Save image as PNG with 32 bit

[Gavin_Williams]

I can't save 32 bit PNG images. Here are the methods I've used...

1) No Alpha channel + Save As / PNG => 24 bit
2) Alpha Channel + Save As / *** No PNG option ***
3) No Alpha channel + Quick Export as PNG => 24 bit
4) Alpha channel + Quick Export as PNG => 24 bit
5) Alpha channel + Export As / PNG with Transparancy => 24 bit
6) Alpha channel + Export As / PNG without Transparency => 24 bit
7) No Alpha channel + Export As / PNG with Transparency => 24 bit

[c.pfaffenbichler]

What do you mean? 32bit per Channel or 32bit overall? 

[Gavin_Williams]

Either really. But probably 8bpc is what I'm focusing on now, although I do eventually need to know how to produce HDR images. Though I'm not sure yet if PNG can be used for such images.

[c.pfaffenbichler]

You may want to use tif or psd/psb for 32bit per channel images. 

As for an »8bit per channel plus transparency«-image: How does a PNG-24 creted via Save for Web fall short of your requirements? 

[Gavin_Williams]

I'm not doing web. And looking at it, it doesn't look like it's going to be useful for me. It has PNG24 which I already have by default. There's jpg and some indexed formats.

[Stephen_A_Marsh]

A common topic:

https://community.adobe.com/t5/forums/searchpage/tab/message?advanced=false&allow_punctuation=false&...

[Gavin_Williams]

That search gives a lot of answers that aren't relevant and the ones I'm reading that directly address the issue are in fact wrong! Photoshop will not save in 32 bit (4 channels) even when I take measures to do so.

[Gavin_Williams]

For example one person suggests ...

If you save a PNG with the transparency option it MUST be using alpha. There's no other way to do it for 8-bit per channel. Does it work?
A) There is no answer, but I will anser it here, no, it does not work.

In another thread it is state 'Since "Quick Export As..." always saves PNGs as 32 bit,'
A) And that is plainly wrong!

[c.pfaffenbichler]

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If you save a PNG with the transparency option it MUST be using alpha. There's no other way to do it for 8-bit per channel. Does it work?

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In Photoshop Alpha Channels and Transparency are two different concepts, so what do you actually mean?

What is the problem with PNG-24?  

[Gavin_Williams]

My image loading pipeline doesn't currently support 24 bit images. I would have to write code to read in 24 bits and output 32 bits. That will probably happen eventually. In the mean-time I'm looking for a way to produce 32 bit images in Photoshop.

As for Alpha and Transparency. For me, there is no distinction, and I don't understand what photoshop means by having 2 kinds of transparency. For my purposes, the alpha channel stores the transparency value. That's why I need those extra 8 bits, for carrying per/pixel transparency.

[c.pfaffenbichler]

Photoshop does not have two kinds of transparency, Photoshop has transparency. 

Alpha Channels are another thing. 

So your pipeline does not accept PNG-24 as created with SfW? 

[Gavin_Williams]

Yeah, Alpha channels are not strictly transparency. But the alpha channel is used to carry transparency. Is that not the case? Or have I forgotten some trick in Photoshop to get transparency in a 4th channel?

[Gavin_Williams]

A few years ago, I was using Photoshop to produce PNG's with transparency. Maybe I've just forgotten how to do it. And I've reverted to the approach that makes sense.

[c.pfaffenbichler]

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@Gavin_Williams wrote:

A few years ago, I was using Photoshop to produce PNG's with transparency. Maybe I've just forgotten how to do it. And I've reverted to the approach that makes sense.

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Save for Web; PNG-24; is there a problem about that? 

[Gavin_Williams]

PNG-24 isn't compatible with my current pipeline. PNG-32 is compatible but there doesn't seem to be any way now to get Photoshop to output such an image. I've just taken a look at some old images of mine. And interestlingly, when I load the PNG32 image into photoshop, it only has 3 channels, and so it must be storing a hidden alpha channel.

[Gavin_Williams]

So following this idea that photoshops alpha layer is hidden. I made a second layer and used the background eraser tool. That gives transparency, even though Photoshop is only showing 3 channels. So the image now has a hidden alpha channel. Saving the image, it's 32 bit.

[c.pfaffenbichler]

Wait … are you saying you never actually created transparency before that? 

If you already created an Alpha Channel you could have used that as a Layer Mask or to delete. 

Could you please post screenshots with the pertinent Panels (Toolbar, Layers, Options Bar, …) visible? 

[Gavin_Williams]

OK. If I create a transparent layer and unlock the default background. I can then use the normal eraser tool to modify the hidden alpha channel. Though I have to actually erase part of the image to force photoshop to actually save as 32 bit, which is really janky. There should be a way just to save as 32 bit. I won't always have transparency. And that's really a hack.

[c.pfaffenbichler]

No need to use the Eraser Tool, just load the Alpha Channel as a Selection and apply it as a Layer Mask. 

[Gavin_Williams]

OK, Let me just work that out.

[Gavin_Williams]

Just to be clear, I need to be able to save 32 bit PNG's without having created a transparent layer and without erasing to transparency. I do need transparency as well, but I also need images without transparency in 32 bit (which would have the alpha channel filled with the value 255).

[Stephen_A_Marsh]

My take is that a PNG file does actually have an alpha channel used for transparency, and then Photoshop will apply that channel to the file and remove it when the image is opened.

Here is a link to the PNG specification, specifically the section on colour and transparency:

https://www.w3.org/TR/2003/REC-PNG-20031110/#6Colour-values

Here is a sample image from a Wikipedia page:

https://en.wikipedia.org/wiki/File:PNG_transparency_demonstration_1.png 

When I run ExifTool on this image, I get this:

[PNG] ColorType: RGB with Alpha

This follows the PNG specification listing in table 6.1 - PNG image types and colour types linked above:

Truecolour with alpha

This is what Apple Preview indicates, there is a boolean value of 1 under "Has Alpha" which does not exist for a flattened PNG without transparency:

And this is what ImageJ reports for both the original and the one saved/exported from Photoshop - 32 Bit RGB (24+8):

However, when opened into Photoshop, there are only the primary RGB channels, there is no alpha channel – but there is transparency already applied.

When re-saving/re-exporting the original file from Photoshop with transparency, both ExifTool, Apple Preview and ImageJ all indicate that the image has an alpha channel. Of course, we can't verify this in Photoshop as it appears to apply and strip the transparency alpha.

I'll try to update these findings with some other software when I have time, perhaps one will show the intact transparency A channel and not just the RGB with transparency applied.

In some ways, this topic reminds me of the one on EXR alpha/transparency:

https://community.adobe.com/t5/photoshop-ecosystem-discussions/change-in-exr-open-from-cs2-to-cs3-ca...

EDIT: GIMP does recognise the Alpha channel for transparency, from both the original file and the one saved/exported from Photoshop.