# Right handed coordinate system... why?

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There must be an explenation for this.. however it completely eludes me. I mean.. Left to right is positive, that just feels right. That's how most people read. Down to up is positive. That feels right too. Up.. yey! It's going up!! I'm adding energy (or whatever) to something and it goes up into the sky. But backwards being positive? That totally doesn't make sense to me. Someone explain please.

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There is more than one right-handed coordinate system; a right handed coordinate system is one where the cross product of the positive X and Y axis vectors form the positive Z axis vector when you "visualize" the cross product with your right hand. A left-handed one is where the same applies, but for your left hand. Armed with that knowledge you should be able to concieve of many different right-handed coordinate frames.

You can construct a right-handed coordinate frame where "backwards" is negative, but that will break the semantic sense you've assigned to at least one of the other axis.

You need a left-handed system to get what you want.

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Aye, I was asking this because i read that XNA will use right handed coordinates (that work like the example i wrote above) by default. Anyways, the answer is that Z is made out of the crossproduct of X and Y then, right?

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Because traditional mathematics and physics are done in right handed coordinate systems. Making XNA compatible with this will make programming physics easier.

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That's nothing, compared to unreals coordinate system. They have the x away from you, the y to the right and the z up.

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Right handed makes more sense when you think about the video hardware. You can make +X point right, +Z away from the screen and +Y down the screen. Down the screen is actually increasing the VRAM address, so +Y == +VRAM Address.

Skizz

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Quote:
 Original post by RedburnI mean.. Left to right is positive, that just feels right. That's how most people read. Down to up is positive.

No, Up to Down is positive, thats how most people read, you start at the top of the page and go down.

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It's not so much a matter of left -> right, down -> up, but which way 'up' is at all. All cartesian coordinate systems (worth mentioning) have increasing magnintude as you travel away from the origin. Also, everybody knows what x and y do in the plane, on a page. The left/right discrepancy comes when we try to work out what 'up' is in three dimensions:

If we hold our page facing us, like a TV screen, the old y coordinate points upwards, towards the sky, but if we put it flat on the table, y (which used to be 'up') now points away from us. The new z coordinates goes where it must, in either case. It is this act of swapping y and z that toggles handedness.

Regards

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Quote:
 Original post by RedburnI mean.. Left to right is positive, that just feels right. That's how most people read. Someone explain please.

off topic...but actually "most people" read right to left considering the population of people that read asian-based languages :).

also, i thought you could specify matrices in DX as either right handed or left handed (eg. D3DXMatrixLookAtRH or D3DXMatrixLookAtLH) giving you the option to define your own coord sys.

i find it amusing that Max is Z up(our designers use) and Maya is Y up(our animators use). that's a lot of fun.

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Take a piece of paper, draw x & y lines on it and put it on the table. The "natural" direction for z is up from the table and you've got a right handed system.

Take the *same* piece of paper and stick it on your monitor. Now the "natural" direction for z is into the monitor and you've got a left handed system.

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