C++ libjpeg:如何获得合适的图像解压缩大小?
我有这样一个常见的代码:C++ libjpeg:如何获得合适的图像解压缩大小?,c++,jpeg,compression,libjpeg,C++,Jpeg,Compression,Libjpeg,我有这样一个常见的代码: struct jpeg_decompress_struct cinfo; jpeg_create_decompress(&cinfo); jpeg_stdio_src(&cinfo, infile); jpeg_read_header(&cinfo, TRUE); cinfo.scale_num = ?; cinfo.scale_denom = ?; needed_width = cinfo.output_width; needed_heig
struct jpeg_decompress_struct cinfo;
jpeg_create_decompress(&cinfo);
jpeg_stdio_src(&cinfo, infile);
jpeg_read_header(&cinfo, TRUE);
cinfo.scale_num = ?;
cinfo.scale_denom = ?;
needed_width = cinfo.output_width;
needed_height = cinfo.output_height;
如何选择缩放数量和缩放密度参数将图像缩放到所需大小。例如,我想把它缩小一倍
如果我设置了scale\u num=1
,scale\u denom=2
。结果是:(1802x1237)到(258x194)
文件说:
Scale the image by the fraction scale_num/scale_denom. Default is
1/1, or no scaling. Currently, the only supported scaling ratios
are 1/1, 1/2, 1/4, and 1/8.
但当我设定这样的比例时,我并没有得到需要的结果
因此问题是:如何设置
scale\u num
和scale\u denom
以获得与所需最大尺寸相似的图像。您应该调用jpeg\u calc\u output\u dimensions(cinfo)以获得正确的输出宽度和输出高度值
为了计算系数刻度,我通常会这样做:
unsigned int intlog2(unsigned int val) {
int targetlevel = 0;
while (val >>= 1) ++targetlevel;
return targetlevel;
}
// ....
// for example,
const int width = 5184;
const int height = 3456;
unsigned int needed_width = 640;
unsigned int needed_height = 480;
unsigned int wdeg = intlog2(width / needed_width);
unsigned int hdeg = intlog2(height / needed_height);
unsigned int scale_den = std::min(1 << (std::min)(wdeg, hdeg), 8);
unsigned int result_width = width / scale_den;
unsigned int result_height = height / scale_den;
unsigned int intlog2(unsigned int val){
int targetlevel=0;
而(val>>=1)++targetlevel;
返回目标级别;
}
// ....
//比如说,,
常数整数宽度=5184;
const int height=3456;
所需的无符号整数_宽度=640;
所需的无符号整数_高度=480;
无符号int wdeg=intlog2(宽度/所需宽度);
无符号int hdeg=intlog2(高度/所需高度);
unsigned int scale\u den=std::min(1您应该调用jpeg\u calc\u output\u dimensions(cinfo)以获得正确的输出宽度和输出高度值
为了计算系数刻度,我通常会这样做:
unsigned int intlog2(unsigned int val) {
int targetlevel = 0;
while (val >>= 1) ++targetlevel;
return targetlevel;
}
// ....
// for example,
const int width = 5184;
const int height = 3456;
unsigned int needed_width = 640;
unsigned int needed_height = 480;
unsigned int wdeg = intlog2(width / needed_width);
unsigned int hdeg = intlog2(height / needed_height);
unsigned int scale_den = std::min(1 << (std::min)(wdeg, hdeg), 8);
unsigned int result_width = width / scale_den;
unsigned int result_height = height / scale_den;
unsigned int intlog2(unsigned int val){
int targetlevel=0;
而(val>>=1)++targetlevel;
返回目标级别;
}
// ....
//比如说,,
常数整数宽度=5184;
const int height=3456;
所需的无符号整数_宽度=640;
所需的无符号整数_高度=480;
无符号int wdeg=intlog2(宽度/所需宽度);
无符号int hdeg=intlog2(高度/所需高度);
unsigned int scale\u den=std::min(1您的输入宽度和高度以及所需的宽度和高度是多少,所需的高度是多少?3264 x 2448到1024 x 768,结果是1632 x 1224。但是libjpeg可以为此图像加载例如1224 x 918。您应该使用1632 x 1224(scale\u den=2)读取图像,然后执行调整大小到所需大小输入的宽度和高度是多少?所需的宽度和高度是多少?3264 x 2448到1024 x 768,结果是1632 x 1224。但是libjpeg可以为该图像加载例如1224 x 918。您应该使用1632 x 1224(比例=2)读取图像,然后执行调整大小到所需大小