Re: Image transforms as a benchmark?

[Arthur Miller]

Alan Third <alan@idiocy.org> writes:

> On Sun, Sep 12, 2021 at 01:45:39PM +0200, Arthur Miller wrote:
>> 
>> I tried to make another little benchmark, I saw with optimization flags, that
>> quite some loops have got unrolled and vectorized in image.c, so I wanted to 
>> see
>> if it matters when doing some transforms on images. I tested so far just with
>> svg.
>> 
>> I wonder if image-rotate is handled completely by external libraries? I see
>> no effect on performance, regardless of how many time I rotate some image. 
>> Is it
>> same situation for scaling down? I see big difference when scaling up images 
>> so
>> I guess that is handled by Emacs own code?
>
> SVG is probably not a great example for testing image transforms, at
> least if you're using the master branch.

I wanted to make few benchmarks that are relevant to normal use. I do some
searches, replacements and similar in a big text buffer (Plat's dialoagues as I
posted a day before), and I did some symbol lookups in Emacs lisp sources. SVG
is getting a bit of uprise lately, so thought it might matter. But I'll guess
I'll skip SVG then.

> Image scaling in SVG is handled at the time the image is rasterized,
> so if you ask for the image to be doubled in size, the rasterizer
> creates a bitmap that is twice the size.

Yes, I saw the code. But I wasn't sure if it does upscaling itself, or it
outsources the entire venture to librsvg & co. I understand the most of image
operations are handled by 3rd party libraries. I saw some code for image
transforms in image.c, but I haven't looked much where it is used and so, maybe
I should have :).

Why is it so drammatic difference then when scaling up compared to scaling down?
It takes like 5 seconds to do that loop in scale-up test, but just a fraction of
a second when scaling down. I tested little different versions, and I see the
effect, so scaling is done.

> A PNG, for example, is not scaled in this way. When we ask the
> graphics toolkit to draw it, we ask for to draw it at twice the size.
> Ideally this should be done in the graphics hardware, so Emacs only
> ever handles the unscaled image data.
>
> The same holds for rotation for both SVG and bitmap formats. The
> toolkit is asked to perform the final rotation, ideally in hardware.
>
> I say toolkit, but in X it's actually the X server itself we ask
> through XRender, which as described above should hand that off to the
> graphics hardware.
>
> I'm unsure what loops might be unrolled within image.c, I'd guess it's
> probably loops relating to creating images and image masks.
>
> You could perhaps try stepping through the frames of an animated GIF.
> Our rendering algorithm is rather... inefficient.

I am not sure how to write such test. I'll see, never really worked with gifs in
Emacs.


Thanks both for clarifications.