--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/opengl/util/ellipse_aa.glsl	Mon Jan 11 14:00:40 2010 +0000
@@ -0,0 +1,58 @@
+uniform vec3 inv_matrix_m0;
+uniform vec3 inv_matrix_m1;
+uniform vec3 inv_matrix_m2;
+
+uniform vec2 ellipse_offset;
+
+// ellipse equation
+
+// s^2/a^2 + t^2/b^2 = 1
+//
+// implicit equation:
+// g(s,t) = 1 - s^2/r_s^2 - t^2/r_t^2
+
+// distance from ellipse:
+// grad = [dg/dx dg/dy]
+// d(s, t) ~= g(s, t) / |grad|
+
+// dg/dx = dg/ds * ds/dx + dg/dt * dt/dx
+// dg/dy = dg/ds * ds/dy + dg/dt * dt/dy
+
+float ellipse_aa()
+{
+    mat3 mat;
+
+    mat[0] = inv_matrix_m0;
+    mat[1] = inv_matrix_m1;
+    mat[2] = inv_matrix_m2;
+
+    vec3 hcoords = mat * vec3(gl_FragCoord.xy + ellipse_offset, 1);
+    float inv_w = 1.0 / hcoords.z;
+    vec2 st = hcoords.xy * inv_w;
+
+    vec4 xy = vec4(mat[0].xy, mat[1].xy);
+    vec2 h = vec2(mat[0].z, mat[1].z);
+
+    vec4 dstdxy = (xy.xzyw - h.xyxy * st.xxyy) * inv_w;
+
+    //dstdxy.x = (mat[0].x - mat[0].z * st.x) * inv_w; // ds/dx
+    //dstdxy.y = (mat[1].x - mat[1].z * st.x) * inv_w; // ds/dy
+    //dstdxy.z = (mat[0].y - mat[0].z * st.y) * inv_w; // dt/dx
+    //dstdxy.w = (mat[1].y - mat[1].z * st.y) * inv_w; // dt/dy
+
+    vec2 inv_r = gl_TexCoord[0].xy;
+    vec2 n = st * inv_r;
+    float g = 1.0 - dot(n, n);
+
+    vec2 dgdst = -2.0 * n * inv_r;
+
+    vec2 grad = vec2(dot(dgdst, dstdxy.xz),
+                     dot(dgdst, dstdxy.yw));
+
+    return smoothstep(-0.5, 0.5, g * inversesqrt(dot(grad, grad)));
+}
+
+void main()
+{
+    gl_FragColor = ellipse_aa().xxxx;
+}