capture projection/view matrices for world-to-screen projection

MatrixCapture keeps the matrices from the level render pass so 2d
overlays (nametags) can project world positions onto the gui-scaled
screen.

src/main/java/cc/fros/mixin/render/MixinLevelRenderer.java

@@ -7,6 +7,7 @@ import com.mojang.blaze3d.vertex.PoseStack;
 import com.mojang.math.Axis;
 import cc.fros.event.impl.render.Render3DEvent;
 import cc.fros.event.impl.render.RenderBlockOutlineEvent;
+import cc.fros.util.render.MatrixCapture;
 import net.minecraft.client.Camera;
 import net.minecraft.client.DeltaTracker;
 import net.minecraft.client.renderer.LevelRenderer;
@@ -37,6 +38,9 @@ public class MixinLevelRenderer {
                         Camera camera, Matrix4f positionMatrix, Matrix4f matrix4f, Matrix4f projectionMatrix,
                         GpuBufferSlice fogBuffer, Vector4f fogColor, boolean renderSky, CallbackInfo ci, @Local ProfilerFiller profiler) {
 
+        MatrixCapture.projection = new Matrix4f(projectionMatrix);
+        MatrixCapture.view = new Matrix4f(positionMatrix);
+
         PoseStack stack = new PoseStack();
         stack.pushPose();
         stack.mulPose(Axis.XP.rotationDegrees(mc.gameRenderer.getMainCamera().xRot()));

src/main/java/cc/fros/util/render/MatrixCapture.java

@@ -0,0 +1,49 @@
+package cc.fros.util.render;
+
+import cc.fros.util.traits.Util;
+import net.minecraft.world.phys.Vec3;
+import org.joml.Matrix4f;
+import org.joml.Vector4f;
+
+/**
+ * Projection/view matrices captured during the level pass, used to project
+ * world positions onto the gui-scaled screen for 2d overlays like nametags.
+ */
+public final class MatrixCapture implements Util {
+    public static Matrix4f projection;
+    public static Matrix4f view;
+
+    private MatrixCapture() {
+        throw new AssertionError();
+    }
+
+    public static float[] worldToScreen(double worldX, double worldY, double worldZ) {
+        if (projection == null || view == null) return null;
+
+        Vec3 camPos = mc.gameRenderer.getMainCamera().position();
+        Matrix4f mvp = projection.mul(view, new Matrix4f());
+
+        Vector4f point = new Vector4f(
+                (float) (worldX - camPos.x()),
+                (float) (worldY - camPos.y()),
+                (float) (worldZ - camPos.z()),
+                1.0f
+        );
+        mvp.transform(point);
+
+        if (point.w <= 0.0f) return null;
+
+        float ndcX = point.x / point.w;
+        float ndcY = point.y / point.w;
+
+        if (ndcX < -1.1f || ndcX > 1.1f || ndcY < -1.1f || ndcY > 1.1f) return null;
+
+        int guiW = mc.getWindow().getGuiScaledWidth();
+        int guiH = mc.getWindow().getGuiScaledHeight();
+
+        return new float[]{
+                (ndcX + 1.0f) / 2.0f * guiW,
+                (1.0f - ndcY) / 2.0f * guiH
+        };
+    }
+}