This article details the information in the sprite text files in the Half-Life pak0.pak file so you can add your own sprites to the HUD. This is closely associated with editing code but I'll leave that to programmers and just explain the text files. This does not explain how to move sprites around in the game HUD, but rather to modify these text files to accommodate your new sprites. This is necessary if you're adding new weapons or if trying to replace existing sprites is too restricting. For a mod, you need to include any edited or new text files in your own pak file.

To edit sprites, see my sprite tutorial. It gives information about the types of sprites, as does Valve's Sprite Tutorial.


Here is a listing of the sprites involved and the list of text files, all in the sprites folder of the pak file. I'll give the details for Hud.txt and one of the weapons, Weapon_9mmar.txt.

Hud.txt
Below is a sampling of the many sprites in hud.txt. It includes important things such as the health numbers, damage icons and weapon icons that appear when you pick up a weapon. Here's a listing of the sprite parameters:

The first number (121) is how many sprites there are in this text file. Then it starts listing the sprites. First is the sprite Index or name that it's called in the code. Then either 320 or 640 is chosen. 320 is for screen resolutions less than 640x480, and 640 is for 640x480 and above. The sprites are organized to list the 320 ones first, then the 640 sprites. Next is the actual sprite filename without the .spr extension. Then comes the top left X coordinate and the top left Y coordinate in that file. Next is the width and height to designate a rectangle which includes the image you'd like to use for that sprite.

To find out more about changing the HUD in Half-Life's source code, see cl_dll\hud.cpp.


Weapon_9mmar.txt
Below are the sprites in weapon_9mmar.txt. It includes when you select a weapon to use, it's ammo and crosshairs. Here's a listing of the sprite parameters:

The first number (12) is how many sprites there are in this text file. Then it starts listing the weapon's sprites. First is the sprite Index or name that it's called in the code. The Sprite Index choices are: ammo, ammo2, autoaim, crosshair, weapon, weapon_s, zoom and zoom_autoaim. Ammo and ammo2 are primary and secondary ammo sprites for when you pick them up. Weapon_s is it's selected state in the HUD, and weapon is not selected, plus it will change color when the weapon is out of ammo. The colors are listed in cl_dll\hud.h as RGB_YELLOWISH, RGB_REDISH and RGB_GREENISH, which can be changed. Then either 320 or 640 is chosen for screen resolutions. Next is the actual sprite filename without the .spr extension. Then comes the top left X coordinate and the top left Y coordinate in that file. Next is the width and height to designate a rectangle which includes the image you'd like to use for that sprite.

To find out more about weapon sprites in Half-Life's source code, see cl_dll\ammo.cpp.


Conclusion
If you're replacing existing sprites and they're not the same size, you can simply change the coordinates in these files. If you want to add new sprite files to the HUD, just add them in to hud.txt and set it's number appropriately. You can create your own text files for new weapons but you'll need to add code to load in the text files.