Cairo Saved Paths with Transforms Render "Backwards" -

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the below code sample in lua, intended used conky. thought compute complicated path, save it, render twice -- once filled region, again stroked path, creating outlined region.

the problem saved path seems render "backwards" compared straightforward rendering.

the first section creates rotated l-shape in green, appears expected.

the second section attempts create same l-shape saved path, render in red. resulting path not come out rotated, since didn't call cairo_rotate() before appending path, suppose makes sense.

the third section renders set of progressively rotated l-shapes in cyan, appears expected.

the fourth section attempts same thing third section, time using saved paths, , rendering in yellow. paths come out going in wrong direction.

my guess cairo transformation functions apply transforms unstroked path components (if any) accumulated in cairo_t context. don't quite grasp why come out "forwards" 1 way , "backwards" other way.

what misunderstanding cairo paths , transforms?

function tests (cr)     local wiggle     local mat_base = cairo_matrix_t:create ()     tolua.takeownership (mat_base)      cairo_set_line_width (cr, 4)     cairo_identity_matrix (cr)      ---     -- direct stroke of l-shaped figure.     cairo_save (cr)          cairo_translate (cr, 128, 128)         cairo_new_path (cr)         cairo_rotate (cr, math.pi / 6)         cairo_move_to (cr, 150, 0)         cairo_line_to (cr, 0, 0)         cairo_line_to (cr, 0, 50)          cairo_set_source_rgb (cr, 0, 1, 0)         cairo_stroke (cr)      cairo_restore (cr)     ---     -- capture l-shaped figure path, render.     cairo_save (cr)          cairo_translate (cr, 384, 128)         cairo_new_path (cr)         cairo_rotate (cr, math.pi / 6)         cairo_move_to (cr, 150, 0)         cairo_line_to (cr, 0, 0)         cairo_line_to (cr, 0, 50)         wiggle = cairo_copy_path (cr)      cairo_restore (cr)     cairo_save (cr)          cairo_translate (cr, 384, 128)         cairo_new_path (cr)         cairo_append_path (cr, wiggle)         cairo_set_source_rgb (cr, 1, 0, 0)         cairo_stroke (cr)          cairo_path_destroy (wiggle)      cairo_restore (cr)     ---     -- direct stroke of set of rotated l-shaped figures.     cairo_save (cr)          cairo_translate (cr, 640, 128)         cairo_get_matrix (cr, mat_base)         cairo_new_path (cr)         = 0, 4             cairo_set_matrix (cr, mat_base)             cairo_rotate (cr, * math.pi / 6)             cairo_move_to (cr, 150, 0)             cairo_line_to (cr, 0, 0)             cairo_line_to (cr, 0, 50)         end         cairo_set_source_rgb (cr, 0, 1, 1)         cairo_stroke (cr)      cairo_restore (cr)     ---     -- capture rotated l-shaped figures path, render.     cairo_save (cr)          cairo_translate (cr, 896, 128)         cairo_get_matrix (cr, mat_base)         cairo_new_path (cr)         = 0, 4             cairo_set_matrix (cr, mat_base)             cairo_rotate (cr, * math.pi / 6)             cairo_move_to (cr, 150, 0)             cairo_line_to (cr, 0, 0)             cairo_line_to (cr, 0, 50)         end         wiggle = cairo_copy_path (cr)      cairo_restore (cr)     cairo_save (cr)          cairo_new_path (cr)         cairo_translate (cr, 896, 128)         cairo_append_path (cr, wiggle)         cairo_set_source_rgb (cr, 1, 1, 0)         cairo_stroke (cr)          cairo_path_destroy (wiggle)      cairo_restore (cr)     -- end

path rendering

when call cairo_copy_path, path expressed in current user space. if first draw path , afterwards call cairo_rotate(), cairo_copy_path(), resulting path include same last rotation.

so in red example, copy path before undoing rotation , hence appears there no rotation.

in yellow example, copy path after rotation , first rotations in different "direction" in blue one.

try replacing a = cairo_copy_path(cr) cairo_save(cr) ; cairo_identity_matrix(cr) ; = cairo_copy_path(cr) ; cairo_restore(cr). way path in surface coordinates seems more expecting.


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