In this tutorial you will learn how to make a wall light that is seen in the Portal test chambers.
Create a room for the light to be in, for the sake of this tutorial we will use a 384*384*256 room with walls 16 units thick. Ensure the roof is touching the walls only by its edges.
Texture the edges of the roof with the same texture you used for your roof.
Shrink the roof by four on each end, so in this case, from 384*384 -> 376*376
Create a 4*16 block with the invisible texture and put it right beside your newly shrunk roof. They should fit so there are no leaks.
Copy the invisible blocks and re-texture them to have nodraw. Move them higher above the invisible blocks so you can easily preform step 6
Texture the bottom of the nodraw 4*16 blocks to light_recessedcool002. Rotate the recessed texture with the face edit tool so it is parallel with the block, with the lighter side of the gradient facing the inside of the room.
Tip: Change the light_recessedcool002 textures light map resolution to 8 to create a more constant glow. Also, you may choose to set the texture scale higher, so the light does not have the breaks in them
Move the nodraw blocks to sit on top of the invisible blocks.
Select all the top walls and pull them up so they enclose the invisible blocks.
Step 9 (Optional)
You do not have to place the overlays, the differences are negligible.
Add an overlay with decals/overlay_light_gradient, and change the vstart/end to .99 instead of 1.
Resize to fit, and move inside of the hole.
Repeat for every wall
Compile your map, and ensure there are no leaks.