Re: Testing native image scaling

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 19 Jan 2019 21:45:43 +0000
> From: Alan Third <alan@idiocy.org>
> Cc: emacs-devel@gnu.org
> 
> On Sat, Jan 19, 2019 at 11:31:34AM +0200, Eli Zaretskii wrote:
> > Alan, could you please tell how you tested native image scaling with
> > the XRENDER extension, and perhaps show some Lisp or existing commands
> > you used for that?  E.g., did the features in thumbs.el work for you
> > in a build without Imagemagick?
> 
> I just used a fairly simple series of commands like this:
> 
> (setq i (create-image "~/image.png" nil nil :scale 0.5))
> (insert-image i)
> (setq ii (create-image "~/image.png" nil nil :scale 0.5))
> (insert "\n")
> (insert-image ii)

Thanks, this was very helpful.  (Actually, just create-image with one
argument is enough: after inserting it, '+' or '-' on the image will
interactively resize it.)

> It look like thumbs.el uses ImageMagick’s convert program directly, so
> it won’t be affected by native scaling.

Ah, okay, I missed that.  image-dired seems to do the same.  I guess
we should at some point update those (and others) to use the native
resizing.

> > I tried to implement this for MS-Windows, but I guess my understanding
> > of the internal workings of this is incomplete/incorrect, or my code
> > is buggy (or both), because I don't seem to be able to cause Emacs to
> > exercise the code when the original image's size and the size
> > requested by scaling differ.  For example, I thought that when scaling
> > is requested, x_set_image_size should be called and compute image
> > dimensions different from the original img->height and img->width, but
> > I seem to be unable to see this.  What am I missing?  Could you
> > perhaps describe the flow of calls when, e.g., the user types '+' on
> > an image in image-mode, and Emacs scales the image at point?
> 
> I think you understand this right.
> 
> x_set_image_size calculates the new sizes, does the conversion, and
> writes those new sizes back into struct image, over‐writing the
> original sizes.
> 
> For NS it also asks the NSImage back‐end to scale the image using
> ns_image_set_size, which in effect does the actual scaling.
> 
> For XRender it sets up an affine transformation matrix, and applies it
> to img->picture, which also in effect does the actual scaling.
> 
> The compositing functions in nsterm.m and xterm.c don’t need to know
> the original image size, just the new size, and NSImage/XRender
> handles the rest.

Well, on w32, the implementation actually resizes when it draws.
Maybe that's sub-optimal, but I know next to nothing about w32 image
display, so what I got looks definitely fine for my ignorance.

So we now have native resizing on all major platforms.

> We could use XRender to rotate images if we really wanted to, and the
> NS port already supports it.

Where's the NS support for that?  AFAICT, :rotate is only handled in
ImageMagick specific portions of the code, what did I miss?

Thanks.