use luaffifb to make it work in places other than luajit

[jcupitt]

See https://github.com/facebook/luaffifb

[rolandlo]

So should lua-vips work with Lua 5.3 when luaffifb is installed?
I did so and added a replacement for bit.band (bitwise and), which looks like the only thing required from other libraries.
However I get a segmentation fault after a short while:

Lua 5.3.6  Copyright (C) 1994-2020 Lua.org, PUC-Rio
> vips = require "vips"
> vips.Image
table: 0x558ea766a7a0
> im = vips.Image.new_from_file("dog.png")
Speicherzugriffsfehler (Speicherabzug geschrieben)

[jcupitt]

Hi @rolandlo, I think it would need some dev work to make this happen. I've not had time, but if you do, a PR would be great.

[rolandlo]

Seems I managed to get it working on Lua 5.3.6. At least the example from the Readme works when I rebase my patches onto lua-vips version 1.1.9 (commit 4115e8e).

The images created (tiny.jpg and x.png) look fine. For this to work I also had to add some changes to Tekenlight's fork of luaffifb.

So far I haven't had time to test further. Without rebasing on version 1.1.9 I get a segfault when executing

image = image = image + {1,2,3}

from the example.

Let me point out that I don't have much experience with Lua and the Lua C API and have not worked with lua-vips before, so most likely my solution can and should be improved a lot.

The code changes are motivated by the following observations:

• Currently (on master) lua-vips uses unpack which should be table.unpack in Lua 5.3. So I replaced it by unpack or table.unpack
• Currently lua-vips compares gv.gtype (which is of type cdata) with Lua numbers like gvalue.gbool_type, so I used tonumber to convert gv.gtype into a Lua number
• Currently luaffifb (both on the Facebook repo and on Tekenlight's fork) doesn't support things like voperation.call (or more generally <ffi-metatype>.<function name>) since the ctype_mt doesn't support the __index metamethod. So I simply added ctype_index by copying the relevant part from cdata_index. That looks a bit hacky, but it works.

[jcupitt]

That's very cool!

Sorry, I have no free time right now :( After July I should have some time to look at this again.

[rolandlo]

A little update: I have used my patched versions of lua-vips and luaffifb to write a Xournal++ plugin, where images in a Xournal++ document can be manipulated in various ways, see xournalpp/xournalpp#4884. It seems to work fine (using Lua 5.3.6 on Ubuntu 23.04).

[rolandlo]

One more update:

• Rebased on master
• Fixed some segfaults related to comparing with nil instead of ffi.NULL
• All examples in the example directory except of array.lua work now. In array.lua,

local ptype = ffi.typeof("typedef unsigned short int[$][?]", im:bands())

results in the error message:

lua: array.lua:14: unknown type typedef on line 1
stack traceback:
	[C]: in function 'ffi.typeof'
	array.lua:14: in main chunk
	[C]: in ?

In watermark.lua I had to replace division / by integer division // in text:replicate (which requires integer arguments)

[rolandlo]

Fixed by #60.