How to create a Decal

1/ Open a picture

2/ Crop the picture to 64 pixel in height and width

3/ Put this picture in black and white (greyscale option)

If the picture is so dark, you can adjust contrast and brightness

The grey element are more or less transparents according to the level (to very transparent at few transparent).

All this white elements blanc become transparents (invisibles) after the negatif.

4/ Make a negatif of picture to get some black or white contours

5/ Save this picture in repertory Half-Life\Logos in bitmap format (.bmp).

It appears automatic in the list of Decals, in the interface of the game.

You can to attribute the color who appears in the game WHEN you launch the game. (customize game)