Trepidor

The tool allows to remove black bars from the game at 21:9 and wider and increase or decrease field of view in real-time at any resolution.
The included presets will adjust the vertical FOV at the gameplay to match that of 16:9, with some deviations at the cutscenes and interactions mostly in favor of higher vertical FOV.
Download and unpack the archive using software like 7zip or PeaZip. The password is pcgw Launch the game, followed by the tool. Press the hotkeys as instructed to make the desired changes. Tested the latest Microsoft Store / Game Pass version at 2560x1080 and briefly at 3440x1440, 3840x1080 and 5760x1080.

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These solutions for 2560x1080, 3440x1440, 32:9, 5:4, 4:3 and 16:10 resolutions will remove the black bars from the game without reducing the field of view. The pre-rendered cutscenes, subtitles and some of the UI elements will be stretched.
Unpack the applicable archive with software like 7zip - the password is pcgw if required.
If using the trainer, make sure the game is launched before it. After applying the fixes with the hotkeys, change the display mode to fullscreen or borderless and apply. If using the patcher with the Steam version, unpack the game executable with Steamless first, rename it back, then patch. Tested on the latest EGS and Steam versions mainly at 2560x1080, 3440x1440 and 3840x1080.
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The tool removes black bars from the in-game cutscenes, cinematic cameras, shops and a number of other areas at 21:9 and wider resolutions. Since version 2.0 it also removes pillarboxing from the photo mode. Since version 3.0, it allows for custom FOV adjustment.
Launch the game first, then the tool. The password for the archive is pcgw
As expected from a trainer based on CE components, this tool does get falsely flagged by some anti-virus software as a trojan, while the rest correctly mark it as a "hacktool".
Use the tool provided here at own risk and ideally avoid playing RDR Online without completely disabling the fix - especially the FOV hack.
Tested at 2560x1080, 3840x1080 and 5760x1080. Additionally confirmed as working by two volunteers from WSGF.
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The tool removes black bars from the game at 21:9, 32:9, 48:9 and other wide resolutions.
Download and unpack the archive using software like 7zip or PeaZip. The password is pcgw Launch the game Launch the tool Press the "Enable" key While in the game, briefly switch to the Borderless or Fullscreen mode. Use the tool to manually adjust the UI size at 32:9 and wider resolutions. Known issues:
Stretched loading screens and rare FMVs. Minor UI display and alignment issues. Use the UI adjustment keys if necessary. The window mode switching requirement is also undesirable, hence the version number!
Tested the latest Epic version at 2560x1080, briefly at 3840x1080, 5760x1080, 5120x2160 and 7680x1080 on a single monitor.
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The small patcher removes black bars from the game at all wide resolutions.
Keep in mind that some of the textures simply do not have enough width to fill the screen, and the menu UI will become cropped the wider it gets from 21:9, although the game appears to be playable even at 48:9.

Tested the latest GOG version at 2560x1080, 3440x1440, 3840x1080 and 5760x1080.

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The trainer displays the current FOV and allows to change it at all resolutions in real time.
Unpack the archive with software like 7zip - the password is pcgw Launch the game, followed by the tool. Press the keys as instructed All trainers based on CE components may trigger some anti-virus software.

Tested the latest full GOG version and the Steam demo.
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