translate example a bit confusing

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lukasz

translate example a bit confusing

in Programming Questions • 11 months ago

trying to understand what is going on this sketch from a book I am reading, a bit confused. The processing documentation says that translate cascades down to all elements after so lines 9 and 12 cancel themselves out? I understand that the transform X is only on X axis and does 180 which essentially flips it upside down and that shouldn't affect the Z since its just forward and back. so why even have line 12?

book says this is to center the camera around the point cloud. however this is very confusing as to how this works.

              
               void draw() {
                 strokeWeight(8); 
                 //println(frameRate);
                 background(0);
                 kinect.update();
                 PImage rgbImage = kinect.rgbImage();
               

                 //pushMatrix();
                 translate(width/2, height/2, -1000);
                 rotateX(radians(180));
               

                 translate(0, 0, 1000);
               

                 rotateY(radians(map(mouseX, 0, width, -180, 180)));
                 stroke(255);
                 PVector[] depthPoints = kinect.depthMapRealWorld();
               

                 for (int i = 0; i < depthPoints.length; i+=10) {
                   PVector currentPoint = depthPoints[i];
                   point(currentPoint.x, currentPoint.y, currentPoint.z);
                 }
               

                 translate(boxCenter.x, boxCenter.y, boxCenter.z);
                 stroke(255, 0, 0);
                 noFill();
                 box(boxSize);
                 //popMatrix(); 
               }

Replies(5)

quarks

Re: translate example a bit confusing

11 months ago

You have not included all the code but this sketch must be using a 3D renderer i.e. P3D or OPENGL. Since its 3D then the translation can be in 3 directions.

Line 1 the transformation matrix is reset to (0,0,0) at the beginning of draw
Line 9 moves the drawing origin from (0,0,0) to (width/2, height/2, -1000)
Line 10 rotates 180 degrees about the X axis
Line 12 translates the drawing origin to (width/2, height/2, 0) Note that translations are additive so (width/2, height/2, -1000) + (0,0,1000) = (width/2, height/2, 0)
Line 14 rotates about the Y axis depending on horizontal position of the mouse
Lines 16 - 21 draws some points based on data from a Kinect device (I assume)
Line 23 translates the drawing origin to the 3D position for a cube (don't forget its added to current position as calculated in last translation (line 12)
Line 26 draws a cube at the current drawing origin

HTH

lukasz

Re: translate example a bit confusing

10 months ago

yeah I left out unimportant things, since it definitely works and they are irrelevant, its line 9 and 12 that interest me.

thanks for the detailed explanation. However, I am feeling like a dummy here because what you said confirms what I said, why not just get rid of line 12 and just do this on 9 translate ( width /2, height /2, 0);

I am confused why zoom out, then in again,
seems very pointless since we are only rotating 180.
wouldn't that be same as: translate(2,2,2) translate(-2,-2,-2); ?

quarks

Re: translate example a bit confusing

10 months ago

U nfortunately I don't have the book, the code or a kinect device so I can't experiment with the code to try it out.
You say the code definitely works so have you tried making the changes that you suggest? If you have and
it makes no difference then the code author missed that fact.

if you were to draw a 3D object at line 11 then it would certainly make a difference.

BTW what is the book?

lukasz

Re: translate example a bit confusing

10 months ago

I have tried it, I am not understanding what goes on though. if I set it to 0, it looks very flat. but the code suggests it should cancel each other out. its very confusing.

its from book making things see.

lukasz

Re: translate example a bit confusing

10 months ago

translate(width/2, height/2, -1000);
rotateX(radians(180));
translate(0, 0, 1000);
rotateY(radians(map(mouseX, 0, width, -180, 180)));

translate(width/2, height/2, 0);
rotateX(radians(180));
//translate(0, 0, 1000);
rotateY(radians(map(mouseX, 0, width, -180, 180)));

here is the entire code: https://gist.github.com/4145333

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