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Just now, AffinityJules said:

If you could get high enough then would see the rainbow as a circle.

No drink or drugs required! :P

14 hours ago, R C-R said:

I have occasionally seen circular rainbows while looking out aircraft windows, & rainbows in mists created by fountains & garden hoses, but never an upside down one.

 


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58 minutes ago, AffinityJules said:

If you could get high enough then would see the rainbow as a circle.

If you get high enough, you might see paisley or other weird patterns in rainbows ... even when there are no rainbows to be seen. 


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18 hours ago, DesignT said:

Sorry I disagree rainbow arc is almost of the time a not complete arc 

An arc is defined as part of a circle, which is just what you see most of the time with a rainbow. If you wish to emulate a natural rainbow, then it should be an arc of a perfect circle. If you want to be even more natural then you should include the shading described above.

John


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16 hours ago, R C-R said:

I have occasionally seen circular rainbows while looking out aircraft windows, & rainbows in mists created by fountains & garden hoses, but never an upside down one.

You would only get a complete rainbow when the sun was lower in the sky than your viewpoint. So an aircraft would be the ideal location. If there was less moisture in the air at higher altitudes, then you could get a true up-side-down rainbow (as opposed to the circumzenithal one.

John


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34 minutes ago, John Rostron said:

An arc is defined as part of a circle, which is just what you see most of the time with a rainbow. If you wish to emulate a natural rainbow, then it should be an arc of a perfect circle. If you want to be even more natural then you should include the shading described above.

You also might need to use masking to emulate how rainbows in nature appear behind objects in the foreground & middle ground, like in the Wikipedia article examples where the rainbow seems to end behind the trees.


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8 hours ago, Alfred said:

When is a rainbow ever not a complete arc?? o.O

Complete arc is in my opinion an arc that will touch the earth on both sides the other arcs are still arcs but don´t touch the earth.


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Well the conversation about rainbows goes well but I do not see any help to get the effect that I need.

Can you give me some tips about how can I get this effect ?

Instead of be over the background image, I will need to be a solid layer but the layer is cut (mask) in the area where we have the rainbow arc (ellipse) like if we see the the rainbow thru a arc mask.

I don´t want that the rainbow overlays the other layer I need the rainbow goes behind the foreground and pass thru an arc cut.

It´s enough to give me an Ideia of the technic that I should to get this effect.

Anyone ?

Thanks you all


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42 minutes ago, DesignT said:

I don´t want that the rainbow overlays the other layer I need the rainbow goes behind the foreground and pass thru an arc cut.

The rainbow layer should be above the other layer(s) but by adjusting either its layer transparency, layer blend mode, or whatever you can let part of the underlying image show through, just like it would if it was a real rainbow. If the foreground is not on a separate layer, you can use masking on the rainbow layer to hide it where it would otherwise appear on top of the foreground.

It might help to study some photos of rainbows, like those in the Wikipedia article.


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11 hours ago, DesignT said:

Complete arc is in my opinion an arc that will touch the earth on both sides the other arcs are still arcs but don´t touch the earth.

According to the Wikipedia article on Arcs in Projective Geometry:

Quote

A k-arc which can not be extended to a larger arc is called a complete arc

However, you do not have to use the Projective Geometry definition. Provided you define your term precisely (as you have) then you can then use it  as you wish. As Humpty Dumpty said in Through the Looking Glass by Lewis Carroll:

Quote

"When I use a word, Humpty Dumpty  said, in rather a scornful tone, "it means just what I choose it to mean—neither more nor less." "The question is," said Alice, "whether you can make words  mean so many different things." "The question is," said Humpty Dumpty, "which is to be master—that's all."

Apologies to those who have seen my allusion to Humpty Dumpty before.

John


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Posted (edited)

I have just read the Wikipedia article on Rainbows, and there is one property of rainbows that has not been mentioned in trying to emulate a 'natural' rainbow: 

Quote

The colour pattern of a rainbow is different from a spectrum, and the colours are less saturated. There is spectral smearing in a rainbow owing to the fact that for any particular wavelength, there is a distribution of exit angles, rather than a single unvarying angle.

Having created a rainbow gradient, then you could either apply a little Gaussian Blur or a little desaturation (or both) to get this effect.

EDIT: Another feature of a 'natural' rainbow is that it is only a couple of degrees wide. Those re-creations that I have seen are all noticeably wider than this. 

John

Edited by John Rostron
Update

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4 minutes ago, John Rostron said:

According to the Wikipedia article on Arcs in Projective Geometry:

Quote

A k-arc which can not be extended to a larger arc is called a complete arc

 

I think you’re complicating the issue by taking us into projective geometry, John! Here’s what the Arc (geometry) article on Wikipedia has to say:

Quote

In Euclidean geometry, an arc (symbol: ) is a closed segment of a differentiable curve. A common example in the plane (a two-dimensional manifold), is a segment of a circle called a circular arc.

 


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2 minutes ago, Alfred said:

I think you’re complicating the issue by taking us into projective geometry,

I gave that source as it did refer to a 'complete arc', which no-where else did. My point being that the term 'complete arc' is essentially one which you can define as you wish, as @DesignT has done.

John


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1 minute ago, Alfred said:

I think you’re complicating the issue by taking us into projective geometry, John!

So ... anybody ever seen a rainbow in nature or a photograph of one that is not a circular arc? I have not, & I am not sure it is even possible.


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4 minutes ago, R C-R said:

So ... anybody ever seen a rainbow in nature or a photograph of one that is not a circular arc? I have not, & I am not sure it is even possible.

No it is not. The rainbow is a pure image derived from optical refraction which ensures that it will always be a pure circular arc.

John


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I have seen white rainbows.


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1 hour ago, GRH said:

I have seen white rainbows.

White rainbows are seen when the light intensity is too low for normal colour vision. See the quoted Wikipedia article for more.

John


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On 5/12/2019 at 9:35 PM, John Rostron said:

White rainbows are seen when the light intensity is too low for normal colour vision. See the quoted Wikipedia article for more.

John

Also known as fogbows, as was the case; brilliant sunshine and slightly foggy.


Dell Inspiron 15 7000 Series 7559 - i7 6700HQ - 16Mb RAM - 128Gb SSD 1Tb HD

Nvidia GEFORCE GTX 960M 4Gb GDDR5 RAM - 4K Touch Screen

___

Asus N56V i7 3630QM 2.40GHz; 8Mb RAM; 1Tb HD; 64 bit.

Nvidia GEFORCE GT 650M 2Gb: 1920 x 1080

2nd Monitor: Asus ProArt  24": 1920 x 1200

------

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