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74 lines
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74 lines
3.3 KiB
ReStructuredText
.. _doc_xr_full_screen_effects:
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XR full screen effects
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======================
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When adding custom full screen effects to your XR application, one approach is
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using a full screen quad and applying effects to that quad's shader.
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Add a :ref:`MeshInstance3D <class_MeshInstance3D>` node
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to your scene as a child of your :ref:`XRCamera3D <class_XRCamera3D>`,
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and set the ``mesh`` property to a :ref:`QuadMesh <class_QuadMesh>`.
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Set the width and height of the quad to ``2``.
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.. image:: img/xr_full_screen_effects_starting_quad.webp
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You can then add a shader to your quad to make it cover the screen. This is done by setting the
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vertex shader's ``POSITION`` built-in to ``vec4(VERTEX.xy, 1.0, 1.0)``.
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However, when creating an effect that is centered straight ahead in the user's view
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(such as a vignette effect), the end result may look incorrect in XR.
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Below shows captures of the right-eye view with a vignette shader, both from the headset and the render target itself.
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The left captures are an unmodified shader; the right captures adjust the full screen quad using the projection matrix.
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While the capture on the left is centered in the render target, it is off-center in the headset view.
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But, after applying the projection matrix, we see that the effect is centered in the headset itself.
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.. image:: img/xr_full_screen_effects_vignette_before_after.webp
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Applying the projection matrix
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------------------------------
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To properly center the effect, the ``POSITION`` of the full screen quad
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needs to take the asymmetric field of view into account. To do this while also ensuring the quad
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has full coverage of the entire render target, we can subdivide the quad and apply the projection matrix
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to the inner vertices. Let's increase the subdivide width and depth of the quad.
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.. image:: img/xr_full_screen_effects_ending_quad.webp
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Then, in the vertex function of our shader, we apply an offset from the projection matrix to
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the inner vertices. Here's an example of how you might do this with the above simple vignette shader:
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.. code-block:: glsl
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shader_type spatial;
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render_mode depth_test_disabled, skip_vertex_transform, unshaded, cull_disabled;
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// Modify VERTEX.xy using the projection matrix to correctly center the effect.
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void vertex() {
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vec2 vert_pos = VERTEX.xy;
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if (length(vert_pos) < 0.99) {
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vec4 offset = PROJECTION_MATRIX * vec4(0.0, 0.0, 1.0, 1.0);
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vert_pos += (offset.xy / offset.w);
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}
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POSITION = vec4(vert_pos, 1.0, 1.0);
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}
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void fragment() {
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ALBEDO = vec3(0.0);
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ALPHA = dot(UV * 2.0 - 1.0, UV * 2.0 - 1.0) * 2.0;
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}
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.. note:: For more info on asymmetric FOV and its purpose, see this
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`Meta Asymmetric Field of View FAQ <https://developers.meta.com/horizon/documentation/unity/unity-asymmetric-fov-faq/>`_.
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Limitations
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-----------
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This full screen effect method has no performance concerns for per-pixel effects such as the above vignette shader.
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However, it is not recommended to read from the screen texture when using this technique.
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Full screen effects that require reading from the screen texture effectively disable all rendering performance optimizations in XR.
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This is because, when reading from the screen texture, Godot makes a full copy of the render buffer;
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this drastically increases the workload for the GPU and can create performance concerns.
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