Re: [Mlt-devel] Consumer embedding, gtk2_preview

[Dan D. <da...@de...>]

please include the mailing list

On Mon, Jan 21, 2013 at 8:16 PM, Harry van Haaren <har...@gm...> wrote:
> On Mon, Jan 21, 2013 at 10:05 PM, Dan Dennedy <da...@de...> wrote:
>>
>> If the consumer is not going to the drawing,
>> then you use an audio-only consumer such as sdl_audio, and in your app
>> you listen to the mlt_event "consumer-frame-show". The event handler
>> will receive a mlt_frame. Then you call mlt_frame_get_image() on that
>> frame to get the image data pointer. Kdenlive and Shotcut's
>> OpenGL-based video widgets are the main examples of that technique.
>
>
> Ok that makes sense, implemented.
>
> I don't fully understand the memory management when using mlt_frame.

It is reference counted. If you want to use the frame or any of its
contents outside of the callback function, then you must increment the
reference with mlt_properties_inc_ref() and then mlt_frame_close() to
release your reference when you are done with it.

>   uint8_t* image = NULL;
> passing in &image makes mlt_frame set the pointer to the location of its
> internal frame memory?

just the image part of that internal frame memory. The image pointer
is only valid as long as the frame is valid (not all references are
released).

> and then its up to the caller to do something useful with that pointer?

yes

> I'm getting segfaults when I try to copy that memory using memcpy:
>   mlt_frame_get_image (frame, &image, &format, &width, &height, false);
>   size_t size = 320*240*4;

This size is only valid if you set format to mlt_image_rgb24a before
the call to mlt_frame_get_image(). The width and height should also be
set to 320 and 240 before the call if the mlt_profile is not 320x240.
If you set width and height to 0 before the call, then you get back
the width and height of the profile.

>   data = (unsigned char*) malloc( size );
>   memcpy( data, image, size );
>
> GDB output is as follows (seems fine to me)
> 79      memcpy( data, image, width * height * 3 );
> (gdb) p data
> $1 = (void *) 0x7fffffffea40
> (gdb) p image
> $2 = (uint8_t *) 0x7fffb8291830 ""
> (gdb) p width
> $3 = 320
> (gdb) p height
> $4 = 240
>
> Can you make any sense of what's going on? I don't have a clue anymore. -H