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Hidden layer after using the "Color to Alpha" function.

Jan 2, 2011, 09:12 (This post was last modified: Jan 2, 2011 09:13 by Gaijin77.)
Post: #1
Hidden layer after using the "Color to Alpha" function.
Hi, I'm encountering a problem creating transparent png files for some T-shirt designs and was hoping someone might know why. Basically, I use the "color to alpha" function to make a certain color transparent and everything looks OK in gimp but when I print the T-shirt I find that the transparent part is still there. At first I thought it was a problem with the printing but when I checked the image in Microsoft Paint I could see that the color that was supposed to be transparent was still there. I'm wondering if anyone on the forum knows why this might be and how I might stop it happening. I have this problem whenever I use the color to alpha function and it is getting quite frustrating. I've attached a file to this thread so that you can see what I mean. When I open this file in gimp the only visible color is yellow and the rest of the image is transparent. However, if I open it in Microsoft Paint I can still see the green outline that I thought I had erased (turned transparent) in Gimp. I'd be very grateful for any advice.

Thanks,
Daniel


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Jan 2, 2011, 10:18
Post: #2
RE: Hidden layer after using the "Color to Alpha" function.
What we get in your picture is a yellow knot on a black background (so your PNG isn't transparent). If I use color-to-alpha to recreate a transparent background and save it as PNG it works... and if I display the resulting PNG in various utilities (including the picture and fax viewer and the Office Picture Editor) the background is indeed transparent. But if I import it in MS-Paint I get the black background back. I conclude that Paint is a complete lemon that barely supports PNG (clue: you can't define the "transparent" color like you can with GIF).

I can only suggest that you use something else to check your pictures and/or print them. IIRC you can print from the picture and fax viewer. However the printing of "transparent" things (and even more of partially transparent things like your de-aliased borders) can be printer driver dependent, some may put a white background...

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Jan 2, 2011, 10:56
Post: #3
RE: Hidden layer after using the "Color to Alpha" function.
Thanks for your quick reply. I totally agree that MS-Paint is a complete lemon. The only reason I use it is to check for any "hidden layers" that might end up on my T-shirts. I use CafePress (a print-on-demand website) to print the T-shirts and for some reason these "hidden layers" I can only see in MS-Paint keep getting printed on the T-shirts. If you click on my attachment, select [Save image as...] to save the original file (not the thumbnail) and open it in MS-Paint what do you see? I see a yellow knot with a green outline against a black background. I understand that the black background is MS-Paint's way of dealing with transparency, but I can't figure out why the green outline that I colored to alpha is still there...
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Jan 2, 2011, 11:57
Post: #4
RE: Hidden layer after using the "Color to Alpha" function.
The PNG you posted must have been changed by MS-Paint because it has a black background. C2A on it doesn't produce a green outline but that may be the black background interfering.

However, I see what you are talking about because that's a frequent problem. The standard color-to-alpha is not 100% correct in many cases. It's OK in the general case, where you remove one color from a picture with many colors, but it doesn't work that well in many cases when you have only two colors because it can leave pixels in an intermediate color (there is room for a specific "color delta" plugin here).

You can try to mitigate the problem by passing a yellow brush in "hue" or "color" mode over the edges of your knot, this will repaint the greenish pixels with the hue (and hue+saturation, for "color", IIRC) of your yellow without changing the rest (lightness and, mostly, transparency)(you can use a broad brush since you don' t need to be accurate, it's all automatic)

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Jan 3, 2011, 04:49
Post: #5
RE: Hidden layer after using the "Color to Alpha" function.
Yeah, it's weird because everything looks fine in GIMP and pretty much every other software I've used except MS-Paint. It even looks fine in the preview software on the CafePress site. The first time I knew there was a problem was when the T-shirts came back printed with the colors I thought I had removed using the color-to-alpha function. I've also noticed that I don't have this problem if I save the file as a GIF, then reopen it and save it as a PNG, so perhaps that is a workaround I can use. The only downside is that the file size seems to get reduced which I'm guessing would indicate a loss of resolution.

Also, just a couple of questions about the terminology you use as I'm new to this forum and want to make sure I understand correctly:

What do you mean by de-aliased borders?
What do you mean by C2A?
What do you mean by IIRC? (I'm guessing you mean "If I remember correctly" but just want to be sure!).

Thanks for your help, it is really appreciated.

Cheers,
Daniel
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Jan 3, 2011, 07:52
Post: #6
RE: Hidden layer after using the "Color to Alpha" function.
Using GIF wouldn't cause a loss of resolution but a loss of colors since GIF only supports 256 colors total in the picture. In your case that shouldn't be a problem if you are careful and use proper software since you have a grand total of one of two colors (but would MS-Paint index the colors correctly)? But I still don't understand for what purpose you are using Paint.

De-aliased: with added semi-transparent pixels so that it doesn't look jagged (ie, with anti-aliasing applied)

C2A: Color-to-Alpha

IIRC: If I Recall/Remember Correctly

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