小编Ond*_*rej的帖子

我需要从网络摄像头的视频流中取出一帧并将其写入文件。在 ffmpeg 中我可以这样做：

ffmpeg -i rtsp://10.6.101.40:554/video.3gp -t 1 img.png

我的 GStreamer 命令：

gst-launch-1.0  rtspsrc location="rtsp://10.6.101.40:554/video.3gp" is_live=true ! decodebin ! jpegenc ! filesink location=img.jpg

问题是，gstreamer进程一直运行并且没有结束。如何在文件写入后仅拍摄一帧并强制流关闭？是否可以从命令行执行此操作，或者我应该在 c/python 等中进行编码...

多谢。

测试代码:

(define-syntax (test-d stx)
  #'(begin 
      (define (callme a)
        (writeln a))))

(define-syntax (test-e stx)
  (datum->syntax stx '(begin 
                         (define (callme2 a)
                           (writeln a)))))


> (test-d)
> (callme 1)
. . callme: undefined;
 cannot reference an identifier before its definition
> (test-e)
> (callme2 1)
1

我不明白test-d和test-e的区别.他们看起来一样平等.仍然,没有定义callme.

甚至宏步也说它是一样的.

Expansion finished
(module anonymous-module racket
  (#%module-begin
   (define-syntax (test-d stx)
     #'(begin (define (callme a) (writeln a))))
   (define-syntax (test-e stx)
     (datum->syntax
      stx
      '(begin (define (callme2 a) (writeln a)))))
   (begin (define (callme a) (writeln a)))
   (begin (define (callme2 …