2 files changed, 54 insertions, 0 deletions
diff --git a/shaders/rect.frag b/shaders/rect.frag
new file mode 100644
index 0000000..d128f37
--- /dev/null
+++ b/shaders/rect.frag
@@ -0,0 +1,36 @@
+#version 330
+
+/* The fragment shader uses the UV coordinates to calculate whether it is in
+   the T, R, B, or L border.  These are then mixed with the border color, and
+   their inverse is mixed with the fill color, to calculate the fragment color.
+   For example, if we are in the top border, then T=1, so the border mix factor
+   TRBL=1, and the fill mix factor (1-TRBL) is 0.
+
+   The use of pixel units here is handy because the border width can be
+   specified precisely in pixels to draw sharp lines.  The border width is just
+   hardcoded, but could be made a uniform or vertex attribute easily enough. */
+
+uniform vec2 u_size;
+uniform vec4 u_borderColor;
+uniform vec4 u_fillColor;
+
+noperspective in vec2 f_uv;
+
+layout(location = 0) out vec4 FragColor;
+
+void
+main()
+{
+	const float border_width = 2.0;
+
+	float t          = step(border_width, f_uv[1]);
+	float r          = step(border_width, u_size.x - f_uv[0]);
+	float b          = step(border_width, u_size.y - f_uv[1]);
+	float l          = step(border_width, f_uv[0]);
+	float fill_mix   = t * r * b * l;
+	float border_mix = 1.0 - fill_mix;
+	vec4  fill       = fill_mix * u_fillColor;
+	vec4  border     = border_mix * u_borderColor;
+
+	FragColor = fill + border;
+}
diff --git a/shaders/rect.vert b/shaders/rect.vert
new file mode 100644
index 0000000..34fe0b3
--- /dev/null
+++ b/shaders/rect.vert
@@ -0,0 +1,18 @@
+#version 330
+
+/* The vertex shader is trivial, but forwards scaled UV coordinates (in pixels)
+   to the fragment shader for drawing the border. */
+
+uniform mat4 MVP;
+uniform vec2 u_size;
+
+in vec2 v_position;
+
+noperspective out vec2 f_uv;
+
+void
+main()
+{
+	f_uv        = v_position * u_size;
+	gl_Position = MVP * vec4(v_position, 0.0, 1.0);
+}