[PATCH] cairo_renderer: use transform() instead of set_matrix() for markers

[Thomas Petazzoni]

From: Thomas Petazzoni <thomas.p...@free-electrons.com>

Mapnik is current using context.set_matrix() to modify the Cairo
transformation matrix to render markers at the right location and with
the right angle on the map. This is typically used to render one-way
arrows with the MapQuest stylesheet.

Unfortunately, context.set_matrix() completely _overrides_ the
existing Cairo transformation matrix instead of making an _additional_
transformation to it. However, in MapOSMatic
(http://www.maposmatic.org), for our map-rendering needs, we pass to
Mapnik a Cairo surface on which we have already applied a
translation+scaling transformation. The usage of .set_matrix() by
Mapnik completely overrides this transformation, with the result of
having completely misplaced one-way arrows.

By using context.transform(), then the transformation matrix is
applied on _top_ of the current one, which fixes the rendering. Now,
the one-way arrows are at the right locations on our translated+scaled
Cairo surface.

Thanks to Gaël Utard <gael....@kisis.fr> for his help in debugging
this problem.

Signed-off-by: Thomas Petazzoni <thomas.p...@free-electrons.com>
---
src/cairo_renderer.cpp | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/src/cairo_renderer.cpp b/src/cairo_renderer.cpp
index 589247e..12f6277 100644
--- a/src/cairo_renderer.cpp
+++ b/src/cairo_renderer.cpp
@@ -1269,7 +1269,7 @@ void cairo_renderer_base::start_map_processing(Map const& map)
double x, y, angle;
while (placement.get_point(&x, &y, &angle)) {
Cairo::Matrix matrix = Cairo::rotation_matrix(angle) * Cairo::translation_matrix(x,y) ;
- context.set_matrix(matrix);
+ context.transform(matrix);
context.add_path(arrow_);
}
}
--
1.7.4.1

[Dane Springmeyer]

Looks like a great patch. However it would be great to have some tests to accompany this. There are a number of places transforms are uses so it will be important to test them all.

Anyway, I am traveling this week and mostly offline. I will take a look at applying next week when back. In the meantime it would be great if you could create an issue and attach this patch.

Thanks!

Dane