[Solved] Rotating subregion of image

[univrsal]

Hey there,

so far I've been getting away with trail and error when it came to finding out how the graphics api of obs works but I now need some help or at least a hint on how to do the following thing:
My plugin loads an image file, then selects a subregion of that file and draws it inside the source at different locations.
I'm currently doing it this way:

/* m_image is a gs_image_file_t* inside the source */

/* This is called in the render method of my source */
gs_effect_set_texture(gs_effect_get_param_by_name(effect, "image"), m_image->texture);
gs_matrix_push();
// Moves the subregion to the position that I want it to be
gs_matrix_translate3f((float)x, (float)y, 1.f);
// key is a struct that contains information about what subregion and where to place it in the source
gs_draw_sprite_subregion(m_image->texture, 0, key->texture_u, key->texture_v + key->h + 3, key->w + 1, key->h + 1);
gs_matrix_pop();

So for rotation I'd go with gs_matrix_rotaa4f(...), but it only rotates around the top left corner and therefore makes it impossible for me to rotate it around the center of a subregion and then move it to the location I want it to be.
Here's what's happening:

Spoiler: Gif

Here's what I'd like to do:

Spoiler: Image

So I'd need a way to get a subregion from the image then rotate it around the red dot and finally move the rotated part to a certain offset (not necessarily in that order).
If anyone can help me with that I'd gladly appreciate it since I don't have that much experience with this kind of stuff.

Thanks in advance!

 

[univrsal]

Okay, so since no one answered (either because I badly explained it or no one knew an answer) I kept fiddling and got away with trail and error again.
Here's what I used:

        gs_matrix_translate3f(x + (key->w / 2.f),  y + (key->h / 2.f), 1.f); // Put it in position
        gs_matrix_translate3f((float)-(key->w / 2), (float)-(key->h / 2), 1.f); // Needed for rotation
        gs_matrix_rotaa4f(0.f, 0.f, 1.f, angle);
        gs_matrix_translate3f((float)-(key->w / 2), (float)-(key->h / 2), 1.f); // Needed for rotation
        gs_draw_sprite_subregion(m_image->texture, 0, key->texture_u, key->texture_v, key->w + 1, key->h + 1);