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Fix handling of projective transforms #199

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Fix handling of projective transforms #199

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a5c68a5
Fix handling of projective transforms
Sep 1, 2020
f80eb04
fixup! Fix handling of projective transforms
Sep 1, 2020
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16 changes: 8 additions & 8 deletions src/graphene-matrix.c
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Original file line number Diff line number Diff line change
Expand Up @@ -921,8 +921,8 @@ graphene_matrix_transform_point (const graphene_matrix_t *m,
vec3 = graphene_simd4f_init (p->x, p->y, 0.0f, 1.0f);
graphene_simd4x4f_point3_mul (&m->value, &vec3, &vec3);

res->x = graphene_simd4f_get_x (vec3);
res->y = graphene_simd4f_get_y (vec3);
res->x = graphene_simd4f_get_x (vec3) / graphene_simd4f_get_w (vec3);
res->y = graphene_simd4f_get_y (vec3) / graphene_simd4f_get_w (vec3);
}

/**
Expand Down Expand Up @@ -951,9 +951,9 @@ graphene_matrix_transform_point3d (const graphene_matrix_t *m,
vec3 = graphene_simd4f_init (p->x, p->y, p->z, 1.f);
graphene_simd4x4f_point3_mul (&m->value, &vec3, &vec3);

res->x = graphene_simd4f_get_x (vec3);
res->y = graphene_simd4f_get_y (vec3);
res->z = graphene_simd4f_get_z (vec3);
res->x = graphene_simd4f_get_x (vec3) / graphene_simd4f_get_w (vec3);
res->y = graphene_simd4f_get_y (vec3) / graphene_simd4f_get_w (vec3);
res->z = graphene_simd4f_get_y (vec3) / graphene_simd4f_get_w (vec3);
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copy/paste bug here I think.

}
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graphene_matrix_unproject_point3d() does:

  inv_w = 1.f / graphene_simd4f_get_w (v);
  v = graphene_simd4f_mul (v, graphene_simd4f_splat (inv_w));

Which seems more efficient than this

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good point. Also interesting that graphene_matrix_unproject_point3d takes two matrices - I get the sense that we are not fully understanding the graphene api and are not using it properly in gtk

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Unprojection requires the viewport, and has a split modelview/projection matrices because most users of such an API will have a separate modelview and projection matrices already.

I guess the issue, here, is that Graphene adheres to a different set of conventions. The documentation should clarify the use cases.

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In 3d apps, the scene graph typically converts objects into camera space, which is a 3d space with the cameras location at origin, looking down -z axis. Then the camera projection matrix is applied on top of that to convert to 2d space. The exact transform there depends on the camera (i.e. perspective or orthonormal, field of vision, etc). When rendering an object we combine all these matrixes into one (object -> world -> camera -> screen) which the GPU applies to each object vector.

However, for unproject we want to go backwards into world space (not object space), so we invert the camera->screen and apply the camera->world matrix on top of that. These are taken as two separate args because those are the two matrixes that the client usually stores, and we can't use the combined matrix because that is from object space (and only available during rendering).

For gtk+, I don't really think we need unprojection, because we don't set up a complicated projection anyway. It just maps the z=0 plane one-to-one. I guess that means the exact z perspective behaviour is just what happens to fall out of the math... Maybe worth looking at.

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We are handling 4x4 matrices here that operate on points in homogeneous coordinates, to go from homogeneous coordinates to points with 3 coordinates (or 2, as the transform_bounds api does) in general requires a division by w, otherwise the results are just wrong.

Given that this api is in use, we should probably just document the fact that passing a matrix with perspective component to transform_bounds and transform_points is going to yield unexpected results.

For GTK, I have added the functions we needed here: https://gitlab.gnome.org/GNOME/gtk/-/blob/master/gsk/gsktransform.c#L2169


/**
Expand Down Expand Up @@ -1034,9 +1034,9 @@ graphene_matrix_transform_bounds (const graphene_matrix_t *m,
graphene_point_t __p; \
graphene_rect_get_ ## corner (rect, &__p); \
__s = graphene_simd4f_init (__p.x, __p.y, 0.f, 1.f); \
graphene_simd4x4f_vec4_mul (&matrix->value, &__s, &__s); \
out_p.x = graphene_simd4f_get_x (__s); \
out_p.y = graphene_simd4f_get_y (__s); } while (0)
graphene_simd4x4f_point3_mul (&matrix->value, &__s, &__s); \
out_p.x = graphene_simd4f_get_x (__s) / graphene_simd4f_get_w (__s); \
out_p.y = graphene_simd4f_get_y (__s) / graphene_simd4f_get_w (__s); } while (0)

TRANSFORM_POINT (m, &rr, top_left, ret[0]);
TRANSFORM_POINT (m, &rr, top_right, ret[1]);
Expand Down