====== Screen Space ======

===== Core Concept =====

Screen Space techniques operate **directly in the final image space** (the screen), rather than in object or world space.

Instead of computing how light interacts with geometry, they **reuse what the camera has already rendered** and manipulate it to produce the desired visual result.

  → If it is not visible on the screen, it does not exist.




===== Mental Model =====

Three main spaces to think in:

  * **World Space** → physical simulation, lighting, PBR
  * **View Space** → camera-relative tricks (Matcap, etc.)
  * **Screen Space** → pixel-perfect control of final image

Screen Space sits at the top of the stack:

  * Not simulation  
  * Not approximation  
  * Final perception control


===== Origin =====

Screen Space techniques emerged in real-time rendering (mid 2000s → early 2010s) as a response to performance limitations.

Full physical simulation (ray tracing, global illumination) was too expensive for real-time systems.

Solution:

  * Only compute what the camera already sees  
  * Reuse existing rendered data  
  * Fake the rest

This led to the rise of:

  * SSAO (Ambient Occlusion)
  * SSR (Reflections)
  * Screen Space shadows and refraction


===== Core Principle =====

All Screen Space techniques share the same law:

<code>
Use the rendered image as the primary data source
→ operate in screen coordinates
→ trade physical accuracy for performance and control
</code>


=== Strength: ===
  * extremely fast
  * visually convincing

=== Weakness: ===

  * blind to anything off-screen
  * can break at edges or occlusions


===== Screen Space Materials =====

A Screen Space material **ignores object-based shading** and instead **displays content aligned with the camera view**.

  → geometry becomes a canvas  
  → the screen becomes the reference

Properties:

  * Not affected by lighting
  * Not affected by environment maps
  * Stable regardless of object orientation
  * Perfect alignment with output

Typical usage:

  * projection mapping (exact output match)
  * overlays / UI in 3D space
  * compositing layers
  * debug / data visualization


==== Key Consequence ====

  → color = texture(screenX, screenY)


The material is evaluated per pixel on screen, not per surface in space.

  * UVs become secondary  
  * normals become irrelevant  
  * lighting is bypassed


===== Screen Space Reflections (SSR) =====

**SSR** uses screen space to **approximate reflections**.

Instead of tracing rays in 3D:

  → sample the already rendered image  
  → reuse visible pixels as reflection data

=== Limitations: ===

  * cannot reflect off-screen objects
  * breaks at screen edges
  * unstable in fast motion

=== Relation to Screen Space materials: ===

  * same coordinate system (screen space)
  * same dependency on visible data
  * different purpose (effect vs display)


===== Other Screen Space Techniques =====

=== Common siblings: ===

  * **SSAO** → fake contact shadows using depth
  * **Screen Space Refraction** → distort background using screen data
  * **SSGI** → approximate bounced light
  * **Screen Space Shadows** → cheap shadow approximation
  * **Screen Space Decals** → project textures after rendering

=== All share: ===

  → operate on the final image  
  → avoid expensive world simulation


===== Real-Time Philosophy =====

Screen Space is **part of a larger doctrine**:

<code>
Reality → Simulation → Approximation → Screen Space → Perception
</code>

=== In real-time systems: ===

  * simulation is expensive  
  * approximation is common  
  * screen space is strategic control


===== Practical Use (Immersive / Media Servers) =====

In immersive environments, Screen Space **shifts from optimization to precision**.

Used for:

  * projection-aligned rendering
  * predictable visual output
  * compositing in 3D pipelines
  * stable overlays and UI elements

=== Key advantage: === 

  → What you see = what you output


===== Limitations =====

  * breaks physical realism
  * flat appearance if misused
  * dependent on camera view
  * no depth awareness beyond visible data

=== Use when: ===

  → control > realism


===== Final Rune =====

Screen Space techniques **do not simulate the world**.

They **control what the audience perceives**, using only what is already visible.

  → Geometry becomes support  
  → The screen becomes truth