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Index › Programming Questions & Help › Syntax Questions › Can I invert the y axis so it starts from bottom
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Can I invert the y axis so it starts from bottom? (Read 1267 times)
liudr
Can I invert the y axis so it starts from bottom?
Apr 8th, 2009, 9:32pm  
How does someone invert the y axis so it starts from bottom left corner and points up instead of starting from top left corner and points down? Right now all I can think of is to add inversion to every single shape I draw and to mouseY. Any easier way out? Thanks
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NoahBuddy
Re: Can I invert the y axis so it starts from bottom?
Reply #1 - Apr 8th, 2009, 11:30pm  
This thread discusses how/why of doing that.

http://processing.org/discourse/yabb_beta/YaBB.cgi?board=OpenGL;action=display;num=1206476051
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PhiLho 
Re: Can I invert the y axis so it starts from bottom?
Reply #2 - Apr 9th, 2009, 6:04am  
For 2D:
size(200, 200);
background(255);
scale(1, -1);
translate(0, -height);
triangle(0, 0, 58, 200, 200, 75);
triangle(30, 75, 58, 20, 86, 75);
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WWW  
liudr
Re: Can I invert the y axis so it starts from bottom?
Reply #3 - Apr 9th, 2009, 6:14am  
Thank you NoahBuddy and PhiLho. I intend to use my programs as class demos so the y-axis needs to be inverted. Otherwise my students will be confused.
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cloister
Re: Can I invert the y axis so it starts from bottom?
Reply #4 - Apr 9th, 2009, 9:28am  
Note that when you flip the Y axis like that, EVERYTHING flips.

Including text, which will appear upside down.

When you want to render right-side-up text, the easiest thing to do is to temporarily translate to the location of the text, scale the y-axis upside-down again, draw the text, and then undo the transformation:

pushMatrix(); // save the current coordinate system
translate(textXPos, textYPos); // move to where you want the text to be
scale(1,-1); // flip things over
text("hello world", 0,0); // draw the text
popMatrix(); // restore the state.
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liudr
Re: Can I invert the y axis so it starts from bottom?
Reply #5 - Apr 9th, 2009, 9:37am  
Here's some test results I got:
First, everything is appearing at the correct place according to a right-side-up vertical axis.
Then, the texts are drawn up side down. I tried to add scale(1,1,1) in front of the text but it didn't work. I then added pushMatrix() and popmatrix(). That worked.

A short summary:
pushMatrix() before you scale(1,-1,1)
Do stuff
popMatrix() before you display text.


import processing.opengl.*;
size(640,480,OPENGL);
pushMatrix();
scale(1,-1,1);
translate(tX,tY); // translate however you like.
// Now do stuff that you want in the right-side-up x-y system.
popMatrix();
//Now get back to computer graphics convension and display some text.
text("Blah",20,20);
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cloister
Re: Can I invert the y axis so it starts from bottom?
Reply #6 - Apr 9th, 2009, 1:57pm  
See my earlier post, but: you had the right idea, except for one thing.

Scale(), translate(), rotate(), and any other coordinate system transformations are _cumulative_.

So "scale(1,-1)" doesn't mean "make the y-axis go up", it means "flip the y-axis over from whatever it is now".  You can think of this particular scale operation as a toggle.

Similarly, "scale(1,1)" doesn't mean "make the y-axis go down", it means "scale the x- and y- axes by 1.0 times whatever their current basis vectors are".  Which, as you might imagine, has no effect whatsoever.
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liudr
Re: Can I invert the y axis so it starts from bottom?
Reply #7 - Apr 10th, 2009, 2:46pm  
Thanks cloister. Now I realize the scale(1,-1) is just an improper rotational matrix instead of a command to demand y to be up. Seems like I did scale(1,1) just to do nothing.

Now, question for you: do you know why the computer graphics seem to be in a left-handed coordinate system? Are the cross products defined with left-hand rule? I bet I can check the Pvector.cross() but seriously if the system is LH I don't know how to do physics in it.
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Endolith
Re: Can I invert the y axis so it starts from bottom?
Reply #8 - May 15th, 2009, 12:30pm  
liudr wrote on Apr 9th, 2009, 6:14am:
Thank you NoahBuddy and PhiLho. I intend to use my programs as class demos so the y-axis needs to be inverted. Otherwise my students will be confused.


Can you create a "virtual coordinate system" and map it onto the real coordinates  That's what the "graphing2dequation" example does, for instance (I can't link to it).  It creates a virtual x,y coordinate system on top of the normal one, and then creates a virtual r,theta polar coordinate system on top of that.  I'm trying to plot math functions, and this seems like a good way to do it.
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