cogl: Add support for clipping regions

This uses the stencil buffer to poke holes in the shape of the
given cairo_region_t that we will draw through.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/867

cogl/cogl/cogl-clip-stack.c

@@ -295,6 +295,30 @@ _cogl_clip_stack_push_primitive (CoglClipStack *stack,
   return (CoglClipStack *) entry;
 }
 
+CoglClipStack *
+cogl_clip_stack_push_region (CoglClipStack   *stack,
+                             cairo_region_t  *region)
+{
+  CoglClipStack *entry;
+  CoglClipStackRegion *entry_region;
+  cairo_rectangle_int_t bounds;
+
+  entry_region = _cogl_clip_stack_push_entry (stack,
+                                              sizeof (CoglClipStackRegion),
+                                              COGL_CLIP_STACK_REGION);
+  entry = (CoglClipStack *) entry_region;
+
+  cairo_region_get_extents (region, &bounds);
+  entry->bounds_x0 = bounds.x;
+  entry->bounds_x1 = bounds.x + bounds.width;
+  entry->bounds_y0 = bounds.y;
+  entry->bounds_y1 = bounds.y + bounds.height;
+
+  entry_region->region = cairo_region_reference (region);
+
+  return entry;
+}
+
 CoglClipStack *
 _cogl_clip_stack_ref (CoglClipStack *entry)
 {
@@ -336,6 +360,13 @@ _cogl_clip_stack_unref (CoglClipStack *entry)
             g_slice_free1 (sizeof (CoglClipStackPrimitive), entry);
             break;
           }
+        case COGL_CLIP_STACK_REGION:
+          {
+            CoglClipStackRegion *region = (CoglClipStackRegion *) entry;
+            cairo_region_destroy (region->region);
+            g_slice_free1 (sizeof (CoglClipStackRegion), entry);
+            break;
+          }
         default:
           g_assert_not_reached ();
         }

cogl/cogl/cogl-clip-stack.h

@@ -48,12 +48,14 @@ typedef struct _CoglClipStack CoglClipStack;
 typedef struct _CoglClipStackRect CoglClipStackRect;
 typedef struct _CoglClipStackWindowRect CoglClipStackWindowRect;
 typedef struct _CoglClipStackPrimitive CoglClipStackPrimitive;
+typedef struct _CoglClipStackRegion CoglClipStackRegion;
 
 typedef enum
   {
     COGL_CLIP_STACK_RECT,
     COGL_CLIP_STACK_WINDOW_RECT,
-    COGL_CLIP_STACK_PRIMITIVE
+    COGL_CLIP_STACK_PRIMITIVE,
+    COGL_CLIP_STACK_REGION,
   } CoglClipStackType;
 
 /* A clip stack consists a list of entries. Each entry has a reference
@@ -162,6 +164,13 @@ struct _CoglClipStackPrimitive
   float bounds_y2;
 };
 
+struct _CoglClipStackRegion
+{
+  CoglClipStack _parent_data;
+
+  cairo_region_t *region;
+};
+
 CoglClipStack *
 _cogl_clip_stack_push_window_rectangle (CoglClipStack *stack,
                                         int x_offset,
@@ -189,6 +198,9 @@ _cogl_clip_stack_push_primitive (CoglClipStack *stack,
                                  CoglMatrixEntry *modelview_entry,
                                  CoglMatrixEntry *projection_entry,
                                  const float *viewport);
+CoglClipStack *
+cogl_clip_stack_push_region (CoglClipStack   *stack,
+                             cairo_region_t  *region);
 
 CoglClipStack *
 _cogl_clip_stack_pop (CoglClipStack *stack);

cogl/cogl/cogl-framebuffer.c

@@ -1761,6 +1761,19 @@ cogl_framebuffer_push_primitive_clip (CoglFramebuffer *framebuffer,
       COGL_FRAMEBUFFER_STATE_CLIP;
 }
 
+void
+cogl_framebuffer_push_region_clip (CoglFramebuffer *framebuffer,
+                                   cairo_region_t  *region)
+{
+  framebuffer->clip_stack =
+    cogl_clip_stack_push_region (framebuffer->clip_stack,
+                                 region);
+
+  if (framebuffer->context->current_draw_buffer == framebuffer)
+    framebuffer->context->current_draw_buffer_changes |=
+      COGL_FRAMEBUFFER_STATE_CLIP;
+}
+
 void
 cogl_framebuffer_pop_clip (CoglFramebuffer *framebuffer)
 {

cogl/cogl/cogl-framebuffer.h

@@ -54,6 +54,7 @@ typedef struct _CoglFramebuffer CoglFramebuffer;
 #include <cogl/cogl-bitmap.h>
 #include <cogl/cogl-texture.h>
 #include <glib-object.h>
+#include <cairo.h>
 
 #include <graphene.h>
 
@@ -624,6 +625,10 @@ cogl_framebuffer_push_primitive_clip (CoglFramebuffer *framebuffer,
                                       float bounds_x2,
                                       float bounds_y2);
 
+void
+cogl_framebuffer_push_region_clip (CoglFramebuffer *framebuffer,
+                                   cairo_region_t  *region);
+
 /**
  * cogl_framebuffer_pop_clip:
  * @framebuffer: A #CoglFramebuffer pointer

cogl/cogl/driver/gl/cogl-clip-stack-gl.c

@@ -115,6 +115,102 @@ add_stencil_clip_rectangle (CoglFramebuffer *framebuffer,
   GE( ctx, glStencilOp (GL_KEEP, GL_KEEP, GL_KEEP) );
 }
 
+static void
+add_stencil_clip_region (CoglFramebuffer *framebuffer,
+                         cairo_region_t  *region,
+                         gboolean         merge)
+{
+  CoglContext *ctx = cogl_framebuffer_get_context (framebuffer);
+  CoglMatrix matrix;
+  int num_rectangles = cairo_region_num_rectangles (region);
+  int i;
+
+  /* NB: This can be called while flushing the journal so we need
+   * to be very conservative with what state we change.
+   */
+  _cogl_context_set_current_projection_entry (ctx, &ctx->identity_entry);
+  _cogl_context_set_current_modelview_entry (ctx, &ctx->identity_entry);
+
+  /* The coordinates in the region are meant to be window coordinates,
+   * make a matrix that translates those across the viewport, and into
+   * the default [-1, -1, 1, 1] range.
+   */
+  cogl_matrix_init_identity (&matrix);
+  cogl_matrix_translate (&matrix, -1, 1, 0);
+  cogl_matrix_scale (&matrix,
+                     2.0 / framebuffer->viewport_width,
+                     - 2.0 / framebuffer->viewport_height,
+                     1);
+  cogl_matrix_translate (&matrix,
+                         - framebuffer->viewport_x,
+                         - framebuffer->viewport_y,
+                         0);
+
+  GE( ctx, glEnable (GL_STENCIL_TEST) );
+
+  GE( ctx, glColorMask (FALSE, FALSE, FALSE, FALSE) );
+  GE( ctx, glDepthMask (FALSE) );
+
+  if (merge)
+    {
+      GE( ctx, glStencilFunc (GL_ALWAYS, 0x1, 0x3) );
+      GE( ctx, glStencilOp (GL_KEEP, GL_KEEP, GL_INCR) );
+    }
+  else
+    {
+      /* Initially disallow everything */
+      GE( ctx, glClearStencil (0) );
+      GE( ctx, glClear (GL_STENCIL_BUFFER_BIT) );
+
+      /* Punch out holes to allow the rectangles */
+      GE( ctx, glStencilFunc (GL_ALWAYS, 0x1, 0x1) );
+      GE( ctx, glStencilOp (GL_KEEP, GL_KEEP, GL_REPLACE) );
+    }
+
+  for (i = 0; i < num_rectangles; i++)
+    {
+      cairo_rectangle_int_t rect;
+      float tl[4], br[4];
+
+      cairo_region_get_rectangle (region, i, &rect);
+
+      tl[0] = rect.x;
+      tl[1] = rect.y;
+      tl[2] = 0.;
+      tl[3] = 1.;
+
+      br[0] = rect.x + rect.width;
+      br[1] = rect.y + rect.height;
+      br[2] = 0.;
+      br[3] = 1.;
+
+      cogl_matrix_transform_point (&matrix, &tl[0], &tl[1], &tl[2], &tl[3]);
+      cogl_matrix_transform_point (&matrix, &br[0], &br[1], &br[2], &br[3]);
+
+      _cogl_rectangle_immediate (framebuffer,
+                                 ctx->stencil_pipeline,
+                                 tl[0], tl[1], br[0], br[1]);
+    }
+
+  if (merge)
+    {
+      /* Subtract one from all pixels in the stencil buffer so that
+       * only pixels where both the original stencil buffer and the
+       * region are set will be valid
+       */
+      GE( ctx, glStencilOp (GL_KEEP, GL_KEEP, GL_DECR) );
+      _cogl_rectangle_immediate (framebuffer,
+                                 ctx->stencil_pipeline,
+                                 -1.0, -1.0, 1.0, 1.0);
+    }
+
+  /* Restore the stencil mode */
+  GE (ctx, glDepthMask (TRUE));
+  GE (ctx, glColorMask (TRUE, TRUE, TRUE, TRUE));
+  GE( ctx, glStencilFunc (GL_EQUAL, 0x1, 0x1) );
+  GE( ctx, glStencilOp (GL_KEEP, GL_KEEP, GL_KEEP) );
+}
+
 typedef void (*SilhouettePaintCallback) (CoglFramebuffer *framebuffer,
                                          CoglPipeline *pipeline,
                                          void *user_data);
@@ -382,6 +478,23 @@ _cogl_clip_stack_gl_flush (CoglClipStack *stack,
                 }
               break;
             }
+        case COGL_CLIP_STACK_REGION:
+            {
+              CoglClipStackRegion *region = (CoglClipStackRegion *) entry;
+
+              /* If nrectangles <= 1, it can be fully represented with the
+               * scissor clip.
+               */
+              if (cairo_region_num_rectangles (region->region) > 1)
+                {
+                  COGL_NOTE (CLIPPING, "Adding stencil clip for region");
+
+                  add_stencil_clip_region (framebuffer, region->region,
+                                           using_stencil_buffer);
+                  using_stencil_buffer = TRUE;
+                }
+              break;
+            }
         case COGL_CLIP_STACK_WINDOW_RECT:
           break;
           /* We don't need to do anything for window space rectangles because

cogl/cogl/meson.build

@@ -478,7 +478,7 @@ if have_introspection
     sources: cogl_introspected_headers,
     nsversion: libmutter_api_version,
     namespace: 'Cogl',
-    includes: ['GL-1.0', 'GObject-2.0', 'Graphene-1.0'],
+    includes: ['cairo-1.0', 'GL-1.0', 'GObject-2.0', 'Graphene-1.0'],
     dependencies: [cogl_deps],
     extra_args: introspection_args + [
       '-UCOGL_COMPILATION',