# Re: [Public WebGL] Re: Get Viewport for converting coordinates

```Hi, I think you were right! it seems to be an
projection-matrix-problem. because now I'm testing the same procedure
with a orthographic projection and it works fine. i will test some
other proj matrices.

thank you

>
>
> On Thu, Aug 19, 2010 at 4:56 AM, Mehmet Sirin <m.sirin07@googlemail.com>
> wrote:
>>
>> hi Gregg,
>>
>> > If you're using these functions?
>> For matrix operations I use http://sylvester.jcoglan.com/api/matrix.
>>
>> Hmm, still having problems here. actually I'm doing exactly the things
>> you posted in the recent emails..
>
> Clearly you are not doing exactly the same things or you'd be getting
> exactly the same results :-D
>
>>
>> maybe i should post the result which are calculated in the single
>> steps, somewhere must be the mistake:
>> I got some object coordinates like 3489860.0, 5557180.0,
>> 457.0790023803711. Then I do the standard viewing pipeline with
>> perspective projection and see my scene on the screen.
>> Now I describe what the coordinates look like when i go backwards from
>> screen coordinates to object coordinates.
>>
>> 1.) Transform screen coordinates to normalized device (where the scene
>> is in a [1,1] cube):
>>   //take the inverse of viewport matrix and multiply with screen
>> coordinates
>>
>>   var v = Vector.create([xclick,yclick,0,1]);   //xclick,yclick=pixel
>> coordinates
>>   var
>> a=getViewportMatrix(canvas.clientWidth,canvas.clientHeight).inverse();
>>   var npc_coord=a.x(v);
>
> I don't quite follow the x function and if it's really a matrix multiply but
> assuming it is it's not dividing by W as the function I posted was doing.
>
>>
>> --> value for clicking lower left of canvas: -1,1,0,1   //fixed values
>> in source code: xclick=canvas.clentWidth,..
>> --> value for clicking lower left of canvas: 1,-1,0,1
>> --> clicking in the middle of the scene: 0.0057, 0.1207, 0, 1
>> //without fixed values..
>> ..strange.
>>
>> 2.) Transform normalized coordinates to view coordinates:
>> //take inverse projection matrix and multiply with normalized coordinates
>>   var p_Inv=pMatrix.inverse();            //inverse projection matrix
>>   var vrc_coord=p_Inv.x(npc_coord);
>>
>> without dividing with 4dComponent
>> --> lower left: -0.5336,0.4040,-1, 5.0000
>> ..i thought the view coordinates should look like object coordinates?
>>
>> 3.) ..makes no sense because stuff above is terribly wrong.
>> ---------
>>
>> All in one step:
>>
>>   var pixel = Vector.create([0,530,0,1]);
>>   var view=getViewportMatrix(canvas.clientWidth,canvas.clientHeight);
>>   var pM=pMatrix.dup();
>>   var mvM=mvMatrix.dup();
>>
>>   var allMultiplied= view.multiply(pM).multiply(mvM);
>>   var allInversed=allMultiplied.inverse();
>>   var multiplyWithPixel=allInversed.multiply(pixel)
>>
>>   var w=multiplyWithPixel.flatten()[3];
>>   var objectCoordinate=...;
>>  --> lower left:  3489859.8932, 5557180.0808, 7236.7802
>>  --> upper right: 3489860.1067, 5557179.9191, 7236.7802 but it should
>> be something like 3489940.00, ..
>> ..and when clicking around on screen nothing but the values after the
>> dot of the number changes.
>>
>> :)
>>
>> thank you for reading (and thinking "what the **** is he doing?! ").
>>
>>
>> > Then I'd guess it's something like
>> > var projection = M4x4.makePerspective(45, canvas.clientWidth /
>> > canvas.clientHeight,  0.1, 20000000.0);
>> > var view = M4x4.makeLookAt(eye, center, up);
>> > var viewProjection = M4x4.mul(view, projection);  (or is it projection,
>> > view?)
>> > It doesn't appear he provided an matrix inverse function
>> > var viewInverseProjection = inverse(viewProjection);
>> > where inverse is
>> > function inverse(m) {
>> >   var tmp_0 = m[2*4+2] * m[3*4+3];
>> >   var tmp_1 = m[3*4+2] * m[2*4+3];
>> >   var tmp_2 = m[1*4+2] * m[3*4+3];
>> >   var tmp_3 = m[3*4+2] * m[1*4+3];
>> >   var tmp_4 = m[1*4+2] * m[2*4+3];
>> >   var tmp_5 = m[2*4+2] * m[1*4+3];
>> >   var tmp_6 = m[0*4+2] * m[3*4+3];
>> >   var tmp_7 = m[3*4+2] * m[0*4+3];
>> >   var tmp_8 = m[0*4+2] * m[2*4+3];
>> >   var tmp_9 = m[2*4+2] * m[0*4+3];
>> >   var tmp_10 = m[0*4+2] * m[1*4+3];
>> >   var tmp_11 = m[1*4+2] * m[0*4+3];
>> >   var tmp_12 = m[2*4+0] * m[3*4+1];
>> >   var tmp_13 = m[3*4+0] * m[2*4+1];
>> >   var tmp_14 = m[1*4+0] * m[3*4+1];
>> >   var tmp_15 = m[3*4+0] * m[1*4+1];
>> >   var tmp_16 = m[1*4+0] * m[2*4+1];
>> >   var tmp_17 = m[2*4+0] * m[1*4+1];
>> >   var tmp_18 = m[0*4+0] * m[3*4+1];
>> >   var tmp_19 = m[3*4+0] * m[0*4+1];
>> >   var tmp_20 = m[0*4+0] * m[2*4+1];
>> >   var tmp_21 = m[2*4+0] * m[0*4+1];
>> >   var tmp_22 = m[0*4+0] * m[1*4+1];
>> >   var tmp_23 = m[1*4+0] * m[0*4+1];
>> >   var t0 = (tmp_0 * m[1*4+1] + tmp_3 * m[2*4+1] + tmp_4 * m[3*4+1]) -
>> >       (tmp_1 * m[1*4+1] + tmp_2 * m[2*4+1] + tmp_5 * m[3*4+1]);
>> >   var t1 = (tmp_1 * m[0*4+1] + tmp_6 * m[2*4+1] + tmp_9 * m[3*4+1]) -
>> >       (tmp_0 * m[0*4+1] + tmp_7 * m[2*4+1] + tmp_8 * m[3*4+1]);
>> >   var t2 = (tmp_2 * m[0*4+1] + tmp_7 * m[1*4+1] + tmp_10 * m[3*4+1]) -
>> >       (tmp_3 * m[0*4+1] + tmp_6 * m[1*4+1] + tmp_11 * m[3*4+1]);
>> >   var t3 = (tmp_5 * m[0*4+1] + tmp_8 * m[1*4+1] + tmp_11 * m[2*4+1]) -
>> >       (tmp_4 * m[0*4+1] + tmp_9 * m[1*4+1] + tmp_10 * m[2*4+1]);
>> >   var d = 1.0 / (m[0*4+0] * t0 + m[1*4+0] * t1 + m[2*4+0] * t2 +
>> > m[3*4+0] *
>> > t3);
>> >   return [d * t0, d * t1, d * t2, d * t3,
>> >        d * ((tmp_1 * m[1*4+0] + tmp_2 * m[2*4+0] + tmp_5 * m[3*4+0]) -
>> >           (tmp_0 * m[1*4+0] + tmp_3 * m[2*4+0] + tmp_4 * m[3*4+0])),
>> >        d * ((tmp_0 * m[0*4+0] + tmp_7 * m[2*4+0] + tmp_8 * m[3*4+0]) -
>> >           (tmp_1 * m[0*4+0] + tmp_6 * m[2*4+0] + tmp_9 * m[3*4+0])),
>> >        d * ((tmp_3 * m[0*4+0] + tmp_6 * m[1*4+0] + tmp_11 * m[3*4+0]) -
>> >           (tmp_2 * m[0*4+0] + tmp_7 * m[1*4+0] + tmp_10 * m[3*4+0])),
>> >        d * ((tmp_4 * m[0*4+0] + tmp_9 * m[1*4+0] + tmp_10 * m[2*4+0]) -
>> >           (tmp_5 * m[0*4+0] + tmp_8 * m[1*4+0] + tmp_11 * m[2*4+0])),
>> >        d * ((tmp_12 * m[1*4+3] + tmp_15 * m[2*4+3] + tmp_16 * m[3*4+3])
>> > -
>> >           (tmp_13 * m[1*4+3] + tmp_14 * m[2*4+3] + tmp_17 * m[3*4+3])),
>> >        d * ((tmp_13 * m[0*4+3] + tmp_18 * m[2*4+3] + tmp_21 * m[3*4+3])
>> > -
>> >           (tmp_12 * m[0*4+3] + tmp_19 * m[2*4+3] + tmp_20 * m[3*4+3])),
>> >        d * ((tmp_14 * m[0*4+3] + tmp_19 * m[1*4+3] + tmp_22 * m[3*4+3])
>> > -
>> >           (tmp_15 * m[0*4+3] + tmp_18 * m[1*4+3] + tmp_23 * m[3*4+3])),
>> >        d * ((tmp_17 * m[0*4+3] + tmp_20 * m[1*4+3] + tmp_23 * m[2*4+3])
>> > -
>> >           (tmp_16 * m[0*4+3] + tmp_21 * m[1*4+3] + tmp_22 * m[2*4+3])),
>> >        d * ((tmp_14 * m[2*4+2] + tmp_17 * m[3*4+2] + tmp_13 * m[1*4+2])
>> > -
>> >           (tmp_16 * m[3*4+2] + tmp_12 * m[1*4+2] + tmp_15 * m[2*4+2])),
>> >        d * ((tmp_20 * m[3*4+2] + tmp_12 * m[0*4+2] + tmp_19 * m[2*4+2])
>> > -
>> >           (tmp_18 * m[2*4+2] + tmp_21 * m[3*4+2] + tmp_13 * m[0*4+2])),
>> >        d * ((tmp_18 * m[1*4+2] + tmp_23 * m[3*4+2] + tmp_15 * m[0*4+2])
>> > -
>> >           (tmp_22 * m[3*4+2] + tmp_14 * m[0*4+2] + tmp_19 * m[1*4+2])),
>> >        d * ((tmp_22 * m[2*4+2] + tmp_16 * m[0*4+2] + tmp_21 * m[1*4+2])
>> > -
>> >           (tmp_20 * m[1*4+2] + tmp_23 * m[2*4+2] + tmp_17 * m[0*4+2]))];
>> > };
>> >
>
>

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