I don’t use Inkscape, but I know that with Affinity Designer transparent PNGs work fine embedded in SVG but if I put them into PDFs* they tend to end up with black backgrounds and no transparency when imported into the GFUI. Perhaps something like that happens in Inkscape as well?
* I use PDF often with Affinity Designer because AD can’t save SVGs with proper units. The only real alternative is using the 20x12 hack.
I can confirm that doing rotations/flips/anything other than uniform scale inside of inkscape will result in a bad engrave. Everything will look fine in Inkscape and in the GFUI even, but when it’s actually running the job, the engrave will be completely foobar. Often the head on the GF will go randomly moving around instead of just sweeping back and forth over the engrave area, and it will have more jerky stops as it moves around. It’s really bad. I ran into this the first time 2 nights ago, when all I did was flip my image horizontally in inkscape.
I have, also, successfully used PNGs with transparency embedded into SVGs with Inkscape and had them work fine. In fact, almost everything I engrave is done this way. I load the image with transparnecy into Inkscape and then add a cut outline shape around it. That all gets saved as an SVG and loaded and run on the GF just fine.
I’ve seen transparency interpreted as black before. It was uncool, the background was supposed to be untouched, but it was instead engraved super hard. Maybe it was something specific to that file, it was a while ago, but I try to keep it as simple as possible. I’m probably wrong about transparency, but I assure you, rotation is baaad.
In Adobe Illustrator you can “Rasterize” your bitmaps before doing the import, and that flattens out all of the transformations, meaning that the flipping, rotating, etc., is done by Illustrator, so that the GlowForge gets a simple bitmap (with no flipping, scaling, rotating, etc.) in the SVG, which it can print just fine.
The only drawback of Rasterizing is that you lose some info when things get flattened down. So I usually make a copy, hide the original (in a different layer), then Rasterize the copy. That way if I need to go back and redo some manipulation, I still have the original version that I can work from, and re-do the copy and Rasterize.
In the absence of any other information (such as real-world units), the GFUI assumes SVGs have a resolution of 96 dpi. When you export out of Designer, be sure the dpi is set to 96 and “Set viewBox” is turned off and your projects will cut at the right size on Glowforge no matter what size the document is. Well, almost. If you happen to have a document that’s the magic 5:3 ratio, the 20x12 hack will kick in and scale your document up to the full 20x12.