Hello! Perhaps it was discussed somewhere, but I did not find it. How to make a sidescan mosaic with transparent edges? I did not succeed. Here is an example picture 1:
Picture 2 shows the sidescan mosaic created by the Lowrance HDS-Live device:

sorry used google translator

Sidescan mosaics exported from Reefmaster are tiled rasters. Generally speaking, tiled rasters have a black boundary around the outside where alpha occurs.

There's a possibility you can get this to work but it's hit or miss based on software version. You'll need to save the mosaic as an image and process it yourself through the raster processor. You'll need to have an underlying "Shaded Relief" layer. You'll need to set the raster type on the mosaic to "Satellite Imagery". You'll need to set the "Alpha Threshold" to some value greater than 0.

Thank you so much gpiverson! It seems I understood how to build a raster for my device thanks to you. Eh! It would be very great if RM kept in .smf format as "Structure"

Forced to continue the topic! I create a "Satellite Imagery" SideScan mosaic, but it is displayed on the device under the vector layer (under water). "Shaded relief" overlaps vector objects with parameters of the attributes of IMG_RENDER = 0. "Satellite Imagery" layer remains under the vector. How to place the "Photo overlay" over the vector layer?

You can disable the rendering of vector features over raster or generally but you can't reorder the layers. The layer ordering (outside RNC raster which can switch depending on settings) are hard coded.

I never managed to make a side scan mosaic with transparency. I exported via "Satellite imagery" -> "alpha threshold = 1". “Shaded Relief” is turned on, but on the chartplotter ме need to set the "Photo Overlay" -> "Transparency" - 10%, if the "Transparency" is “0” then black tiles again. It just breaks the brain. 2019 year. A watch on a hand for $ 100 - shows transparency, a chartplotter (3 kg of brains) $ 3,000 - no.

Advise what to do?

That's the best you'll be able to display transparent raster pixels within the current system..