Monthly Archives: January 2019

Skies and Backgrounds 2

The other simple plugin now in the Laboratory is the Sky Gradient.  The default settings for the Realistic Sky don’t look like any sky I’ve ever seen except perhaps through sunglasses and tinted windows.  This wide angle camera shows the transition from the sky overhead to the horizon.

Default Sky Settings

The Sky Color Control can easily be adjusted of course but the gradient from the zenith down to the horizon doesn’t look right for a clear day even with the Haze turned right down.  What I had been using was a Bi Gradient to add more blue into the mix but lately it struck me that the transition was still not right and the sky should be bolder bluer lower down.  A formula could be used but it would be pretty cumbersome compared to a plugin with colour chips and a slider made off a template.  By taking the Zentith as 1 and the Horizon as 0 then putting the background value to a power of function (or exponent) the response is 1 when linear and the same as the Bi Gradient.  Values less than 1 will push the bolder blue Zenith colour down toward the horizon and values greater than 1 will push the paler Horizon colour up into the sky.  I found that values of 0.5-0.75 were best but I need to make more observations when the weather is perfectly clear.  Any points below the horizon return as black.

Sky Gradient

 

Skies and Backgrounds 1

I’ve uploaded 2 plugins to the Laboratory; an old one I’d forgotten to upload, the Rotation Map, and a simple new one the Sky Gradient.  The Rotation Map had a bug in it anyway which I only found now with more thorough testing.

Firstly the Rotation Map was made specifically for help with creating night skies.  Finding a good  full 360º star map and a view of the Milky Way this could be projected onto the inside of a giant sphere but it wouldn’t interact well with the Realistic Sky.  Note that any colour in the Background is added to the Realistic Sky when it is rendered.

The rotation controls allow for adjustments to how the sky map appears.  With a few tricks it’s possible to properly animate the path of the stars in the night sky.  A sky map will need to be altered so that the Celestial Poles are aligned with the X-axis.  You’ll need to find the North Star in the image and the Southern Celestial Pole.  Finding the stars in that image was harder for me than in the real night sky.  If the original sky map is rotated around X it will be around whatever is in the middle of the image.

I’ve made the selection of Axis to be consistent with the Realistic Sky in Carrara where North is -X and East is +Y.

Corrections in 2D on the image can’t be made because the top and bottom have distorted and stretched out the stars so that these look fine when they are wrapped around a sphere but not once they are moved.  So the method is to put a Sphere primitive into the scene with it’s centre at the origin and apply the sky map image to its Color channel.  Rotate the sphere until the North Star lines up with the -X axis in the Right side view and the Southern Celestial Pole lines up with the +X axis in the Left side view.

It can help to make a copy of the image and then mark the Celestial poles with a cross in the 2D editor and use that first.  Set the scene Ambient to 100% with no other lights and do a spot render and little adjustments to get as close as possible in the orthogonal views.

Now put a Spherical Camera into the scene and set its position to Left then move it to the origin.  Render the image with the same resolution as the largest original dimension and with square proportions.  The spherical render will now take the undistorted stars and make a new distorted 2D spherical projection with different stars at the top and bottom and with the Milky Way wrapped around the map at an angle rather than in the middle.  Importantly the Southern Celestial Pole will be in the middle of the edge of the image on the left hand side.

Using the Rotation Map plugin as the Scene Background the new adjusted version of the sky map can be loaded.  When the 3D Rotation value for X is animated though 0-360º the stars will make a full revolution around to represent the passing of one day.  The X axis can also be adjusted for the correct time of night and year – this map was about right for an evening in April at 0º so with a 90º rotation is has the Southern Cross in the right place for January.

The 3D Rotation in the Y axis can be adjusted for the latitude of where the scene is supposed to be taking place.  At the Equator this will be zero and for the Southern Hemisphere, in say Sydney, this will be +33º   So for London this would be about 51º North and so use 309º according to the way the map is setup.

I’ll be improving this plugin by adding in freely set spin values for each of the axes for use with animations and full ±360º sliders.  Amateur Carrara Astronomers in the Northern Hemisphere please give feedback…