I take it you got an error?
It's a pretty simple process for what you need to do.
1: put both bloodgulch.map and bitmaps.map in the same folder.
2: Open demo hack
3: press load map and select your bloodgulch.map
4: press to full (no need to even quit after this)
5: open HMT (not Halo Modding Tools 0.5)
6: press Load and select the same bloodgulch.map
7: click the disclosure triangle to the left of bitm
8: select the meta of the bitmap you want to edit (if you don't have bitmaps.map in the same folder HMT will error)
9: press Preview and wait for the image to be loaded
10: press Export (if it's a DXT2/DXT3 you should get 2 images) and save the image(s) where you want them to be
11: edit the image using your preferred image editor
12: press Inject on HMT (if it created two images, put the first one in Texture, the second one in Mask. you have to copy the image file to the clip board and highlight the box and paste into that box)
13: press Inject on the popup window
14: if it worked HMT will crash after a few seconds or when you click on a different tag. if it didn't work, HMT will give an error saying it was not applied correctly. Repeat from 9 until it does work (aka, HMT crashes with a NilObject Exception)
15: go through steps 5 to 9 to make sure your image loaded the way it was supposed to
16: quit HMT
17: go to demo hack which ought to still be open
18: press to demo
19: quit demo hack
20: put both bloodgulch.map AND bitmaps.map in your maps folder (probably a good idea to have backed-up the original versions)
The reason for the AND is because HPC relies on shared assests like bitmaps and sounds rather than storing them per map unlike the xbox and CE versions. So you have to change bitmaps.map by accessing the files in it using the data from a game play map. Any changes to a bitmap will be noticeable for all maps that use that image.
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