aseprite | A parser for the aseprite sprite editor files | Editor library

The solution I ended up with: writing your own blend mode with glsl and using shaders. This way you can create and formulas without the need to think about how to bring it to sfml, because it already supports shaders, that can be easily passed to any draw call.

From the Lua Reference Manual, like on page 1-2

There are eight basic types in Lua: nil, boolean, number, string, function, userdata, thread, and table.

The type userdata is provided to allow arbitrary C data to be stored in Lua variables. A userdata value represents a block of raw memory. There are two kinds of userdata: full userdata, which is an object with a block of memory managed by Lua, and light userdata, which is simply a C pointer value. Userdata has no predefined operations in Lua, except assignment and identity test. By using metatables, the programmer can define operations for full userdata values (see §2.4). Userdata values cannot be created or modified in Lua, only through the C API. This guarantees the integrity of data owned by the host program and C libraries.

So no, there is not just tables.

How might I print all attributes and method names available to me in this 'object'?

image is a ImageObj, but Image should be a table that provides functions that you can use on ImageObjs.

Try for k,v in pairs(Image) do print(k,v) end

Or simply refer to the API documentation.

https://github.com/aseprite/api/blob/master/api/image.md#image

Just in case, make sure all submodules are initialized:

use
print(x, f, "===", docs)

instead of
print(x, f, "===", os.path.dirname(docs))

I fixed this by changing the environment variable CC to cl