1. Navigating huge (UE4 (rendering)) code
UMBC Graphics for Games

2. UE4 is BIG For the 4.20.2-release tag in github C++ (*.h, *.cpp)
51,074 files
14,494,615 lines of C++ code
C# (*.cs)
1813 files
496,550 lines of C# code
Shaders (*.ush, *.usf)
332 files
86,359 lines of HLSL code

3. Navigating large software
Mostly not really just about UE4 or just about rendering
At some point, you will need to make changes to a large pre-existing software base
Tools and strategies are the same

4. Tools of the trade Paper or text editor for notes IDE File search
Know how to bookmark / set breakpoints
Know how to step through & examine code
For UE4, VS extensions in Engine/Extras
File search
VS Find in Files / Xcode search
VS Visual Assist extension (VAX)
External search tool like ripgrep

5. Types of knowledge Big picture Local view
Basic idea of what it is doing
Don’t sweat the details
Local view
How does one thing work
Usually to make changes there or create something similar
Find & understand the local code
Understand who calls it, and with what assumptions
Understand what it calls, and what they do (but not necessarily how)

6. Big picture: Know what to expect
What algorithms do you expect to see?
What data, inputs and outputs?
For rendering, this class

7. Example Two common rendering strategies UE4 supports both
Forward shading
Deferred shading
UE4 supports both

8. Forward shading Compute per-pixel shading while rendering objects
Redundant shading computation for pixels that are overwritten
More per-object control over shading
Supports transparency for sorted objects
Usually limits on type and number of lights

9. Deferred shading
When rendering objects, just save shading parameters per-pixel
In later passes, apply shading and lighting computations per pixel
Culling lights by coverage allows tons of lights
Base shading code needs to be similar for all objects
Rise of Physically Based Shading in games
Transparent objects in a separate forward pass

10. Big picture: Look for documentation
Rendering overview
docs.unrealengine.com/latest/INT/Programming/Rendering/Overview
Some independent blog posts
interplayoflight.wordpress.com/2017/10/25/how-unreal-renders-a-frame
Ask someone who knows

11. Big picture: See what it does
PC: RenderDoc
Install RenderDoc
Enable plugin
Restart UE4
UE4: r.CompositionGraphDebug
Shows all passes
Use vis console command to see pass outputs

12. Big picture: Step through the code
Constrain to the level you care about
Optimizers make life hard
Reorder code, eliminate variables, limit variable lifetime, …
Turn off optimization in just one file
VS: #pragma optimize( "", off )
Xcode: #pragma clang optimize off
DON’T CHECK THIS IN!!!
Breakpoint & step
FDeferredShadingSceneRenderer::Render
FPostProcessing::Process

13. Details: Where to start
Find a starting point in the code
Build your understanding out from there
Keep the window small
You’re changing a black box in a chain of black boxes
Just be sure to honor the interfaces and assumptions

14. Details: Finding code Find in files
VS: Solution search box, Ctrl-Shift-F, Ctrl-T
Xcode: Magnifying glass, search bar
Limit by path, file type, previous search results
Try a few different words
Looking for a page or two of results. Too many isn’t useful.
If you know a log string or cvar name, start with that.
For blueprint or material node, look for node name
Given several choices, look for the more unique one

15. Details: Understanding code you find
Read the code
Add breakpoints
Look at the call stack on break
Step through
May help to add ONETHREAD command line argument
For shader compiler, bAllowCompilingThroughWorkers=False

16. Details: Finding something similar
How to make a material node
Find a uniquely named one, similar to what you want
Copy & rename everything it does
Modify to new purpose
How to make a blueprint node
Find a uniquely named one , similar to what you want
How to make a new postprocessing pass