itch.io

No problem at all BC Game.

To set the game to a lower resolution without using shaders, I used the following settings in Coppercube's "Publish Settings" for windows.exe (target):

"Fixed/Exclusive" (YES)
"Window Resolution" =800x600
"Colour Depth" =32Bit  
"Target FPS" =260
"Fullscreen" (YES)
"Use Desktop Resolution for Screen?" (NO)
"V-sync?" (No)
"Antialiasing" (No)
"Physics Engine" (Collision Only)

The other step I took was to open all the textures in xnresize (free software) and batch convert all the textures to much lower resolution versions.

https://www.xnview.com/en/xnresize/

In Rebel County (for example) I down-scaled all the textures from 2048 pixels, down to 512 pixels or smaller. This had a huge impact on speeding up the game.

If you resize the original textures (without renaming them or changing the file-location), the resolution changes will automatically be reflected/updated when you next re-open your project in coppercube.

If you don't have the original textures any more, it will be a bit more work, but you can export them all using coppercube's texture window:

by right clicking the individual textures and saving as "image-name.png" (make sure you specify a suitable file-extension otherwise they'll be exported as BMP (bitmap) instead.

Save them all to a new folder, then you can batch-resize them all (using xnresize as before).

You can also manually refresh textures in coppercube (without closing/opening the project) by right-clicking a texture in coppercube's texture window and then clicking "reload texture".

It's good practice to keep a backup of the original files too, instead of overwriting them - just in case you change your mind or need the originals for any other reason later on.

A final thing I did on Rebel county was to add a scan-line.png. To do this, I just added a simple 2D overlay with the following settings:
"x Width" = 101%
"y Height" = 101%
"x Position" = -1%
"y Position" = -1%
"Draw Background?" (NO)
"Use image as background?" (YES)
"Keep image ratio?" (NO)
"Sharp Image" (NO)

If you have several 2D overlays already in your game, to make sure the scan-lines are on top of all the other layers, simply drag the scan-line overlay (in the scene-graph editor) to become a child of the others - this changes the layer ordering (parent to back, child to front).

Here are some example "scan-line overlay" images - or you can search online for "scan-line overlay.png"...
https://forums.libretro.com/t/some-scanline-overlays-w-fade-effect/18457

Make sure the image is a .png format - otherwise you wont be able to see your game through it.

You can use exactly the same technique for a "vignette" effect or a "bezel" effect, or "scratches", "HUDs", "Gun-sights", "blood-splatters", "cracked screen", "Bullet holes".. etc.
and you can turn their visibility on and off to use them as a basic visual effect (such as explosion flashes or a "player-hit" effect).

One thing worth noting - once you add a 2D overlay, when you try to click on anything in the Coppercube editor, it will select the 2D overlay instead (basically it can get in the way of editing your game). To prevent this, simply hide the 2D overlay until you publish your game (I usually just hide it and add a "Make visible: 2D overlay" instruction to the "On first Draw" action, so I never forget to enable it by accident.