【LibJPEG专题系列二十二】提取图像大小

席萌0209

              Section - [22] How can my program extract
                    image dimensions from a JPEGfile?

The header of a JPEG file consists of a series of blocks, called "markers". The image height and width are stored
in a marker of type SOFn (Start Of Frame, type N).  To find the SOFn you must skip over the preceding markers;
you don't have to know what's in the other types of markers, just use their length words to skip over them.  The
minimum logic needed is perhaps a page of C code.  (Some people have recommended just searching for the byte
pair representing SOFn, without paying attention to the marker block structure. This is unsafe because a prior
marker might contain the SOFn pattern, either by chance or because it contains a JPEG-compressed thumbnail
image.  If you don't follow the marker structure you will retrieve the thumbnail's size instead of the main image
size.)  A profusely commented example in C can be found in rdjpgcom.c in the IJG distribution (see part 2, item
15).  Perl code can be found in wwwis, from http://www.tardis.ed.ac.uk/~ark/wwwis/.

席萌0209

                                   Section 22—提取图像尺寸

        JPEG文件的头文件包含一系列的块，被称为“标记”（ markers）。图像的高度和宽度都存储在SOFn
类型（帧图像开始，N型）标记。为了找到SOFn必须跳过前面的标记；你不必知道其他类型的标记包含什么，
只需要使用它们的length words来跳过即可。所需的最低的逻辑也许只是一页C代码。（一些人曾建议只要搜
索一对一对的字节来表示SOFn，而不用把注意力放在块结构化标记上。但这是不保险的，因为以前的标记可
能包含SOFn模式，无论是偶然还是因为它包含了一个JPEG压缩的缩略图。如果不遵循标记结构，你检索的
将是缩略图的大小而非主图像的大小。）在IJG分区（part 2, item 15）的rdjpgcom.c中可以找到很多注释过
的例子。在wwwis中可找到Perl代码，可见于http://www.tardis.ed.ac.uk/~ark/wwwis/。