UI: Draw round corners between the editors

This makes easier to distinguish between different editors
(as oppose to an editor and its regions).

Note action zones look a bit strange with this. I recommend we do next:
* Make sure all 4 corners can be used as action zones.
* Remove their drawing code (or show them only on mouse hover).

source/blender/editors/include/UI_interface.h

@@ -317,6 +317,7 @@ typedef enum {
  * Used for code that draws its own UI style elements. */
 
 void UI_draw_anti_tria(float x1, float y1, float x2, float y2, float x3, float y3, const float color[4]);
+void UI_draw_anti_fan(float tri_array[][2], unsigned int length, const float color[4]);
 
 void UI_draw_roundbox_corner_set(int type);
 void UI_draw_roundbox_aa(bool filled, float minx, float miny, float maxx, float maxy, float rad, const float color[4]);

source/blender/editors/interface/interface_widgets.c

@@ -526,6 +526,40 @@ void UI_draw_anti_tria(float x1, float y1, float x2, float y2, float x3, float y
 	glDisable(GL_BLEND);
 }
 
+void UI_draw_anti_fan(float tri_array[][2], unsigned int length, const float color[4])
+{
+	float draw_color[4];
+
+	copy_v4_v4(draw_color, color);
+	draw_color[3] *= 2.0f / WIDGET_AA_JITTER;
+
+	glEnable(GL_BLEND);
+
+	unsigned int pos = GWN_vertformat_attr_add(immVertexFormat(), "pos", GWN_COMP_F32, 2, GWN_FETCH_FLOAT);
+	immBindBuiltinProgram(GPU_SHADER_2D_UNIFORM_COLOR);
+
+	immUniformColor4fv(draw_color);
+
+	/* for each AA step */
+	for (int j = 0; j < WIDGET_AA_JITTER; j++) {
+		immBegin(GWN_PRIM_TRI_FAN, length);
+		immVertex2f(pos, tri_array[0][0], tri_array[0][1]);
+		immVertex2f(pos, tri_array[1][0], tri_array[1][1]);
+
+		/* We jitter only the middle of the fan, the extremes are pinned. */
+		for (int i = 2; i < length - 1; i++) {
+			immVertex2f(pos, tri_array[i][0] + jit[j][0], tri_array[i][1] + jit[j][1]);
+		}
+
+		immVertex2f(pos, tri_array[length - 1][0], tri_array[length - 1][1]);
+		immEnd();
+	}
+
+	immUnbindProgram();
+
+	glDisable(GL_BLEND);
+}
+
 static void widget_init(uiWidgetBase *wtb)
 {
 	wtb->totvert = wtb->halfwayvert = 0;

source/blender/editors/screen/screen_draw.c

@@ -28,9 +28,13 @@
 #include "GPU_immediate.h"
 #include "GPU_matrix.h"
 
+#include "BLI_math.h"
+
 #include "WM_api.h"
 #include "WM_types.h"
 
+#include "UI_interface.h"
+
 #include "screen_intern.h"
 
 /**
@@ -208,6 +212,106 @@ static void draw_join_shape(ScrArea *sa, char dir, unsigned int pos)
 	}
 }
 
+#define CORNER_RESOLUTION 10
+static void drawscredge_corner_geometry(
+        int sizex, int sizey,
+        int corner_x, int corner_y,
+        int center_x, int center_y,
+        double angle_offset)
+{
+	const int radius = ABS(corner_x - center_x);
+	const float color[4] = {0.0f, 0.0f, 0.0f, 1.0f};
+	const int line_thickness = U.pixelsize;
+
+	if (corner_x < center_x) {
+		if (corner_x > 0.0f) {
+			/* Left (internal) edge. */
+			corner_x += line_thickness;
+			center_x += line_thickness;
+		}
+	}
+	else {
+		/* Right (internal) edge. */
+		if (corner_x < sizex - 1) {
+			corner_x += 1 - line_thickness;
+			center_x += 1 - line_thickness;
+		}
+		else {
+			/* Corner case, extreme right edge. */
+			corner_x += 1;
+			center_x += 1;
+		}
+	}
+
+	if (corner_y < center_y) {
+		if (corner_y > 0.0f) {
+			/* Bottom (internal) edge. */
+			corner_y += line_thickness;
+			center_y += line_thickness;
+		}
+	}
+	else {
+		/* Top (internal) edge. */
+		if (corner_y < sizey) {
+			corner_y += 1 - line_thickness;
+			center_y += 1 - line_thickness;
+		}
+	}
+
+	float tri_array[CORNER_RESOLUTION + 1][2];
+
+	tri_array[0][0] = corner_x;
+	tri_array[0][1] = corner_y;
+
+	for (int i = 0; i < CORNER_RESOLUTION; i++) {
+		double angle = angle_offset + (M_PI_2 * ((float)i / (CORNER_RESOLUTION - 1)));
+		float x = center_x + (radius * cos(angle));
+		float y = center_y + (radius * sin(angle));
+		tri_array[i + 1][0] = x;
+		tri_array[i + 1][1] = y;
+	}
+	UI_draw_anti_fan(tri_array, CORNER_RESOLUTION + 1, color);
+}
+
+#undef CORNER_RESOLUTION
+
+static void drawscredge_corner(ScrArea *sa, int sizex, int sizey)
+{
+	int size = 10 * U.pixelsize;
+
+	/* Bottom-Left. */
+	drawscredge_corner_geometry(sizex, sizey,
+	                            sa->v1->vec.x,
+					            sa->v1->vec.y,
+					            sa->v1->vec.x + size,
+					            sa->v1->vec.y + size,
+	                            M_PI_2 * 2.0f);
+
+	/* Top-Left. */
+	drawscredge_corner_geometry(sizex, sizey,
+	                            sa->v2->vec.x,
+	                            sa->v2->vec.y,
+	                            sa->v2->vec.x + size,
+	                            sa->v2->vec.y - size,
+	                            M_PI_2);
+
+	/* Top-Right. */
+	drawscredge_corner_geometry(sizex, sizey,
+	                            sa->v3->vec.x,
+	                            sa->v3->vec.y,
+	                            sa->v3->vec.x - size,
+	                            sa->v3->vec.y - size,
+	                            0.0f);
+
+	/* Bottom-Right. */
+	drawscredge_corner_geometry(sizex, sizey,
+	                            sa->v4->vec.x,
+	                            sa->v4->vec.y,
+	                            sa->v4->vec.x - size,
+	                            sa->v4->vec.y + size,
+	                            M_PI_2 * 3.0f);
+}
+
 /**
  * Draw screen area darker with arrow (visualization of future joining).
  */
@@ -307,7 +411,7 @@ void ED_screen_draw_edges(wmWindow *win)
 	if (U.pixelsize > 1.0f) {
 		/* FIXME: doesn't our glLineWidth already scale by U.pixelsize? */
 		glLineWidth((2.0f * U.pixelsize) - 1);
-		immUniformColor3ub(0x50, 0x50, 0x50);
+		immUniformColor3ub(0, 0, 0);
 
 		for (sa = screen->areabase.first; sa; sa = sa->next) {
 			drawscredge_area(sa, winsize_x, winsize_y, pos);
@@ -323,6 +427,10 @@ void ED_screen_draw_edges(wmWindow *win)
 
 	immUnbindProgram();
 
+	for (sa = screen->areabase.first; sa; sa = sa->next) {
+		drawscredge_corner(sa, winsize_x, winsize_y);
+	}
+
 	screen->do_draw = false;
 }