Render highlight vertex markers as sphere impostors

Add a geometry-shader program that expands each highlight point into a
camera-facing quad shaded as a lit sphere (hemisphere normal per fragment,
discard outside the disc). ShaderProgram gains optional geometry-stage
support; HighlightOverlay splits draw into drawLines/drawPoints so points
use the impostor program while edges keep the flat scene program.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CityDoctorParent/Extensions/CityDoctorGUI/src/main/java/de/hft/stuttgart/citydoctor2/gui/gl/GLViewport.java

@@ -25,6 +25,7 @@ public class GLViewport {
 
     private ShaderProgram sceneProgram;
     private ShaderProgram pickProgram;
+    private ShaderProgram pointProgram;
     private boolean initialized;
 
     // state set off-thread, consumed in render callback
@@ -114,6 +115,7 @@ public class GLViewport {
         if (!initialized) {
             sceneProgram = new ShaderProgram(gl, "scene.vert", "scene.frag");
             pickProgram = new ShaderProgram(gl, "pick.vert", "pick.frag");
+            pointProgram = new ShaderProgram(gl, "point.vert", "point.geom", "point.frag");
             overlay.init(gl);
             gl.glEnable(GL.GL_DEPTH_TEST);
             gl.glEnable(GL.GL_MULTISAMPLE); // anti-aliasing for the multisampled framebuffer
@@ -160,7 +162,14 @@ public class GLViewport {
         sceneProgram.use(gl);
         gl.glUniformMatrix4fv(sceneProgram.uniform(gl, "uMVP"), 1, false, mvp, 0);
         scene.draw(gl);
-        overlay.draw(gl, pointScale, lineScale);
+        // edge markers reuse the flat scene program (pos + colour)
+        overlay.drawLines(gl, lineScale);
+        // vertex markers use the sphere-impostor program (geometry shader -> camera-facing quads)
+        pointProgram.use(gl);
+        gl.glUniformMatrix4fv(pointProgram.uniform(gl, "uMVP"), 1, false, mvp, 0);
+        gl.glUniform2f(pointProgram.uniform(gl, "uViewport"), (float) w, (float) h);
+        gl.glUniform1f(pointProgram.uniform(gl, "uPointSize"), pointScale);
+        overlay.drawPoints(gl);
 
         double[] req = pickRequest;
         if (req != null && currentScene != null) {

CityDoctorParent/Extensions/CityDoctorGUI/src/main/java/de/hft/stuttgart/citydoctor2/gui/gl/HighlightOverlay.java

@@ -74,20 +74,27 @@ public class HighlightOverlay {
         gl.glVertexAttribPointer(loc, comp, GL.GL_FLOAT, false, 0, 0L);
     }
 
-    public void draw(GL3 gl, float pointSize, float lineWidth) {
-        if (!created) {
+    /** Draws the edge markers as lines. The caller must bind a program that reads pos+color. */
+    public void drawLines(GL3 gl, float lineWidth) {
+        if (!created || lineVertexCount == 0) {
             return;
         }
-        if (lineVertexCount > 0) {
         gl.glLineWidth(lineWidth);
         gl.glBindVertexArray(lineVao);
         gl.glDrawArrays(GL.GL_LINES, 0, lineVertexCount);
+        gl.glBindVertexArray(0);
+    }
+
+    /**
+     * Draws the vertex markers as GL_POINTS. The caller must bind the sphere-impostor program (a
+     * geometry shader expands each point into a camera-facing quad shaded as a sphere).
+     */
+    public void drawPoints(GL3 gl) {
+        if (!created || pointCount == 0) {
+            return;
         }
-        if (pointCount > 0) {
-            gl.glPointSize(pointSize);
         gl.glBindVertexArray(pointVao);
         gl.glDrawArrays(GL.GL_POINTS, 0, pointCount);
-        }
         gl.glBindVertexArray(0);
     }
 

CityDoctorParent/Extensions/CityDoctorGUI/src/main/java/de/hft/stuttgart/citydoctor2/gui/gl/ShaderProgram.java

@@ -15,14 +15,26 @@ public class ShaderProgram {
     private final int program;
 
     public ShaderProgram(GL3 gl, String vertexResource, String fragmentResource) {
+        this(gl, vertexResource, null, fragmentResource);
+    }
+
+    /** Builds a program with an optional geometry stage ({@code geometryResource} may be null). */
+    public ShaderProgram(GL3 gl, String vertexResource, String geometryResource, String fragmentResource) {
         int vs = compile(gl, GL3.GL_VERTEX_SHADER, read(vertexResource));
+        int gs = geometryResource != null ? compile(gl, GL3.GL_GEOMETRY_SHADER, read(geometryResource)) : 0;
         int fs = compile(gl, GL3.GL_FRAGMENT_SHADER, read(fragmentResource));
         program = gl.glCreateProgram();
         gl.glAttachShader(program, vs);
+        if (gs != 0) {
+            gl.glAttachShader(program, gs);
+        }
         gl.glAttachShader(program, fs);
         gl.glLinkProgram(program);
         checkLink(gl, program);
         gl.glDeleteShader(vs);
+        if (gs != 0) {
+            gl.glDeleteShader(gs);
+        }
         gl.glDeleteShader(fs);
     }
 

CityDoctorParent/Extensions/CityDoctorGUI/src/main/resources/de/hft/stuttgart/citydoctor2/gui/gl/point.frag

+#version 330 core
+in vec2 vQuad;
+in vec3 vColor;
+out vec4 fragColor;
+
+void main() {
+    float r2 = dot(vQuad, vQuad);
+    if (r2 > 1.0) {
+        discard; // outside the sphere's silhouette
+    }
+    float z = sqrt(1.0 - r2);
+    vec3 normal = normalize(vec3(vQuad.x, vQuad.y, z));
+    vec3 lightDir = normalize(vec3(0.3, 0.3, 1.0));
+    float diff = max(dot(normal, lightDir), 0.0);
+    float ambient = 0.45;
+    vec3 color = vColor * (ambient + (1.0 - ambient) * diff);
+    fragColor = vec4(color, 1.0);
+}

CityDoctorParent/Extensions/CityDoctorGUI/src/main/resources/de/hft/stuttgart/citydoctor2/gui/gl/point.geom

+#version 330 core
+layout(points) in;
+layout(triangle_strip, max_vertices = 4) out;
+
+in vec3 vColorG[];
+
+uniform vec2 uViewport;   // framebuffer size in pixels
+uniform float uPointSize; // sphere diameter in pixels
+
+out vec2 vQuad;           // billboard-local coords in [-1, 1]
+out vec3 vColor;
+
+void main() {
+    vec4 c = gl_in[0].gl_Position;
+    if (c.w <= 0.0) {
+        return; // behind the camera
+    }
+    // half-extent in clip space for a constant on-screen pixel size
+    vec2 h = vec2(uPointSize / uViewport.x, uPointSize / uViewport.y) * c.w;
+    vColor = vColorG[0];
+
+    gl_Position = c + vec4(-h.x, -h.y, 0.0, 0.0); vQuad = vec2(-1.0, -1.0); EmitVertex();
+    gl_Position = c + vec4( h.x, -h.y, 0.0, 0.0); vQuad = vec2( 1.0, -1.0); EmitVertex();
+    gl_Position = c + vec4(-h.x,  h.y, 0.0, 0.0); vQuad = vec2(-1.0,  1.0); EmitVertex();
+    gl_Position = c + vec4( h.x,  h.y, 0.0, 0.0); vQuad = vec2( 1.0,  1.0); EmitVertex();
+    EndPrimitive();
+}

CityDoctorParent/Extensions/CityDoctorGUI/src/main/resources/de/hft/stuttgart/citydoctor2/gui/gl/point.vert

+#version 330 core
+layout(location = 0) in vec3 aPos;
+layout(location = 1) in vec3 aColor;
+uniform mat4 uMVP;
+out vec3 vColorG;
+void main() {
+    gl_Position = uMVP * vec4(aPos, 1.0);
+    vColorG = aColor;
+}