ogt_vox: specify the target coordinate system and do the conversions inside the lib

[mgerhardy]

that would most likely ease the life of the users that are doing this on their own each time. I have a lot of code doing this conversion and I suppose a lot of other users suffer from the same code duplication.

[jpaver]

I'm not sure we should scope the library to do coordinate system conversions within the existing APIs. I think it's probably okay if you added an API that would modify the transforms in a writeable scene, but I'd have to see the API first to see whether it's likely to be broadly useful for many folk. Any examples of what you had in mind?

[mgerhardy]

it could also go into a separated ogt_vox_tools.h header. Or if the context discussion from the other tickets gets implemented, it would also be possible to add this to the context without changing any api.

[mgerhardy]

the ufbx library allows to specify the coordinate axes like this

// Coordinate axes the scene is represented in.
// NOTE: `front` is the _opposite_ from forward!
typedef struct ufbx_coordinate_axes {
    ufbx_coordinate_axis right;
    ufbx_coordinate_axis up;
    ufbx_coordinate_axis front;
} ufbx_coordinate_axes;

and defines some defaults like this

const ufbx_coordinate_axes ufbx_axes_right_handed_y_up = {
    UFBX_COORDINATE_AXIS_POSITIVE_X, UFBX_COORDINATE_AXIS_POSITIVE_Y, UFBX_COORDINATE_AXIS_POSITIVE_Z,
};
const ufbx_coordinate_axes ufbx_axes_right_handed_z_up = {
    UFBX_COORDINATE_AXIS_POSITIVE_X, UFBX_COORDINATE_AXIS_POSITIVE_Z, UFBX_COORDINATE_AXIS_NEGATIVE_Y,
};
const ufbx_coordinate_axes ufbx_axes_left_handed_y_up = {
    UFBX_COORDINATE_AXIS_POSITIVE_X, UFBX_COORDINATE_AXIS_POSITIVE_Y, UFBX_COORDINATE_AXIS_NEGATIVE_Z,
};
const ufbx_coordinate_axes ufbx_axes_left_handed_z_up = {
    UFBX_COORDINATE_AXIS_POSITIVE_X, UFBX_COORDINATE_AXIS_POSITIVE_Z, UFBX_COORDINATE_AXIS_POSITIVE_Y,
};

this would maybe also be useful for tools like vox2obj

[jpaver]

I'd like to avoid coordinate conversion in this library - keeping it narrowly scoped to reading/writing and merging lets it stay simple and good at those things without becoming too complicated.

As a bit of a middle-ground, I've actually exposed the ogt_vox_transform_multiply function in this PR: https://github.com/jpaver/opengametools/pull/56 - it'll allow the client to do whatever coordinate conversion they like by just doing another multiply on top of a global/flattened transform. eg.

const ogt_vox_transform coordinate_transform = { whatevs... };
ogt_vox_transform instance_transform = ogt_vox_transform_multiply( instance->transform, coordinate_transform );