Transparent PNG image imported to SU 2021 and 2022 shows black where it should be clear

I’ve just scanned a couple of black and white images into SU Pro, both 2021 and 2022.

Both SU versions behave the same way.

Here’s one of the PNG images as viewed in Mac Monterey 12.3 Preview:

The transparent parts of the image come in as black:

On all previous (but not very recent) occasions, the transparency has stayed transparent when imported as a texture (as this image was - I just checked that).

And when I directly uploaded the PNG file to this forum post, it too showed a black background, unlike the image above which is a screen capture pasted in.

Is this some change in Mac OS X, something recent in SU or something else?

Can you put the actual png into a zip file and upload that? The forum changes the file.

Yes, here it is, zipped:
Studio lighting grid 1 (76.3 KB)

Heading for bed now but will try tomorrow.

Thanks. I wonder if this is a Mac thing? It used to work using exactly the same method and tools - Preview and SU only.

No idea where the black pixels came from. It looks on my machine almost as if it’s turned into a negative.

Not sure what’s going on at your end. Maybe a change in Preview?

Well, your transparent download works for me - comes in transparent to SU.

Maybe it is something in Preview? Weird. No time to explore further tonight, but I’ll try another image editor to convert the scan to transparent background.

Very quick reaction: I’ve seen this kind of problem before on a Mac, and I don’t have an answer right away. It might help to open and resave in another application like Photoshop or something. It’s as if there’s a LUT (look up table) problem at times with either absolute white or absolute black or transparent in some look up table attached to the file, and maybe using another app to save that file might handle it better? No guarantee.

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It is possible to have an 8 bit PNG with transparency, but normally PNGs are 32 bit, 24 of which is RGB, and 8 bits for transparency. Your image is a grayscale one. It seems to have lost any transparency along the way. What was the original image you started with?

I started with a black and white image on tracing paper, scanned into Preview from my Epson WF 7720 multifunction A3 printer/scanner and saved as PNG.

In Preview, I used the magic wand tool to select all the white areas in turn, and pressed the backspace key to delete the colour and make these areas transparent.

Saved the file.

But when I reopen it in Preview, the background supposed to be transparent has gone black.

A thought. Previously, with other paper images I saved the file from the scanner as a jpg and when I selected and deleted the background white, I was asked whether to convert to png. I wonder if the directly saved png from the scanner doesn’t have transparency properly enabled?

I’ll rescan tomorrow and see if that is the difference from former successes.

Your comment, @colin made me think of that possibility.

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Preview’s magic wand tool is amazing, but with the steps you took it seems not to give a good transparent image. Here’s the same steps with Photoshop, see how this goes.

By the way, this is not likely to be better than the one Dave did earlier.

Can’t get to sleep (3:10am here in UK) so tried scanning and saving as jpg before using magic wand and converting to png.

No difference, unfortunately. Image after making background transparent looks fine in Preview, but when saved and reopened, ‘transparent’ image sections still go black.

Will try Apple forum later.

Not a SU problem, apparently.

I don’t have any version of Photoshop to try.

Thanks @DaveR and @colin for converting part of my scan to transparent png. Could either of you convert the other part for me if it isn’t too time consuming, to let me progress with this project? Thanks, both, for your help so far.

I’ve attached both the original saved jpg file, and the one theoretically made transparent and saved as a png, for the second part of my scan.

And I can confirm that the checkbox Alpha in the Save dialogue was checked.

For the longer term, I’ll try to get help from the Apple support forum to see why it isn’t working properly any more.
Studio lighting grid (168.6 KB)

Studio lighting grid (68.5 KB)

If the goal is to trace over the dark lines in the image, then directly use the JPEG, and don’t worry about making it transparent.

That is one option I was considering and might well have used.

The other was just to use a transparent image, with no actual geometry. But that now won’t work.

Spent some time today with Apple online chat support, so far to no avail, to try to understand the issue in Preview. Seems to me like a bug.

Will defer further effort on this, as it turns out the drawing is at least three decades old, and was never built as drawn!

We’ll have to measure the existing lighting grid in situ - which won’t be easy. It’s about 16ft above the floor.

But for other purposes I’d like to be able to create and use transparent PNG textures in SU again.

Some time ago, I researched and put work into making a Photoshop action that converts grayscale to transparency, but it’s complicated and I still struggle with it for some reason. I checked Affinity Photo again, and it’s utterly easy-peasy with that. Open the file, go Filters> Colors> Erase White Paper. How about that? They have a specific command just for that operation. It doesn’t look transparent in Preview, but it does in Affinity Photo and Photoshop, and it imports to SU as transparent at least when I tried it.

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I think I’ve worked out what went wrong. This was the first time I’d ever tried to scan and make transparent parts of a black and white image.

I have re-scanned the tracing paper image with the scanner set to Colour, not Black and White.

Now when I make parts transparent, accept the option to change the format to .png, and save it as a .png format file, when I reopen the saved .png file it stays transparent.

I don’t know the underlying reason for this, but it seems to me that the combination of scanner set to B&W, and (attempted) transparency in the .png file, doesn’t work well. The specification for .png files allows the use of grayscale with alpha channel, but the scanner doesn’t seem to create it although it was apparently set to do so.

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PS. And now it turns from ‘internal evidence’ that the undated diagram was nearly three decades old. It purported to show an arrangement of pipes and angle iron near the ceiling, to support theatre lighting. And it was never implemented in our Studio theatre!

The ‘internal evidence’ is the phone number given for the company that prepared the diagram: a London number starting with 01.

Back in 1995, London phone numbers were changed to start with the prefix 020.

See PhONEday - Wikipedia.

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