使用libjpeg.framework压缩UIImage

+(void)writeFile:(NSString *)filePath withQuality:(int)quality
{

//初始化图片参数
UIImage *image=[UIImage imageNamed:@"testimg.bmp"];
JSAMPLE *image_buffer = (JSAMPLE *)[self RGBDataForImage:image];
int image_width = image.size.width;
int image_height= image.size.height;
int image_components=3;
//输出图片参数
const char * filename=[filePath UTF8String];
/* 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 */

/* 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);

/* 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.
*/
if ((outfile = fopen(filename, "wb")) == NULL) {
fprintf(stderr, "can't open %s\n", filename);
exit(1);
}
jpeg_stdio_dest(&cinfo, outfile);

/* 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 = image_width;     /* image width and height, in pixels */
cinfo.image_height = image_height;
cinfo.input_components =image_components;        /* # 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 */);

/* 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);

/* 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 = image_width * 3;    /* JSAMPLEs per row in image_buffer */

while (cinfo.next_scanline < cinfo.image_height) {
/* 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_buffer[cinfo.next_scanline * row_stride];
(void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
}

/* Step 6: Finish compression */

jpeg_finish_compress(&cinfo);
/* After finish_compress, we can close the output file. */
fclose(outfile);

/* Step 7: release JPEG compression object */

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

+(unsigned char *)RGBDataForImage:(UIImage *)image
{
// Create a pixel buffer in an easy to use format
CGImageRef imageRef = [image CGImage];
int width = (int)CGImageGetWidth(imageRef);
int height = (int)CGImageGetHeight(imageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

unsigned char *m_PixelBuf = malloc(sizeof(unsigned char) * height * width * 4);
unsigned char *outPixel= malloc(sizeof(unsigned char) * height * width * 3);

int bytesPerPixel = 4;
int bytesPerRow = bytesPerPixel * width;
int bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(m_PixelBuf, width, height,
bitsPerComponent, bytesPerRow, colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);

CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGContextRelease(context);

for (int y=0; y<height; y++)
{
for (int x=0; x<width; x++)
{
int byteIndex = (bytesPerRow * y) + x * bytesPerPixel;
int outIndex=(3*width*y)+x*3;
outPixel[outIndex+0]= m_PixelBuf[byteIndex+0];
outPixel[outIndex+1]= m_PixelBuf[byteIndex+1];
outPixel[outIndex+2]= m_PixelBuf[byteIndex+2];
}
}

CGColorSpaceRelease(colorSpace);
free(m_PixelBuf);
free(outPixel);
return outPixel;

}