Re: [pygame] pygame.pixelcopy.array_to_surface(Dest, Src) not working

On Sat, Dec 24, 2016 at 9:39 AM, Mikhail V <mikhailwas@xxxxxxxxx> wrote:

> --Are you tied to palettal color?

Yes in many cases. I need also palettal rendering for higher
depths. Ideally graphical frontend should support also 16 bit and 32 bit
palettal rendering. And ideally the final colorisation should happen on GPU
namely I send an integer array to video system and video system
must resolve the values to the end format and ideally
this should be done in most effective way.
But this is not what Pygame can do.

So currently for higher depths I use numpy to convert integers
to composite colors in each loop cycle. So Pygame provides only 8 bit
paletted, but actually for many applications it is good enough.
Even if I'll write commercial arcade game, it would not suffer
much from being 8 bit color. Most of the the time it is not
about smooth gradients, it is about action and fun.

What I mean is, you can have an "8-bit" game emulated using 24-bit graphics. You just only use 256 (or fewer) colors instead of the whole 16777216. Once again, this is more performant, both in software (SDL, PyGame) and in hardware (GL, Vulkan).

> The copy buffer trick you're using looks a little suspect to me.

But since there is no other method which is comliant with numpy order,
and as I see now, it works as fast as pixelcopy.
Look what I mean, if I use get_buffer() and then I simply copy a
memory chunk from one place to another(?), it works and the result
shows up correctly. So one numpy *row* is mapped to one image *row*
on the screen.

> In particular, if the internal layouts differ at all (for example, if the NumPy
> array is contiguous, and the SDL surface has row padding, both of which are likely)
> then the copy will fail (fatally crash) in some cases. I don't know whether PyGame
> is smart enough to anticipate that. Odd width/height surfaces,
> and especially truecolor surfaces, would be a reassuring test.

Yes there are a lot of alignment nuances here. As as understand
GPU surface must be memory aligned, so odd shapes must be padded
into aligned rectangels. Especially 24 bit formats makes it
even harder to convert.

It's not alignment so much (IIRC SDL uses malloc on uint8, so it's byte-aligned) as row stride. If the row size doesn't work out to a nice number, then SDL will insert padding bytes. So copying from a contiguous array might create weird striated patterns, while copying from a surface could actually segfault the underlying NumPy backend. So it's not just inelegant--it's unsafe.

Once again, I don't know if PyGame is smart enough to detect this and handle it in the buffer API (and if it did handle it, it would require an allocation+copy, which would make it slow), so I would err on the side of caution.

On Sat, Dec 24, 2016 at 5:12 PM, Mikhail V <mikhailwas@xxxxxxxxx> wrote:

Probably there is more criterias here that I am not aware of
and objective arguments to prefer "FORTRAN" order, apart
from having more traditional [x,y] notation?

The argument I think comes from building/slicing matrices out of (column) vectors. You see this a lot in numerical work. If the row is of pointers, you can build sparse systems that reference underlying vector without doing any copying (you can do this with row data instead, but then you need row vectors, and that would be morally wrong). This is important since building sparse systems can be very slow if you're not careful.

I still avoid FORTRAN order because it's not mathy. E.g., the matrix element "a_{0,2}" should be accessed as "a[0][2]". For an objective argument, I'll note that graphics hardware--in particular VGA/VBE hardware, which influenced latter standards, e.g. HDMI--is row-major, top-to-bottom raster order. This has been hugely influential, and is more-or-less expected today by graphics programmers. It explains everything from most windowing systems today having GUI controls at the top and left, to why GL takes padded scanlines as texture input.

One way or another, at this point, changing the order in PyGame is probably a bad idea (backwards compatibility and suchlike). At the very least, it would needs to be deferred to a major update with breaking API changes.

Ian