So here are all the explanations
For fs2004, I've created a lot of large photo sceneries (about 15000 km2 each) with the fs2004 resampler to create the bgl and fs2000 resample to slice 256x256 textures. All these sceneries work perfect in fsx. The resampling typical areas are lod8 cells, so each image is 8192x8192 pixels large.
Then I was waiting for fsx to be able to create such sceneries but in a best resolution (2.5m/pixel and more...). So I've tried the fsx resampler to do the same. First I've created the base layer for one example area image, a lod8 cell in a standard 4.75 m/pixel resolution. Ok it fits perfectly, but now I'd like to create a smaller area into this lod8 project. So I've created the same type of fsx project, with a 2.375 m/pixel resolution, and placed it in the same directory that the 4.75 m/pixel bgl. This project is resampled with a blend mask to get a smooth transition with the underlaying 4.75 project. It works but there's a problem all arround the alpha channel area : black stripes, electric blue/red pixels and large black areas replacing the alpha channel when distance from aircraft is increasing !
I've crated another 1m/pixel project in the same way, and it's the same problem.
It seems like the fsx engine cannot recognize the ontop layer, and mix all the layers underlaying !
A solution still exists with keeping fs2004 projects and replacing desired textures in a higher resolution ! it works perfectly without any bug, but the main problem is to slice image (do you know a software that could slice a 24 bits 8192x8192 image in multiples smaller images) and work with dxt3 equivalent transparency.
I'm using bmp files with .inf example descripted here. All the projects except the 4.75 one use a blend mask :
[Destination]
DestDir = .
DestBaseFileName = scenery\img_240
DestFileType = BGL
UseSourceDimensions = 1
LOD = Auto
[Source]
Type = MultiSource
NumberOfSources = 2
[Source1]
Type = BMP
Layer = Imagery
SourceDir = .
SourceFile = img_240.bmp
Variation = Day
ulyMap = 45.285644531250000
ulxMap = 5.786132812500000
xDim = 0.0000286102294921875
yDim = 0.0000214576721191406
Channel_BlendMask = 2.0
[Source2]
Type = BMP
Layer = None
SourceDir = .
SourceFile = img_240_blend.bmp
Variation = Day
ulyMap = 45.285644531250000
ulxMap = 5.786132812500000
xDim = 0.0000286102294921875
yDim = 0.0000214576721191406
SamplingMethod = Gaussian
I'll post screenshots later.