[gimp] app: add gimp_display_shell_constrain_angle()

[Ell <ell src gnome org>]

commit 8c1a277007ec149cfb4ad091d28cb27c32353a85
Author: Ell <ell_se yahoo com>
Date:   Thu May 14 23:44:50 2020 +0300

    app: add gimp_display_shell_constrain_angle()
    
    ... which constrains an angle to discrete increments in screen
    space, similarly to gimp_display_shell_constrain_line().

 app/display/gimpdisplayshell-utils.c | 20 +++++++++++++++++++
 app/display/gimpdisplayshell-utils.h | 38 ++++++++++++++++++++----------------
 2 files changed, 41 insertions(+), 17 deletions(-)
---
diff --git a/app/display/gimpdisplayshell-utils.c b/app/display/gimpdisplayshell-utils.c
index 912bcc8a66..b8089635c2 100644
--- a/app/display/gimpdisplayshell-utils.c
+++ b/app/display/gimpdisplayshell-utils.c
@@ -89,6 +89,26 @@ gimp_display_shell_constrain_line (GimpDisplayShell *shell,
                        xres, yres);
 }
 
+gdouble
+gimp_display_shell_constrain_angle (GimpDisplayShell *shell,
+                                    gdouble           angle,
+                                    gint              n_snap_lines)
+{
+  gdouble x, y;
+
+  g_return_val_if_fail (GIMP_IS_DISPLAY_SHELL (shell), 0.0);
+
+  x = cos (angle);
+  y = sin (angle);
+
+  gimp_display_shell_constrain_line (shell,
+                                     0.0, 0.0,
+                                     &x,  &y,
+                                     n_snap_lines);
+
+  return atan2 (y, x);
+}
+
 /**
  * gimp_display_shell_get_line_status:
  * @status:    initial status text.
diff --git a/app/display/gimpdisplayshell-utils.h b/app/display/gimpdisplayshell-utils.h
index d397ed6a18..3eb52e383b 100644
--- a/app/display/gimpdisplayshell-utils.h
+++ b/app/display/gimpdisplayshell-utils.h
@@ -19,23 +19,27 @@
 #define __GIMP_DISPLAY_SHELL_UTILS_H__
 
 
-void    gimp_display_shell_get_constrained_line_params (GimpDisplayShell *shell,
-                                                        gdouble          *offset_angle,
-                                                        gdouble          *xres,
-                                                        gdouble          *yres);
-void    gimp_display_shell_constrain_line              (GimpDisplayShell *shell,
-                                                        gdouble           start_x,
-                                                        gdouble           start_y,
-                                                        gdouble          *end_x,
-                                                        gdouble          *end_y,
-                                                        gint              n_snap_lines);
-gchar * gimp_display_shell_get_line_status             (GimpDisplayShell *shell,
-                                                        const gchar      *status,
-                                                        const gchar      *separator,
-                                                        gdouble           x1,
-                                                        gdouble           y1,
-                                                        gdouble           x2,
-                                                        gdouble           y2);
+void      gimp_display_shell_get_constrained_line_params (GimpDisplayShell *shell,
+                                                          gdouble          *offset_angle,
+                                                          gdouble          *xres,
+                                                          gdouble          *yres);
+void      gimp_display_shell_constrain_line              (GimpDisplayShell *shell,
+                                                          gdouble           start_x,
+                                                          gdouble           start_y,
+                                                          gdouble          *end_x,
+                                                          gdouble          *end_y,
+                                                          gint              n_snap_lines);
+gdouble   gimp_display_shell_constrain_angle             (GimpDisplayShell *shell,
+                                                          gdouble           angle,
+                                                          gint              n_snap_lines);
+
+gchar   * gimp_display_shell_get_line_status             (GimpDisplayShell *shell,
+                                                          const gchar      *status,
+                                                          const gchar      *separator,
+                                                          gdouble           x1,
+                                                          gdouble           y1,
+                                                          gdouble           x2,
+                                                          gdouble           y2);
 
 
 #endif  /*  __GIMP_DISPLAY_SHELL_UTILS_H__  */