libjpeg库使用
libjpeg库是专门用于jpeg图片格式解压和压缩的库
程序实例如下：
#include <stdio.h>
#include <setjmp.h>
#include <string.h>
#include <stdlib.h>
#include "jpeg-lib/include/jpeglib.h"
#define PUT_2B(array,offset,value) \
(array[offset] = (char) ((value) & 0xFF), \
array[offset+1] = (char) (((value) >> 8) & 0xFF))
#define PUT_4B(array,offset,value) \
(array[offset] = (char) ((value) & 0xFF), \
array[offset+1] = (char) (((value) >> 8) & 0xFF), \
array[offset+2] = (char) (((value) >> 16) & 0xFF), \
array[offset+3] = (char) (((value) >> 24) & 0xFF))
void write_bmp_header(j_decompress_ptr cinfo, FILE *output_file)
{
char bmpfileheader[14];
char bmpinfoheader[40];
long headersize, bfSize;
int bits_per_pixel, cmap_entries;
int step;
/* Compute colormap size and total file size */
if (cinfo->out_color_space == JCS_RGB) {
if (cinfo->quantize_colors) {
/* Colormapped RGB */
bits_per_pixel = 8;
cmap_entries = 256;
} else {
/* Unquantized, full color RGB */
bits_per_pixel = 24;
cmap_entries = 0;
}
} else {
/* Grayscale output. We need to fake a 256-entry colormap. */
bits_per_pixel = 8;
cmap_entries = 256;
}
step = cinfo->output_width * cinfo->output_components;
while ((step & 3) != 0) step++;
/* File size */
headersize = 14 + 40 + cmap_entries * 4; /* Header and colormap */
bfSize = headersize + (long) step * (long) cinfo->output_height;
/* Set unused fields of header to 0 */
memset(bmpfileheader, 0, sizeof(bmpfileheader));
memset(bmpinfoheader, 0 ,sizeof(bmpinfoheader));
/* Fill the file header */
bmpfileheader[0] = 0x42;/* first 2 bytes are ASCII 'B', 'M' */
bmpfileheader[1] = 0x4D;
PUT_4B(bmpfileheader, 2, bfSize); /* bfSize */
/* we leave bfReserved1 & bfReserved2 = 0 */
PUT_4B(bmpfileheader, 10, headersize); /* bfOffBits */
/* Fill the info header (Microsoft calls this a BITMAPINFOHEADER) */
PUT_2B(bmpinfoheader, 0, 40); /* biSize */
PUT_4B(bmpinfoheader, 4, cinfo->output_width); /* biWidth */
PUT_4B(bmpinfoheader, 8, cinfo->output_height); /* biHeight */
PUT_2B(bmpinfoheader, 12, 1); /* biPlanes - must be 1 */
PUT_2B(bmpinfoheader, 14, bits_per_pixel); /* biBitCount */
/* we leave biCompression = 0, for none */
/* we leave biSizeImage = 0; this is correct for uncompressed data */
if (cinfo->density_unit == 2) { /* if have density in dots/cm, then */
PUT_4B(bmpinfoheader, 24, (INT32) (cinfo->X_density*100)); /* XPels/M */
PUT_4B(bmpinfoheader, 28, (INT32) (cinfo->Y_density*100)); /* XPels/M */
}
PUT_2B(bmpinfoheader, 32, cmap_entries); /* biClrUsed */
/* we leave biClrImportant = 0 */
if (fwrite(bmpfileheader, 1, 14, output_file) != (size_t) 14) {
printf("write bmpfileheader error\n");
}
if (fwrite(bmpinfoheader, 1, 40, output_file) != (size_t) 40) {
printf("write bmpinfoheader error\n");
}
if (cmap_entries > 0) {
}
}
void write_pixel_data(j_decompress_ptr cinfo, unsigned char *output_buffer, FILE *output_file)
{
int rows, cols;
int row_width;
int step;
unsigned char *tmp = NULL;
unsigned char *pdata;
row_width = cinfo->output_width * cinfo->output_components;
step = row_width;
while ((step & 3) != 0) step++;
pdata = (unsigned char *)malloc(step);
memset(pdata, 0, step);
tmp = output_buffer + row_width * (cinfo->output_height - 1);
for (rows = 0; rows < cinfo->output_height; rows++) {
for (cols = 0; cols < row_width; cols += 3) {
pdata[cols + 2] = tmp[cols + 0];
pdata[cols + 1] = tmp[cols + 1];
pdata[cols + 0] = tmp[cols + 2];
}
tmp -= row_width;
fwrite(pdata, 1, step, output_file);
}
free(pdata);
}
int read_jpeg_file(const char *input_filename, const char *output_filename)
{
struct jpeg_decompress_struct cinfo;
struct jpeg_error_mgr jerr;
FILE *input_file;
FILE *output_file;
JSAMPARRAY buffer;
int row_width;
unsigned char *output_buffer;
unsigned char *tmp = NULL;
cinfo.err = jpeg_std_error(&jerr);
if ((input_file = fopen(input_filename, "rb")) == NULL) {
fprintf(stderr, "can't open %s\n", input_filename);
return -1;
}
if ((output_file = fopen(output_filename, "wb")) == NULL) {
fprintf(stderr, "can't open %s\n", output_filename);
return -1;
}
jpeg_create_decompress(&cinfo);
/* Specify data source for decompression */
jpeg_stdio_src(&cinfo, input_file);
/* Read file header, set default decompression parameters */
(void) jpeg_read_header(&cinfo, TRUE);
/* Start decompressor */
(void) jpeg_start_decompress(&cinfo);
row_width = cinfo.output_width * cinfo.output_components;
buffer = (*cinfo.mem->alloc_sarray)
((j_common_ptr) &cinfo, JPOOL_IMAGE, row_width, 1);
write_bmp_header(&cinfo, output_file);
output_buffer = (unsigned char *)malloc(row_width * cinfo.output_height);
memset(output_buffer, 0, row_width * cinfo.output_height);
tmp = output_buffer;
/* Process data */
while (cinfo.output_scanline < cinfo.output_height) {
(void) jpeg_read_scanlines(&cinfo, buffer, 1);
memcpy(tmp, *buffer, row_width);
tmp += row_width;
}
write_pixel_data(&cinfo, output_buffer, output_file);
free(output_buffer);
(void) jpeg_finish_decompress(&cinfo);
jpeg_destroy_decompress(&cinfo);
/* Close files, if we opened them */
fclose(input_file);
fclose(output_file);
return 0;
}
int main(int argc, char *argv[])
{
if (argc < 3) {
read_jpeg_file("tt.jpg", "tt.bmp");
} else {
read_jpeg_file(argv[1], argv[2]);
}
return 0;
}
程序主要实现将一个jpeg格式文件转换成bmp格式文件，主要参考了libjpeg库中的example.c和djpeg.c
