Hello:
IIUC, this may be sorted out by examining a few Effect (*.Fx) files, as discussed in FS Special Effects SDK docs:
https://msdn.microsoft.com/en-us/library/cc526969.aspx#InsertMenu
1.) FSX SDK Doc "
Creating Special Effects.html" > "
ParticleAttributes.0" > "
uv1="
file:///[FSX SDK install path]/Environment%20Kit/Special%20Effects%20SDK/Creating%20Special%20Effects.html#uv1=
"
uv1=0.00, 0.50
The location of the lower-left corner of the texture on the texture page (X, Y) expressed as percentages. The lower-left corner of this texture starts at the left side of the page and 50 percent up its side."
file:///[FSX SDK install path]/Environment%20Kit/Special%20Effects%20SDK/Creating%20Special%20Effects.html#uv2=
"
uv2=0.50, 1.00 The location of the upper-right corner of the texture lies on the texture page (X, Y) expressed as percentages. The upper-right corner of this texture is 50 percent to the right and at the very top of the page. See the diagram below."
2.) [FSX install path]\Effects\
fx_navyel.fx
FYI: Effect 'basic' and Effect 'controller'
*.fx files (including those from OrbX) can be opened / examined in Windows NotePad instead of using FSX SDK FX Tool etc.
< Sub-section excerpt: "
fx_navyel.fx" >
[ParticleAttributes.0]
Blend Mode=
2
Texture=
fx_2.bmp
Bounce=0.00
Color Start=150, 150, 0,
1
Color End=150, 150, 0,
0
Jitter Distance=0.00
Jitter Time=0.00
uv1=0.00, 0.00
uv2=0.01, 0.01
X Scale Goal=0.00
Y Scale Goal=0.00
Z Scale Goal=0.00
Extrude Length=0.00
Extrude Pitch Max=0.00
Extrude Heading Max=0.00
3.) [FSX install path]\Effects\Texture\
fx_2.bmp < ...see image above under "
1.)" >
AFAIK, in other (
ex: terrain / landclass scenery or MDL-based) scenery object texture file scenarios, FS bitmap pixel coordinates are referenced as starting at the
Upper Left corner, and ending at the
Lower Right corner.
But
Effects pixel coordinates are referenced as starting at the Lower Left, and ending at the
Upper Right corner of a mapped texture bitmap.
<
Perhaps because Fx-mapped texture BMPs are "vertically-flipped" and FS reads them from a 'meta-object' DirectX graphics buffer address ? 
>
PS: For info on Effect transparency, see...
4.) FSX SDK Doc "
Creating Special Effects.html" > "
ParticleAttributes.0" > "
Blend Mode="
file:///[FSX SDK install path]/Environment%20Kit/Special%20Effects%20SDK/Creating%20Special%20Effects.html#Blend Mode=
"
Blend Mode=1/2/3
Blend mode specifies which blending mode is used to display the texture, which should be one of:
0: No Alpha
1: Alpha blending
2: Additive blending
3: Multiply blending
Alpha textures are full color and include a transparency mask (alpha channel).
Additive textures, on the other hand, have no alpha channel. The transparency of an additive texture is based off of color values within the texture. Color values in the texture are added to the background color. If an additive texture is triggered against pure white, nothing happens because the values are already maximized at 255, 255, 255, which represents the RGB value for white. Any part of the texture that you want to be invisible must be pure black < at 0,0,0, > (or else you’ll just get expanding squares). Additive textures are best used when the effect is simulating something bright—like fire or a light."
[
EDITED]
5.) FSX SDK Doc "
Creating Special Effects.html" > "
ParticleAttributes.0" > "
Color Start="
file:///[FSX SDK install path]/Environment%20Kit/Special%20Effects%20SDK/Creating%20Special%20Effects.html#Color Start=
"
Color Start=000, 000, 000, 000
The initial color of a particle. The first three numbers define the red, green, and blue (RGB) values and the fourth number specifies the amount of alpha transparency (R, G, B, [00-255])."
6.) FSX SDK Doc "
Creating Special Effects.html" > "
ParticleAttributes.0" > "
Color End="
file:///[FSX SDK install path]/Environment%20Kit/Special%20Effects%20SDK/Creating%20Special%20Effects.html#Color End=
"
Color End=000, 000, 000, 000
The final color of a particle. The first three numbers specify the red, green, and blue (RGB) values, and the fourth number specifies the amount of alpha transparency (R, G, B, [00-255])."
[
END_EDIT]
NOTE: Alterative MS-FSX / ESP URL for
Special Effects SDK Documentation:
http://msdn.microsoft.com/en-us/library/cc526969.aspx#Properties
Hope this might help with mapping / displaying a texture 'cell' area from "
fx_2.bmp" with the desired transparency !
Regards,
GaryGB
