libjpeg压缩图片，产生的缩略图颜色有失真

[xuhui_7810]

各位大神：
我用libjpeg压缩图片，产生的缩略图颜色有失真，原图和缩略图如下：


我的代码如下：

struct my_error_mgr {
  struct jpeg_error_mgr pub;	/* "public" fields */

  jmp_buf setjmp_buffer;	/* for return to caller */
};

typedef struct my_error_mgr * my_error_ptr;



//读取Jpeg图片的数据并返回，如果出错，返回NULL
unsigned char* App::ReadJpeg(char* path, int& width, int& height)
{
	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);

	 struct my_error_mgr my_err;

	 info.err = jpeg_std_error(&my_err.pub);
	 my_err.pub.error_exit = my_error_exit;

	 /* Establish the setjmp return context for my_error_exit to use. */
	 if (setjmp(my_err.setjmp_buffer)) {
		 /* If we get here, the JPEG code has signaled an error.
		 * We need to clean up the JPEG object, close the input file, and return.
		 */
		 printf("Error occured\n");
		 jpeg_destroy_decompress(&info);
		 fclose(file);
		 return NULL;
	 }

	jpeg_create_decompress( &info ); //fills info structure
	jpeg_stdio_src( &info, file );        //void

	int ret_Read_Head = jpeg_read_header( &info, 1 ); //int

	if(ret_Read_Head != JPEG_HEADER_OK){
		printf("jpeg_read_header failed\n");
		fclose(file);
		jpeg_destroy_decompress(&info);
		return NULL;
	}

	bool bStart = jpeg_start_decompress( &info );
	if(!bStart){
		printf("jpeg_start_decompress failed\n");
		fclose(file);
		jpeg_destroy_decompress(&info);
		return NULL;
	}
	int w = width = info.output_width;
	int h = height = info.output_height;
	int numChannels = info.num_components; // 3 = RGB, 4 = RGBA
	printf("ReadJpeg out_color_space =%d\n", info.out_color_space);
	unsigned long dataSize = w * h * numChannels;

	// read RGB(A) scanlines one at a time into jdata[]
	unsigned char *data = (unsigned char *)malloc( dataSize );
	if(!data) return NULL;

	unsigned char* rowptr;
	while ( info.output_scanline < (unsigned)h )
	{
		rowptr = data + info.output_scanline * w * numChannels;
		jpeg_read_scanlines( &info, &rowptr, 1 );
	}

	jpeg_finish_decompress( &info );    

	fclose( file );

	return data;
}

/*参数为：
 *返回图片的宽度(w_Dest),
 *返回图片的高度(h_Dest),
 *返回图片的位深(bit_depth),
 *源图片的RGB数据(src),
 *源图片的宽度(w_Src),
 *源图片的高度(h_Src)
 */
unsigned char* App::do_Stretch_Linear(int w_Dest,int h_Dest,int bit_depth,unsigned char *src,int w_Src,int h_Src)
{
	int sw = w_Src-1, sh = h_Src-1, dw = w_Dest-1, dh = h_Dest-1;
	int B, N, x, y;
	int nPixelSize = bit_depth/8;
	unsigned char *pLinePrev,*pLineNext;
	unsigned char *pDest = new unsigned char[w_Dest*h_Dest*bit_depth/8];
	unsigned char *tmp;
	unsigned char *pA,*pB,*pC,*pD;

	for(int i=0;i<=dh;++i)
	{
		tmp =pDest + i*w_Dest*nPixelSize;
		y = i*sh/dh;
		N = dh - i*sh%dh;
		pLinePrev = src + (y++)*w_Src*nPixelSize;
		//pLinePrev =(unsigned char *)aSrc->m_bitBuf+((y++)*aSrc->m_width*nPixelSize);
		pLineNext = (N==dh) ? pLinePrev : src+y*w_Src*nPixelSize;
		//pLineNext = ( N == dh ) ? pLinePrev : (unsigned char *)aSrc->m_bitBuf+(y*aSrc->m_width*nPixelSize);
		for(int j=0;j<=dw;++j)
		{
			x = j*sw/dw*nPixelSize;
			B = dw-j*sw%dw;
			pA = pLinePrev+x;
			pB = pA+nPixelSize;
			pC = pLineNext + x;
			pD = pC + nPixelSize;
			if(B == dw)
			{
				pB=pA;
				pD=pC;
			}

			for(int k=0;k<nPixelSize;++k)
			{
			    pA++;
				pB++;
				pC++;
				pD++;
				*(tmp++) = ( unsigned char )( int )(
					( B * N * ( (*pA) - (*pB) - (*pC) + (*pD) ) + dw * N * (*pB)
					+ dh * B * (*pC) + ( dw * dh - dh * B - dw * N ) * (*pD)
					+ dw * dh / 2 ) / ( dw * dh ) );
			}
		}
	}
	return pDest;
}

bool App::write_JPEG_file (char * filename, unsigned char* image_buffer, int quality,int image_height, int image_width)
{

	if(filename == NULL || image_buffer == NULL) return false;

	struct jpeg_compress_struct cinfo;
	struct jpeg_error_mgr jerr;
	FILE * outfile;		/* target file */
	JSAMPROW row_pointer[1];	/* pointer to JSAMPLE row[s] */
	int row_stride;		/* physical row width in image buffer */
	cinfo.err = jpeg_std_error(&jerr);
	/* Now we can initialize the JPEG compression object. */
	jpeg_create_compress(&cinfo);

	if ((outfile = fopen(filename, "wb")) == NULL) {
		fprintf(stderr, "can't open %s\n", filename);
		return false;
	}
	jpeg_stdio_dest(&cinfo, outfile);

	cinfo.image_width = image_width; 	/* image width and height, in pixels */
	cinfo.image_height = image_height;
	cinfo.input_components = 3;		/* # of color components per pixel */
	cinfo.in_color_space = JCS_EXT_BGR; 	/* colorspace of input image */

	jpeg_set_defaults(&cinfo);

	

    cinfo.comp_info[0].v_samp_factor = 1;
    cinfo.comp_info[0].h_samp_factor = 1;	
	jpeg_set_quality(&cinfo, quality, TRUE /* limit to baseline-JPEG values */);

	jpeg_start_compress(&cinfo, TRUE);

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

	while (cinfo.next_scanline < cinfo.image_height) {
		row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
		(void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
	}

	jpeg_finish_compress(&cinfo);
	fclose(outfile);

	jpeg_destroy_compress(&cinfo);

	return true;
}

//生成图片的缩略图（图片的一个缩小版本）
 bool App::generate_image_thumbnail(char* inputFile, char* outputFile)
{
	if(inputFile == NULL || outputFile == NULL) return false;

	//读取jpeg图片像素数组
	int w=0,h=0;
	unsigned char* buff = ReadJpeg(inputFile,w,h);
	if(buff == NULL) {
		printf("ReadJpeg Failed\n");
		return false;
	}
	printf("generate_image_thumbnail src w=%d, h=%d\n", w, h);
	//缩放图片，缩放后的大小为(tb_w,tb_h)
	int tb_w = 384, tb_h = 208;
	unsigned char * img_buf = do_Stretch_Linear(tb_w,tb_h,24,buff,w,h);
	free(buff);

	//将缩放后的像素数组保存到jpeg文件
	bool bRetWrite = write_JPEG_file(outputFile,img_buf,90, tb_h, tb_w);
	delete[] img_buf;

	if(bRetWrite){
		return true;
	}else{
		printf("GenerateImageThumbnail: write failed\n");
		return true;
	}
}

/*
 * Here's the routine that will replace the standard error_exit method:
 */

void
App::my_error_exit (j_common_ptr cinfo)
{
  /* cinfo->err really points to a my_error_mgr struct, so coerce pointer */
  my_error_ptr myerr = (my_error_ptr) cinfo->err;

  /* Always display the message. */
  /* We could postpone this until after returning, if we chose. */
  (*cinfo->err->output_message) (cinfo);

  /* Return control to the setjmp point */
 longjmp(myerr->setjmp_buffer, 1);
}

void main(void)
{
        generate_image_thumbnail("/src.jpg", "/dest.jpg");
}

之前我将cinfo.in_color_space 设为 JCS_RGB，一样有颜色失真，之前是红色和蓝色反了，所以改成了cinfo.in_color_space = JCS_EXT_BGR;，但这样改之后，颜色变成了这个样子。请问现在该怎么处理呢？ （我这边不方便编译libjpeg，改不了它的源码）

[xuhui_7810]

JPEG 算法本身，将通常屏幕上表示颜色的 RGB（红绿蓝）数值，转换为 YUV 数值（亮度，蓝色分量，红色分量）。正常情况下这个算法是轻微有损的。 可以试试原生的libjpeg-turbo 你好，请问libjpeg有没有其他方法有所改善呢？我这压缩出来的缩略图，看起来不像是轻微有损，颜色失真比较严重。如果是轻微有损，作为缩略图还勉强可以接受

[xuhui_7810]

各位大神，不要让这贴子沉了啊

[cocoabird]

JPEG 算法本身，将通常屏幕上表示颜色的 RGB（红绿蓝）数值，转换为 YUV 数值（亮度，蓝色分量，红色分量）。正常情况下这个算法是轻微有损的。 可以试试原生的libjpeg-turbo

[xuhui_7810]

请大神们再帮忙分析一二，

[xuhui_7810]

请大神们指点一二