image - 我应该在数据流的最开始期待 JPEG SOI 标记吗？

Question

... 或者我应该深入到数据流中寻找 0xFF 0xD8序列？

来自 this Q ，我已经了解到什么 APPn 不必立即遵循 SOI。是否存在符合规范的 JPEG 情况，其中 SOI 位置 != 流的开头？

规范中的引用(附件 B，第 1.1.2 节):

Markers serve to identify the various structural parts of the compressed data formats. Most markers start marker segments containing a related group of parameters; some markers stand alone. All markers are assigned two-byte codes: an X’FF’ byte followed by a byte which is not equal to 0 or X’FF’ (see Table B.1). Any marker may optionally be preceded by any number of fill bytes, which are bytes assigned code X’FF’.

Answer 1

libjpeg在 SOI 之前不允许垃圾:

/* Like next_marker, but used to obtain the initial SOI marker. */
/* For this marker, we do not allow preceding garbage or fill; otherwise,
* we might well scan an entire input file before realizing it ain't JPEG.
* If an application wants to process non-JFIF files, it must seek to the
* SOI before calling the JPEG library.
*/

发件人: Random libjpeg mirror .

例如。 go implementation也不允许前面的垃圾。

但是，如果有疑问，请坚持 Postel 定律:

Be liberal in what you accept, and conservative in what you send

虽然，您不想太自由，否则您最终可能不是从流中提取实际的 JPEG，而是提取嵌入的 EXIF 缩略图或类似的东西。