Why does software often use an inverse coordinate system compared to regular math coord system?

Question

As you likely know, in c#, the origin (0,0) of a plane is the upperleft corner. Going to the right and/or under is regarded as +, while going to the left and/or up is -. Opposed to this is the regular math coordsystem:

(0,0)=mid of plane, going up/right = +, down/left = -.

It's kinda counter-intuitive and can be annoying sometimes, since we're used (for years) to using the regular math coords, and you have to recalculate coords as well.

Is this a fundamental design flaw? And do you get used to it after a while? And which other languages use a different coord system like c#?

all computer graphics coordinate system have the upper left corner as (0, 0), unless there is some strange systems I don't know — phuclv, Jul 19 '15 at 03:39
@samgak It is (and the axis are) what I tell it to be :} Hooray for space transformations. — user2864740, Jul 19 '15 at 04:18
@user2864740: You can define a mapping from a custom coordinate system onto the OpenGL coordinates (using a view matrix), but the OpenGL coordinates don't change. — Ben Voigt, Jul 19 '15 at 04:19
@BenVoigt in the case of glReadPixels 0,0 is the bottom left. For NDC you're right it's the middle. — samgak, Jul 19 '15 at 04:38

Scott Chamberlain · Accepted Answer · 2015-07-19T03:31:55.290

9

It is not C# but the display that uses a inverse coordinate system, this comes from the days back when the display was drawn in using a CRT and the image was drawn in top to bottom, left to right. That is why the coordinate system OS's use match that.

Languages like C# are just wrapping the underlying OS's API and that is why C# uses it too.

edited Jul 19 '15 at 03:31

answered Jul 19 '15 at 03:21

Scott Chamberlain

124,994
33
282
431

Had the same answer almost word for word but you beat me to it :) – DavidG Jul 19 '15 at 03:21
CRTs could have swept bottom to top just as easily -- the origin of the top to bottom numbering system is line numbers on a page of text. – Ben Voigt Jul 19 '15 at 03:35
@BenVoigt Maybe, maybe not. I was under the impression it was because VGA defined its data stream as top down. – Scott Chamberlain Jul 19 '15 at 03:46
I think it goes back even earlier than CRTs, to line printers, essentially automated typewriters. They printed in page-reading order, from left to right then from top to bottom. CRTs inherited their coordinate system, and from there so did many applications. – Edward Doolittle Jul 19 '15 at 04:03
@EdwardDoolittle: Exactly. – Ben Voigt Jul 19 '15 at 04:10
@ScottChamberlain: VGA was a late comer to the world of computer displays. – Ben Voigt Jul 19 '15 at 04:11
1

Early display output (CGA, VGA, whatever) were effectively just 'bitmaps' onto some prespecified memory region. When viewing these regions (and after being exposed as such) as simple 2D arrays a top-left origin and ascending x/y make sense wrt pixel/cell indexing. – user2864740 Jul 19 '15 at 04:13
@user2864740 Thats what I was trying to convey with my last comment. – Scott Chamberlain Jul 19 '15 at 16:30

TaW · Answer 2 · 2015-07-19T10:15:41.820

The mathematical graph plane is a virtual thing, which expands in all directions without limits.

The screen is a real thing, which can not really expand at all.

Instead we use the concept of scrolling and we are used to doing it from a starting point down.

So conceptually the graphics systems all use the same system as a (left-to-right & top-to-bottom) textblock or page in a book . It is about how we scroll to expand/advance the display area.

But it could be defined in any other way; after all e.g. negative coordinates do make sense as opposed to a negative line number..

score 0 · Answer 3 · answered Jul 28 '15 at 00:42

0

If you don't like the coordinate system on the screen, you can create wrapper methods to re-map the coordinates any way you like.

answered Jul 28 '15 at 00:42

Edward Doolittle

4,002
2
14
27

Why does software often use an inverse coordinate system compared to regular math coord system?

3 Answers3