Video format callbacks

[sherington]

I've been using libvlc_video_set_format_callbacks successfully for some time, it works but I have a question about the implementation.

I added some debug statements recently and noticed the following behaviour:

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setup chroma=VAOP, size=1280x720 cleanup setup chroma=VDV0, size=1280x720 cleanup setup chroma=I420, size=1280x738

So, when I play media (in this case an mp4), I get the format callback three times with different parameters (two of them get a corresponding cleanup), before playback starts.

My callback implementation each time simply sets up the buffer format to be RV32 with same width and height specified.

The question then is while my code appears to work correctly, I'm wondering if this behaviour is normal or if I'm supposed to be doing something differently with these different buffer formats?

[Rémi Denis-Courmont]

VLC is probing different formats, until it finds one that works.

[sherington]

Thank you for replying.

What I don't get is that in the first callback VLC probes VAOP and I return RV32, why does it then keep going and probing the other two formats? Why doesn't it accept the RV32 I returned the first time?

I'm just trying to determine if my implementation/understanding of this callback is wrong.

[Rémi Denis-Courmont]

It cannot find a working path to convert VAOP to RV32.

[sherington]

It's clear now, thanks a lot.

[Tibweizh]

Hi Sherington,

I'm facing some problems with my implementation of video format callback, could you please share yours ? Or tell me what you did ?

Thanks

[sherington]

What do you want to know? My implementation is deeply integrated in a Java library and I can't just cleanly pull the code out unfortunately.

[Tibweizh]

In your format setup callback, your basics steps. Because mine does not work as I wish

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unsigned VLCReader::cb_setup(void **opaque, char *chroma, unsigned *width, unsigned *height, unsigned *pitches, unsigned *lines) { VLCReader *p = (VLCReader*)opaque; memcpy(chroma, "RV24", sizeof("RV24") - 1); p->m_width = *width; p->m_height = *height; *pitches = (*width) * 3; *lines = *height; return 1; // indicates the number of buffers allocated }

This version is crashing but when I remove the multiplication it works but video does not show up entirely (only squares on the top part of my window)

[sherington]

From a quick look, the only thing I do different is use RV32 format and a pitches value of width*4.

(I'm not saying RV32 is necessary but it was the only useful format I could get to work with the UI toolkit I was using).

You are definitely allocating enough memory for your *3 buffer size?

[Tibweizh]

Humm I think so but I'll work on this point, thanks mate !

If I succeed I'd tell you

[Tibweizh]

Ok definitely it came from memory allocation, my buffer was too small but... I found something strange with libvlc_video_get_size

Here it is : the size obtained by libvlc_video_get_size was not the same as the one I get from the calllback setup (unsigned *width and unsigned *height). Is this a bug or whatever else ?

Thanks again mate !

[sherington]

I am no expert on this, but I suppose certain buffer formats might require e.g. aligning to 8/16/32 bit boundaries and that could explain the discrepancies you are seeing.

So e.g. if the video height is 500, you'd actually need a buffer height of 504 to hold it (504/8 = 63.0 whereas 500/8 = 62.5). I'm just guessing.

In practical terms does it matter? I don't think so.

[unidan]

It's the difference between visible_size (video size in VLC) and real buffer size (buffer size in VLC). What you get in setup is the size of the video without the padding.

[Tibweizh]

Oh that's it ! Thank you very much Unidan, it's quite clear now for me