Clarifying Differences in Line Rendering Between OpenGL and DX12
I’d like to clarify some points regarding the visual differences in line rendering between the old OpenGL engine and the modern DX12 engine, especially focusing on anti-aliasing (AA) and driver settings.
Resolution and Line Drawing :
Both engines produce lines with the same resolution and thickness.
The provided screenshots are at 1080p with 2x zoom to illustrate differences before any anti-aliasing (AA) is applied. 
Impact of Anti-Aliasing (AA) :
The differences in appearance are primarily due to the varying methods and amounts of AA applied by the graphics drivers and APIs.
AA can smooth out the jagged edges of lines, but different implementations can lead to different visual results.
Driver Settings and Gamma Correction :
On one of my setups, AA gamma correction is enabled by default in the Nvidia driver, which affects the OpenGL engine more noticeably.
With 8x MSAA (Multisample Anti-Aliasing) enabled, the lines appear lighter in the OpenGL engine due to this gamma correction.
Consistency Across Engines :
Disabling AA gamma correction in the Nvidia Control Panel (NVCP) can make the line appearance consistent across both OpenGL and DX12 engines.
Hardware and Driver Variability :
The impact of AA gamma correction is hardware-dependent; not all GPUs and drivers support this adjustment.
The differences can also vary for users with different hardware setups, such as Mac users or those using AMD hardware.
User Perception :
The differences may be non for many users due to these variations in hardware and driver capabilities.
Conclusion:
The visual differences in line rendering between the old OpenGL engine and the modern DX12 engine are influenced by the Nvidia driver’s handling of AA gamma correction. Disabling this feature can align the appearance of lines across both engines. However, since this setting is hardware-specific, not all users will experience the same results.
I hope this explanation helps clarify why some users might see differences while others do not.
For futher reading on Gamma corrected MSAA:
What’s Gamma Correct AA? - NVIDIA’s GeForce 8800 (G80): GPUs Re-architected for DirectX 10 (anandtech.com)