Add PickingFramebuffer for color-pick readback

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import com.jogamp.opengl.GL;
+import com.jogamp.opengl.GL3;
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+
+/** Off-screen framebuffer that stores picking ids as RGBA8; supports single-pixel readback. */
+public class PickingFramebuffer {
+
+    private int fbo;
+    private int colorTex;
+    private int depthRbo;
+    private int width;
+    private int height;
+    private boolean created;
+
+    public void resize(GL3 gl, int w, int h) {
+        if (w <= 0 || h <= 0 || (w == width && h == height && created)) {
+            return;
+        }
+        dispose(gl);
+        width = w;
+        height = h;
+
+        int[] tmp = new int[1];
+        gl.glGenFramebuffers(1, tmp, 0);
+        fbo = tmp[0];
+        gl.glBindFramebuffer(GL.GL_FRAMEBUFFER, fbo);
+
+        gl.glGenTextures(1, tmp, 0);
+        colorTex = tmp[0];
+        gl.glBindTexture(GL.GL_TEXTURE_2D, colorTex);
+        gl.glTexImage2D(GL.GL_TEXTURE_2D, 0, GL.GL_RGBA8, w, h, 0, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, null);
+        gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MIN_FILTER, GL.GL_NEAREST);
+        gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MAG_FILTER, GL.GL_NEAREST);
+        gl.glFramebufferTexture2D(GL.GL_FRAMEBUFFER, GL.GL_COLOR_ATTACHMENT0, GL.GL_TEXTURE_2D, colorTex, 0);
+
+        gl.glGenRenderbuffers(1, tmp, 0);
+        depthRbo = tmp[0];
+        gl.glBindRenderbuffer(GL.GL_RENDERBUFFER, depthRbo);
+        gl.glRenderbufferStorage(GL.GL_RENDERBUFFER, GL.GL_DEPTH_COMPONENT16, w, h);
+        gl.glFramebufferRenderbuffer(GL.GL_FRAMEBUFFER, GL.GL_DEPTH_ATTACHMENT, GL.GL_RENDERBUFFER, depthRbo);
+
+        gl.glBindFramebuffer(GL.GL_FRAMEBUFFER, 0);
+        created = true;
+    }
+
+    public void bind(GL3 gl) {
+        gl.glBindFramebuffer(GL.GL_FRAMEBUFFER, fbo);
+        gl.glViewport(0, 0, width, height);
+    }
+
+    /**
+     * Reads the picking id at canvas pixel (x, y) measured from the top-left. GL's origin is bottom-left,
+     * so the row is flipped.
+     */
+    public int readId(GL3 gl, int x, int y) {
+        if (!created || x < 0 || y < 0 || x >= width || y >= height) {
+            return 0;
+        }
+        ByteBuffer buf = ByteBuffer.allocateDirect(4).order(ByteOrder.nativeOrder());
+        gl.glBindFramebuffer(GL.GL_FRAMEBUFFER, fbo);
+        gl.glReadPixels(x, height - 1 - y, 1, 1, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, buf);
+        gl.glBindFramebuffer(GL.GL_FRAMEBUFFER, 0);
+        int r = buf.get(0) & 0xFF;
+        int g = buf.get(1) & 0xFF;
+        int b = buf.get(2) & 0xFF;
+        return PickingCodec.decode(r, g, b);
+    }
+
+    public void dispose(GL3 gl) {
+        if (created) {
+            gl.glDeleteFramebuffers(1, new int[]{fbo}, 0);
+            gl.glDeleteTextures(1, new int[]{colorTex}, 0);
+            gl.glDeleteRenderbuffers(1, new int[]{depthRbo}, 0);
+            created = false;
+        }
+    }
+}