[FFmpeg-devel] [RFC] Affine video transformation filter

[Michael Niedermayer]

On Wed, Jul 04, 2012 at 01:08:56PM -0600, Michael Bradshaw wrote:
> I've been thinking a bit about adding an affine video transformation
> filter, but I could use some feedback on this.
> 
> Originally, I just wanted to add a rotation video filter. There are
> rotting patches on the mailing list for this, but I thought I might
> start from scratch. While I'm really curious to see what kind of
> results Dartmouth's 1996 paper "High Quality Alias Free Image
> Rotation" [1] provides, I decided a simple rotation matrix (with
> nearest, bilinear, etc. sampling/interpolation methods) would be a
> good place to start and could be used as a baseline comparison to
> other rotation algorithms. And if I'm using a rotation matrix, I might
> as well just do a proper affine transformation matrix and provide
> additional operations.
> 
> However, I'm not sure how to best handle the overlapping functionality
> of affine transformations and existing filters. We already have
> transpose, scale, v/hflip (and maybe others?) video filters, which
> makes reflection and scaling with an affine transform redundant.
> Additionally, I'm thinking of how potential future filters (maybe an
> individual rotation filter that uses a particular algorithm) could
> overlap with this. It's this redundancy that I'm trying to decide how
> to handle, and where I really need the most input on.
> 
> Individual filters (like the scale and transpose filters) have the
> advantage that they can be optimized for speed and quality. The
> general purpose affine transformation filter would have the advantage
> (aside from providing new functionality like rotation and shearing)
> that it could combine several operations into one filter and perform
> all of them in a single pass (so a
> vflip->rotate->translate->scale->crop filter could just become
> affine->crop). I don't know how realistic it is that someone would
> apply all those affine transformations to one video, however.
> 
> What are your thoughts on the overlapping functionality? Should some
> note along the lines of "if you are applying only a single
> affine/linear transformation with an existing corresponding filter,
> consider using that one filter, as it is likely optimized in its speed
> and quality for that transformation" be put in the docs or something?
> Should overlapping functionality be removed (i.e. disable scaling)
> from the affine transformation filter? I'm leaning towards the former
> rather than the latter.
> 
> While I've got your attention, what seems like a sane way to pass
> arguments to an affine transformation filter? I'm thinking something
> along the lines of
> "affine=scale=1.1\,1.3:rotate=15:translate=4\,10:shear=1.2\,2:reflect=-1\,0:sampling=bilinear"
> for example where order matters and options can be repeated (and maybe
> allowing "affine=matrix=x11:x12:x13:x21:x22:x23:x31:x32:x33"). I
> haven't thought of a way to specify the background fill color that I
> like, though.
> 
> Your thoughts are appreciated.

this sounds similar to libmpcodecs/vf_perspective.c


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