First, enter the game, and set your graphics preferences. Save and quit.
Then, go to:
My Documents\Command and Conquer Generals Data
Edit the file "options.ini." You will find a line beginning with the word "Resolution." Change it to set your resolution.
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Thanks to neoWidescreen and KOJiN ... ... here is an optional tweak ...
* RESOLUTION - Change it by editing OPTIONS.INI under your "my documents\Command and Conquer Generals Zero Hour Data" folder as previously listed above.
Unzip GAMEDATA.INI into your "Command & Conquer Generals Zero Hour\Data\INI" folder. (typically under Program Files)
Edit MaxCameraHeight to adjust the FOV. Default setting is 310. Setting it to 410 is approximately the same as a 4:3 monitor, and 510+ gives you a widescreen advantage. ;-)
This Gamedata.ini file is set at maxCameraHeight of 510, however 610 looks sensational at 1920x1200.
Notes:
Zero Hour ... the addon for Generals, creates its own seperate install. So instead of editing the value in "inizh.big" (which is where "gamedata.ini" mentioned above comes from) you would edit the same field in "ini.big" in the generals root dir.
Zero Hour creates it's own installation, you *must* still have the original Generals installed - otherwise ZH doesn't load.
When you unpack the gamedata.ini you DO NOT have to re-insert it into "INIZH.BIG". You can if you want to be super-leet, but it's easier to edit when left in the INI folder.
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Extra:
I had been playing on my 24" in 1920x1200 with MaxCameraHeight set to 700. This loses the terrain at the very screen edges, but you live with it (it looked worse in cutscenes, but who cares!) Now I've got a 30" in 2560x1600, I've got MaxCameraHeight set to 1000 and the terrain problem means it only draws the terrain for half the screen.
A fix for this is to set
DrawEntireTerrain = Yes
(also in GameData.ini). This doesn't seem ideal - you want to extend the camera distance the terrain draws to, not make it draw the entire map. But hey, it at least works.
Now with such a huge camera height, the planes look more ridiculous than ever, they're so close to the ground. Found their heights defined in Locomotor.ini - most of them by default are 100. Last game I tried I had helicopters set at 150 and planes set at 300, however this gives a glitch with the planes landing VTOL style sometimes (which causes them to take ages to land), so I think 250 should be safe. Up to you if you want to post this last bit of info... since this is modifying the game more than just making it widescreen