，但是当我从我的main函数调用这个函数时，会出现内存问题（linux上的错误分段错误）。