带开罗的 Gouraud 阴影三角形

Question

我正在尝试使用 here 中解释的方法获得带有 Cairo 的 Gouraud 阴影三角形。

我有以下代码：

cairo_surface_t *surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 240, 240);
cairo_t *cr = cairo_create(surface);

cairo_pattern_t * pattern = cairo_pattern_create_mesh();
cairo_mesh_pattern_begin_patch (pattern);
cairo_mesh_pattern_move_to (pattern, 100, 100);
cairo_mesh_pattern_line_to (pattern, 130, 130);
cairo_mesh_pattern_line_to (pattern, 130,  70);
cairo_mesh_pattern_set_corner_color_rgb(pattern, 0, 1, 0, 0);
cairo_mesh_pattern_set_corner_color_rgb(pattern, 1, 0, 1, 0);
cairo_mesh_pattern_set_corner_color_rgb(pattern, 2, 0, 0, 1);
cairo_mesh_pattern_end_patch(pattern);
cairo_set_source(cr, pattern);
cairo_fill(cr);

cairo_surface_write_to_png (surface,"test.png");

然而 test.png 只是黑色。

Answer 1

cairo_mesh_pattern_line_to() 不会在实际绘图发生的地方创建几何体。它仅指定模式将发生的位置。因此，要真正看到某些东西，您必须指定几何形状，例如cairo_line_to().

所以这应该有效：

cairo_surface_t *surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 240, 240);
cairo_t *cr = cairo_create(surface);

cairo_pattern_t * pattern = cairo_pattern_create_mesh();
cairo_mesh_pattern_begin_patch (pattern);
cairo_mesh_pattern_move_to (pattern, 100, 100);
cairo_mesh_pattern_line_to (pattern, 130, 130);
cairo_mesh_pattern_line_to (pattern, 130,  70);
cairo_mesh_pattern_set_corner_color_rgb(pattern, 0, 1, 0, 0);
cairo_mesh_pattern_set_corner_color_rgb(pattern, 1, 0, 1, 0);
cairo_mesh_pattern_set_corner_color_rgb(pattern, 2, 0, 0, 1);
cairo_mesh_pattern_end_patch(pattern);
cairo_set_source(cr, pattern);
cairo_move_to(cr, 100, 100);
cairo_line_to(cr, 130, 130);
cairo_line_to(cr, 130, 70);
cairo_fill(cr);

cairo_surface_write_to_png (surface,"test.png");

带开罗的 Gouraud 阴影三角形

Gouraud-shaded triangle with Cairo

c

cairo