Forums > Jitter

exact positioning of elements in gl.sketch

December 15, 2008 | 11:04 am

Hi all,

I was wondering how I can position a couple of videoplanes on a pixel-
accurate position within an opengl environment.
Here’s the thing : I have a background image (on a videoplane filling
the screen), with 2 holes cut out, using alphachannels. I know where
and how big these holes are in pixels, but how do I translate this to
opengl so I can position 2 other videoplanes on the exact location?

Trial and error gives me the right spot, but there’s got to be someway
to do this without guessing, right?

regards,

Brecht.


December 15, 2008 | 11:30 am

GL will normalize
coordinates across [-1, 1] x [-1, 1]

HTH


December 15, 2008 | 11:41 am

so divide 1 by the input

0.0 0.0 will be the middle

Anybody tell me how many decimal points GL will use for positioning?

If I divide 1 by four then everything is easy as you get 0.25
BUT if I have 1 by 14 then I get 7.142857142857e-02

So for accurate positioning how many places do I use say for a position message?


December 15, 2008 | 12:50 pm

Ok. That was simple… so basically it could be done like this :
trigger display to get resolution, input pixels, output gl coords.

– Pasted Max Patch, click to expand. –

On 15 Dec 2008, at 12:30, Carl Rohumaa wrote:

>
> GL will normalize
> coordinates across [-1, 1] x [-1, 1]
>
> HTH


December 15, 2008 | 12:54 pm

Well, sort of works for me, there’s some bugs in there :) Too quick
with it.

On 15 Dec 2008, at 13:50, Brecht Debackere wrote:

> Ok. That was simple… so basically it could be done like this :
> trigger display to get resolution, input pixels, output gl coords.
>
>

– Pasted Max Patch, click to expand. –

>
>
>
> On 15 Dec 2008, at 12:30, Carl Rohumaa wrote:
>
>>
>> GL will normalize
>> coordinates across [-1, 1] x [-1, 1]
>>
>> HTH
>


December 15, 2008 | 1:09 pm

Can’t open your patch as I am only on Max4.6

You will need to get the scale right on the videoplane and then send messages like @position 0.5 -0.5 to get stuff in the right place.

I am using @texture_reset 2 message in the videoplane in my context


December 15, 2008 | 1:15 pm

I have started drawing this stuff on paper so I can visualize what I am doing before I start inputting numbers. BTW the coordinates will be from the middle of your object so you might have to take this into account.

So anybody tell me how many decimal places to use? So for example if my calculations give me 0.3141592653589793 etc


December 15, 2008 | 1:45 pm

you could of course send jit.gl.render the ‘screentoworld x y z’
message and get the gl coords out of its dumpout

On 15 Dec 2008, at 14:15, Carl Rohumaa wrote:

>
> I have started drawing this stuff on paper so I can visualize what I
> am doing before I start inputting numbers. BTW the coordinates will
> be from the middle of your object so you might have to take this
> into account.
>
> So anybody tell me how many decimal places to use? So for example if
> my calculations give me 0.3141592653589793 etc


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