How to create custom sprites
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How can I make my own Sprites for Doom?
First get yourself a Wad editor (doesn't matter which check the WAD file section for more information on editors) and find the sprites you are trying to replace - extract them and open them in a graphics editor now resize the image to at least 4x (8x might be better) the original size keeping the aspect ratio (this should mean Doomsday won't distort your sprite frame when it's placed in engine).
Now make a note of the dimensions of the frame (width and height) and filename and make a new file (this is important for copyright reasons) and either draw the image from scratch. You may wish to take further positioning notes so that custom sprites will line up with the originals. Another method (that was used by id themselves) is to make or find a 3D model of the sprite and take pictures of it from different angles (and different states: attacking, dead etc‚Ä¶) and paste them in.
I'd recommend saving the files as 24bit pngs, but if you feel confident you can try saving space by using custom palettes with 8 bit images. You should already have the right filename because you used the original file for the aspect ratio. Now simply place them in your patches folder to replace the original just like any other patch.