This is odd mapping problem

Processing
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I’ve been playing with navigational star charts, photo maps, star databases, processing for about a year now. With lots of help, I’ve learned how to project an image onto a sphere/globe and plot points onto the image. My coding style is very sloppy and needs improvement, but I have managed to learn how to manipulate the placement of points(stars) some.

Here’s what I found odd. I went to https://aa.usno.navy.mil/data/docs/Nav_Star_Chart.php to get a star chart that I could project onto a sphere. I used paint to crop the image and eliminate extraneous stuff, leaving just the essential chart to be projected. This seemed to work, cropped pretty accurately. I did have to use paint to reverse the image so that when inside the sphere, letters and words were not backwards. I plotted a few stars and some circles centered on these stars and noticed something. The location of everything was off by 180 degrees, but the words weren’t printed backwards.

I went here https://staratlas.com/ to see how the stars for my location and time are oriented. Yup, my cropped USNO chart is off by 180 degrees. I wrote a little if then else and manipulated my circles and star points, adding 180 degrees and my stars landed on the chart’s positions for those stars.

My sketch rotates according to sidereal time for my location. If I plot my circles and stars on a NASA image, it agrees with the staratlas.com.

Does anybody have an explanation?

Here are two screenshots illustrating.

Normal
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Normal180Plus
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One more screenshot showing how when I use the NASA image, it agrees with staratlas.com.

NormalNASA
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I should have known, to turn it 180 degrees, flip the image vertically. There is more to consider if the points/stars are still screwy, but I know how to change signs and subtract. That part I know very well.

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Glad you solved it!

Possibly relevant: Keep in mind that for many graphics systems 0,0 is the lower left (like a graph) and for Processing it is the upper left (like a book). so flipped orientations on loaded data aren’t too surprising when the occur…

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Thanks, @jeremydouglass. It was a whole lot of flipping back and forth, try something, test it, etc… An image projected onto a sphere is different when viewed from the inside vs. outside. I had dealt with that a while ago and forgot how to handle it.

The puzzle has changed a little, recently. I’m using a formula to calculate sidereal time(think of it as how the stars are oriented at local time). To get the local orientation, I rotateY using the angle of the hour and my longitude. It compares with Stellarium but seems off by about 10 minutes. If I remember right, that 10 minutes is about the difference between sidereal and ordinary time. Its very confusing to try to nail it down.