Python + GStreamer：将视频缩放到窗口

Question

我在将 GStreamer 的视频输出重新缩放到显示视频的窗口尺寸（保持视频的纵横比）时遇到了一些问题。问题是我首先需要预卷视频以便能够通过检索协商的上限来确定其尺寸，然后计算它需要显示的尺寸以适合窗口。一旦我预卷了视频并获得了尺寸上限，我就不能再更改视频的尺寸了。设置新的上限仍会导致视频以其原始大小输出。我必须怎么做才能解决这个问题？

只是为了完整。在当前的实现中，我无法渲染到可以轻松解决此问题的 OpenGL 纹理，因为您可以简单地将输出渲染到纹理并对其进行缩放以适应屏幕。我必须在 pygame 表面上绘制输出，它需要具有正确的尺寸。 pygame 确实提供了缩放其表面的功能，但我认为这样的实现（就像我现在所做的那样）比直接从 GStreamer 检索正确大小的帧要慢（我说的对吗？）

这是我用于加载和显示视频的代码（我省略了主循环的内容）：

def calcScaledRes(self, screen_res, image_res):
    """Calculate image size so it fits the screen
    Args
        screen_res (tuple)   -  Display window size/Resolution
        image_res (tuple)    -  Image width and height

    Returns
        tuple - width and height of image scaled to window/screen
    """
    rs = screen_res[0]/float(screen_res[1])
    ri = image_res[0]/float(image_res[1])

    if rs > ri:
        return (int(image_res[0] * screen_res[1]/image_res[1]), screen_res[1])
    else:
        return (screen_res[0], int(image_res[1]*screen_res[0]/image_res[0]))

def load(self, vfile):
    """
    Loads a videofile and makes it ready for playback

    Arguments:
    vfile -- the uri to the file to be played
    """
    # Info required for color space conversion (YUV->RGB)
    # masks are necessary for correct display on unix systems
    _VIDEO_CAPS = ','.join([
        'video/x-raw-rgb',
        'red_mask=(int)0xff0000',
        'green_mask=(int)0x00ff00',
        'blue_mask=(int)0x0000ff'
    ])

    self.caps = gst.Caps(_VIDEO_CAPS)

    # Create videoplayer and load URI
    self.player = gst.element_factory_make("playbin2", "player")        
    self.player.set_property("uri", vfile)

    # Enable deinterlacing of video if necessary
    self.player.props.flags |= (1 << 9)     

    # Reroute frame output to Python
    self._videosink = gst.element_factory_make('appsink', 'videosink')      
    self._videosink.set_property('caps', self.caps)
    self._videosink.set_property('async', True)
    self._videosink.set_property('drop', True)
    self._videosink.set_property('emit-signals', True)
    self._videosink.connect('new-buffer', self.__handle_videoframe)     
    self.player.set_property('video-sink', self._videosink)

    # Preroll movie to get dimension data
    self.player.set_state(gst.STATE_PAUSED)

    # If movie is loaded correctly, info about the clip should be available
    if self.player.get_state(gst.CLOCK_TIME_NONE)[0] == gst.STATE_CHANGE_SUCCESS:
        pads = self._videosink.pads()           
        for pad in pads:            
            caps = pad.get_negotiated_caps()[0]
            self.vidsize = caps['width'], caps['height']
        else:
            raise exceptions.runtime_error("Failed to retrieve video size")

# Calculate size of video when fit to screen
    self.scaledVideoSize = self.calcScaledRes((self.screen_width,self.screen_height), self.vidsize) 
# Calculate the top left corner of the video (to later center it vertically on screen)  
    self.vidPos = ((self.screen_width - self.scaledVideoSize [0]) / 2, (self.screen_height - self.scaledVideoSize [1]) / 2)

    # Add width and height info to video caps and reload caps
    _VIDEO_CAPS += ", width={0}, heigh={1}".format(self.scaledVideoSize[0], self.scaledVideoSize[1])
    self.caps = gst.Caps(_VIDEO_CAPS)
    self._videosink.set_property('caps', self.caps)  #??? not working, video still displayed in original size

def __handle_videoframe(self, appsink):
    """
    Callback method for handling a video frame

    Arguments:
    appsink -- the sink to which gst supplies the frame (not used)
    """     
    buffer = self._videosink.emit('pull-buffer')        

    img = pygame.image.frombuffer(buffer.data, self.vidsize, "RGB")

    # Upscale image to new surfuace if presented fullscreen
    # Create the surface if it doesn't exist yet and keep rendering to this surface
    # for future frames (should be faster)

    if not hasattr(self,"destSurf"):                
        self.destSurf = pygame.transform.scale(img, self.destsize)
    else:
        pygame.transform.scale(img, self.destsize, self.destSurf)
    self.screen.blit(self.destSurf, self.vidPos)

    # Swap the buffers
    pygame.display.flip()

    # Increase frame counter
    self.frameNo += 1

Answer 1

我很确定您的问题是（自从您提出这个问题以来已经很久了）您从未连接过总线来查看发出的消息。
这个代码通常是这样的：

    def some_function(self):
    #code defining Gplayer (the pipeline)
    #
    # here
        Gplayer.set_property('flags', self.GST_VIDEO|self.GST_AUDIO|self.GST_TEXT|self.GST_SOFT_VOLUME|self.GST_DEINTERLACE)
    # more code
    #
    # finally
        # Create the bus to listen for messages
        bus = Gplayer.get_bus()
        bus.add_signal_watch()
        bus.enable_sync_message_emission()
        bus.connect('message', self.OnBusMessage)
        bus.connect('sync-message::element', self.OnSyncMessage)
#   Listen for gstreamer bus messages
    def OnBusMessage(self, bus, message):
        t = message.type
        if t == Gst.MessageType.ERROR:
            pass
        elif t == Gst.MessageType.EOS:
            print ("End of Audio")
        return True

    def OnSyncMessage(self, bus, msg):
        if msg.get_structure() is None:
            return True
        if message_name == 'prepare-window-handle':
            imagesink = msg.src
            imagesink.set_property('force-aspect-ratio', True)
            imagesink.set_window_handle(self.panel1.GetHandle())    

问题的关键在于为同步消息设置回调，并在该回调中将属性 force-aspect-ratio 设置为 True。
此属性可确保视频始终适合正在显示的窗口。

请注意 self.panel1.GetHandle() 函数命名您正在显示视频的面板。

感谢您继续前进，但希望这会帮助其他人翻阅档案。