[Spice-devel] [PATCH spice 1/3] dcc_compress_image: Handle NULL drawable

[Christophe Fergeau]

On Thu, Jan 14, 2016 at 10:27:02AM -0500, Frediano Ziglio wrote:
> Had a small discussion with Pavel.
> We agree that original code is quite complicated and is hard to understand
> the final compression format used.
> 
> So we would like to have some public discussion about the topic.
> 
> I personally agree we should have a single code deciding the compression
> to use.

I definitely agree here. For one, having different compression being
used depending on whether the qxl driver is used or not is unexpected
(eg if you set image compression to glz, lz will still be used during
initial bootup, and then will 'switch' to glz later on. I haven't looked
at the code, so there might be good reasons for that).

> 
> This is the list of actual compressions:
> - AUTO_GLZ;
> - AUTO_LZ;
> - QUIC;
> - GLZ;
> - LZ;
> - LZ4.
> A client can also decide to disable compression.
> 
> The AUTO_XXX looks like they should use QUIC as a fallback if XXX is not
> possible or if an image with high graduality is detected.

(side question, do we have numbers on compression ratio and cpu usage
for quic/lz/glz/lz4?)

> Is not clear when JPEG is used. Looks like is used instead of QUIC if lossy
> compression is enabled. But not always (for instance when we are sending an
> image with red_marshall_image and we selected QUIC as default compression).
> The LZ/GLZ algorithms do not support images with extra stride. But the
> fallback if this happens is not clear (can be none or QUIC).
> For LZ4 there is a client capability (SPICE_DISPLAY_CAP_LZ4_COMPRESSION) but
> there is no such capability for LZ/GLZ. Looks like LZ/GLZ are used only if
> AUTO_GLZ/AUTO_LZ/GLZ/LZ are selected (so the image compression setting in
> this case define the capability).

lz/glz have been there forever, lz4 was added 'recently', so
client/servers have to be able to know whether lz4 is supported or not.
That's the reason there is a lz4 cap, but no lz/glz cap.

Christophe
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