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Re: Image transformations

From: Richard Copley
Subject: Re: Image transformations
Date: Thu, 13 Jun 2019 20:35:19 +0100

On Thu, 13 Jun 2019 at 20:01, Eli Zaretskii <address@hidden> wrote:
> Date: Thu, 13 Jun 2019 17:12:15 +0100
> From: Alan Third <address@hidden>
> Cc: Alp Aker <address@hidden>, address@hidden

> One thing that may also be confusing is that there are two different
> approaches to this. XRender applies the transforms to the image,
> whereas NS applies the tranforms to the surface the image is to be
> drawn to. I have a suspicion, having read some Windows API
> documentation (but not much) that Windows works the same way as NS.
> This is unfortunate as it’s harder to understand what’s going on.

If everybody and their dog work differently, why are we doing this
according to XRender, and not the other way around?

> The key difference is that for NS I have to invert the transformation
> matrix.

What do you mean by "invert", and where is that NS code?

And if you figured out how to map what XRender does to what NS does,
you probably understand well what the XRender matrix means, right?

> I could be wrong, but it may explain why things aren’t doing what
> you’re expecting. Or it could simply be down to the fact I transposed
> the rows and columns. I think it’s probably a good idea for us to deal
> with the transposition first before making any definite statements on
> this.

Not sure what you mean by "deal with transposition".

I doubt it will end up being important to understand the matrix inverse
and transpose operations.

The difference between NS and XRender is that one stores the matrix in
row-major format and the other in column-major format.

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