Hi there, I am new to the forums and indeed new to ImageMagick. But I had this problem and was brought in here so I thought I would add my findings in case they help.
I am not totally sure what is going on either but I wanted to change the DPI and size and then convert from PNG to TIF and I got the same results as on here (When loading back into photoshop CS I see a black background and not a transparent background)
My 2 commands are quite simple:
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exec("convert -density 300x300 -units PixelsPerInch -resize 3600x4500 altResampledImages/72/LJMTest72.png altResampledImages/300/LJMTestTry1.png");
exec("convert altResampledImages/300/LJMTestTry1.png altResampledImages/300/LJMTestTry1.tif");
I split the 2 commands out purely to make sure the PNG2TIF wasnt affected by all the DPI changes and wotnot.
I am also not messing around with CMYK or RGB and Alpha channels.
So like I say, in Photoshop CS I get the black background as well. However, when I right click the file and say "View in Windows Photo Gallery" Then the background is white. So that says to me that the actual TIF file isnt specifying a black background, it is just Photoshop that cant handle the background.
I am now going to the customer to see if they use photoshop for their needs for the result image. If they dont and they use something else that does in fact see a transparent background then there is probably no need for me to try and fix this issue. We can just put it down to being Photoshops fault heheh.
One issue is that MS Paint also doesnt open the TIF file which is a little nerve racking however that could just be due to the 300dpi setting of it perhaps?