[Gimp-user] Layer addition - bug, feature, or user misunderstanding?

[Elle Stone]

Using GIMP 2.9 updated yesterday, two different ways of adding layers 
produce different results. But it seems to me that the two ways should 
produce the same results. Here is a screenshot:

http://ninedegreesbelow.com/bug-reports/gimp29/layer-addition/addition-results-vary.png,

Are the different results a bug, a feature, or am I making an obvious 
mistake or just not understanding something?


Looking at the screenshot, the "channel" layers were produced by making 
a solid white layer and dragging the Red, Blue, and Green channels over 
to the layer stack. So of course each channel layer also has R=G=B=1.0.


The channel layers are added using layer percent opacities of 22.2 for 
the Red channel layer, 71.7% opacity for the Green channel layer, and 
6.1% opacity for the Blue channel layer. The percentages are the correct 
percentages for producing a Luminance conversion to black and white by 
adding the Red, Green, and Blue channels together as layers.


The result of adding the three layers should be white, R=G=B=1.0, which 
is what happens with the second way of adding the layers. But the first 
way, using the more obvious "add each layer to the layers below", 
produces R=G=B=0.793257.


Here's a download link for the actual XCF file:
http://ninedegreesbelow.com/bug-reports/gimp29/layer-addition/white-gimpdefault.xcf 



You'll need to reset *all* of the layer opacities to the values given 
above, because for some reason saving to disk and reopening causes layer 
opacities to shift slightly (for example, the Blue layer opacities shift 
to 5.9% instead of staying at 6.1%).


The image is an sRGB image and the precision is 32-bit floating point 
(linear) in order to get the layers to properly add up to R=G=B=1.0, 
which they should anyway for solid white, but results would be wrong for 
colors other than solid white or solid black.


Elle, puzzled
--
http://ninedegreesbelow.com
Color management and free/libre photography
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Re: [Gimp-user] [Gimp-developer] libmypaint

[Thorsten Stettin]

Am 06.10.2015 um 23:07 schrieb Michael Natterer:

On Tue, 2015-10-06 at 15:59 +0200, Jehan Pagès wrote:

Hi,

On Sun, Oct 4, 2015 at 2:55 PM, Thorsten Stettin
 wrote:

Am 03.10.2015 um 11:49 schrieb Michael Natterer:

On Sat, 2015-10-03 at 00:47 +0200, Thorsten Stettin wrote:

Hello,

I'm just integrate my homebrew Ubuntu package called
libmypaint. But
what's the impact regarding Gimp-2.9.x?

With libmypaint it builds the mypaint brush tool which you need
to enable in prefs -> playground. It's very experimental :)

Ok, I'll do my very best. :-D

I'd like to add that it is still very very slow, not really usable
yet
in real use cases. Just saying since I believe your purpose is to do
a
package, and users should be aware where they are getting into.
Let me just clarify: First of all there are build dependencies. In this 
case you need libmypaint-dev and libjson-c-dev in order to build GIMP 
2.9 against libmypaint under Ubuntu and likely Debian as well.
And therefore there are regarding runtime dependencies. IMHO you have no 
choice. :-D

It has to be activated on a prefs page that says "experimental
playground", so who cares... If they activate it let them have it.

Regards,
--Mitch





--
Lao-Tse sagt: Nichtstun ist besser, als mit viel Mühe nichts zu schaffen.
Und er sagt auch: Ich habe drei Schätze, die ich hüte und hege.
Der eine ist die Liebe, der zweite ist die Genügsamkeit, der dritte ist die 
Demut.
Nur der Liebende ist mutig, nur der Genügsame ist großzügig, nur der Demütige 
ist fähig zu herrschen.

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[Gimp-user] Free Select Lasso Tool - Please help me

[Ladystar]

HI , I am an amateur gimp users, self taught through you tube, about five years
in now.  Something has changed and no matter how many times i scrape gimp off my
drive and reinstall (registry entries too), the issue remains the same.

I can find no settings or options to affect this issue, so it must be something
stupid I have changed or don't understand about the program.

let's say as an example i paste a picture into my gimp frame and from within
that picture I want to cut something out, using my lasso tool by clicking
multiple points around the object closely and then CTRL X to cut it out.

This tool has always worked 100 percent, but now when I attempt to laydown
selection dots around my object, before i am finished laying down the dots, the
dots behind me dissapear, or the dots close the loop in some intersected way
that I didn't choose because i didn't get to finish going around my object.

I am litterally losing my mind over this issue.  I have hit hte tool reset
button, i have used all th different tool selection settings, I can't get it to
let me finish selecting the object i want to cut out, without it finishing it or
me too early and cutting across my object.

I have attached a picture showing how the lasso tool closed the loop on me while
I was still trying to go around the object with selection dots.

Has anyone experienced this, can someone help me.

thank you
Ladystar

Attachments:
* http://www.gimpusers.com/system/attachments/223/original/lassotool.png

-- 
Ladystar (via www.gimpusers.com/forums)
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Re: [Gimp-user] Crop Marks and Bleed

[Jehan Pagès]

Hi,

On Sat, Oct 10, 2015 at 1:40 PM, Partha Bagchi  wrote:
> It's supposed to be in the print dialog box image settings.

There you can add crop marks, but not bleed. Indeed it would be nice
to have a bleed concept within a GIMP canvas since a lot of GIMP users
are working with the printing world. Unfortunately as far as I know,
this does not exist currently.

Right now, your best replacement is to compute yourself the bleed size
in pixel (depending on your resolution), make your image bigger than
the finale size and add guides to exact pixel positions where the
bleeds would be for guiding.

Jehan

> On Fri, Oct 9, 2015 at 8:30 AM, Sarah Grant
>  wrote:
>> Hi there
>>
>> Is there a way to put crop marks and bleed onto a file in GIMP?
>>
>> If you could let me know asap, that would be great.
>>
>> Thanks
>> Sarah
>> Sarah Grant
>> Marketing Director
>>
>> DD: +44(0) 1928 352 084   |   M: +44(0) 7581 625 097   |   E: 
>> sarah.gr...@traverseassociates.co.uk
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Re: [Gimp-user] Free Select Lasso Tool - Please help me

[Pat David]

When using that tool, double-clicking will attempt to close the path from
that location - are you accidentally double-clicking by chance?

On Mon, Oct 12, 2015 at 5:36 PM Ladystar  wrote:

> HI , I am an amateur gimp users, self taught through you tube, about five
> years
> in now.  Something has changed and no matter how many times i scrape gimp
> off my
> drive and reinstall (registry entries too), the issue remains the same.
>
> I can find no settings or options to affect this issue, so it must be
> something
> stupid I have changed or don't understand about the program.
>
> let's say as an example i paste a picture into my gimp frame and from
> within
> that picture I want to cut something out, using my lasso tool by clicking
> multiple points around the object closely and then CTRL X to cut it out.
>
> This tool has always worked 100 percent, but now when I attempt to laydown
> selection dots around my object, before i am finished laying down the
> dots, the
> dots behind me dissapear, or the dots close the loop in some intersected
> way
> that I didn't choose because i didn't get to finish going around my object.
>
> I am litterally losing my mind over this issue.  I have hit hte tool reset
> button, i have used all th different tool selection settings, I can't get
> it to
> let me finish selecting the object i want to cut out, without it finishing
> it or
> me too early and cutting across my object.
>
> I have attached a picture showing how the lasso tool closed the loop on me
> while
> I was still trying to go around the object with selection dots.
>
> Has anyone experienced this, can someone help me.
>
> thank you
> Ladystar
>
> Attachments:
> * http://www.gimpusers.com/system/attachments/223/original/lassotool.png
>
> --
> Ladystar (via www.gimpusers.com/forums)
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Re: [Gimp-user] Layer addition - bug, feature, or user misunderstanding?

[Richard]

The problem is that a layer's opacity doesn't add -- it multiplies, like this:

Result = (opacity) * (this layer) + (100%-opacity) * (result of layers below)

This formula holds true regardless of the layer's assigned blending mode (and 
it's recursive, with the "result of layers below" defined by inserting the next 
layer down into the same formula).

So, if you want multiple translucent layers to have specific 'overall' 
contributions to the final image you need to work out the opacities for each 
individual layer, namely with lower layers having more opacity to compensate 
for their lower position in the stack.

Here is what happens with your example on the left -- Red layer has opacity 
22.2%, Green has opacity 71.7%, Blue has opacity 6.1% (all this on top of a 
solid black background), so the overall image is blended like this:

Image = 22.2% * (red) + (100%-22.2%) * (green,blue)
= 22.2% * (red) + 77.8% * (71.7% * green + (100%-71.7%) * (6.1% * blue) )
= 22.2% * (red) + 77.8% * (71.7% * green + 28.3% * (6.1% * blue) )
= 22.2% * (red) + 55.8% * (green) + 1.3% * (blue) 

And since we know that your red, green, and blue are all white, this means the 
overall image is:

= (22.2% + 55.8% + 1.3%) * (white)
= 79.3% white

Doesn't that 79% look rather familiar? :)  

Aside - the left half of your image is totally reproducible on GIMP 2.8 .  (I 
can't seem to reproduce the right half in 2.8, but I haven't examined the 
actual XCF either, so I don't have all the details.)

Now to fix the values ... first, Red is on top so it can keep the 22.2%; this 
leaves a translucency of 77.8% for everything below it.
For Green, below Red, divide its opacity by Red's translucency (above):  
Green's opacity should be (71.7% / 77.8%) = 92.1%.  This, in turn, leaves 7.9% 
of translucency for Blue below it.
For Blue (which is below both Green and Red), divide its opacity by the overall 
translucencies of both Red and Green.  You can do the math if you want (6.1% / 
77.8% / 7.9%), but it conveniently works out to exactly 100% opacity -- i.e. 
Blue doesn't need any translucency for itself because with both Red and Green 
on top of it (at the above opacities) only 6.1% of Blue will be visible anyway.

To prove it, just plug the new opacities back into the above formula:

Image = 22.2% * (red) + 77.8% * (92.1% * (green) + (100% - 92.1%) * (100% * 
blue) )
= 22.2% * (red) + 77.8% * (92.1% * (green) + 7.9% * (blue) )
= 22.2% * red + 71.7% * green + 6.1% * blue

PS - Keep in mind the above math only works when all three of your R/G/B layers 
are visible; if you toggle even one of them off the opacity of the ones 
underneath it will look off because they depended on the layer(s) above them to 
blend correctly.

-- Stratadrake
strata_ran...@hotmail.com

Numbers may not lie, but neither do they tell the whole truth.


> From: ellest...@ninedegreesbelow.com
> To: gimp-user-list@gnome.org
> Date: Mon, 12 Oct 2015 11:22:46 -0400
> Subject: [Gimp-user] Layer addition - bug, feature, or user misunderstanding?
> 
> Using GIMP 2.9 updated yesterday, two different ways of adding layers 
> produce different results. But it seems to me that the two ways should 
> produce the same results. Here is a screenshot:
> http://ninedegreesbelow.com/bug-reports/gimp29/layer-addition/addition-results-vary.png,
> 
> Are the different results a bug, a feature, or am I making an obvious 
> mistake or just not understanding something?
> 
> Looking at the screenshot, the "channel" layers were produced by making 
> a solid white layer and dragging the Red, Blue, and Green channels over 
> to the layer stack. So of course each channel layer also has R=G=B=1.0.
> 
> The channel layers are added using layer percent opacities of 22.2 for 
> the Red channel layer, 71.7% opacity for the Green channel layer, and 
> 6.1% opacity for the Blue channel layer. The percentages are the correct 
> percentages for producing a Luminance conversion to black and white by 
> adding the Red, Green, and Blue channels together as layers.
> 
> The result of adding the three layers should be white, R=G=B=1.0, which 
> is what happens with the second way of adding the layers. But the first 
> way, using the more obvious "add each layer to the layers below", 
> produces R=G=B=0.793257.
> 
> Here's a download link for the actual XCF file:
> http://ninedegreesbelow.com/bug-reports/gimp29/layer-addition/white-gimpdefault.xcf
>  
> 
> 
> You'll need to reset *all* of the layer opacities to the values given 
> above, because for some reason saving to disk and reopening causes layer 
> opacities to shift slightly (for example, the Blue layer opacities shift 
> to 5.9% instead of staying at 6.1%).
> 
> The image is an sRGB image and the precision is 32-bit floating point 
> (linear) in order to get the layers to properly add up to R=G=B=1.0, 
> which they should anyway for solid white, but results would be wrong for 
> colors other than solid white or solid black.
> 
> Elle,