Cannot write specific values to UVWTag

C4DS

Hi,
I am trying to create precise UV coordinates, but am not able to store the wanted UV values.

Say I create a single polygon object (primitive plane with subdivision 1x1, make editable). And want to map a texture bitmap of 1920x1080.
I then use following script to modify the plane's UV to fit exactly to the top 72 pixels of my bitmap.
The bottom UV coordinates should be 72 / 1080 = 0.06666666...

import c4d
from c4d import gui

def readUV(uvwtag):
    for i in xrange(uvwtag.GetDataCount()):
        uvwdict = uvwtag.GetSlow(i)
        uvwA = uvwdict["a"]
        uvwB = uvwdict["b"]
        uvwC = uvwdict["c"]
        uvwD = uvwdict["d"]
        print "read UV", i, ":"
        print " A: %.15f %.15f" % (uvwA.x, uvwA.y)
        print " B: %.15f %.15f" % (uvwB.x, uvwB.y)
        print " C: %.15f %.15f" % (uvwC.x, uvwC.y)
        print " D: %.15f %.15f" % (uvwD.x, uvwD.y)
    return

# Main function
def main():
    # reads the UVs of the object
    if op == None:
        print "No active object!"
        return
    if not op.CheckType(c4d.Opolygon):
        print "Active object is not Opolygon"
        return

    # get the UVW tag of the object
    uvwtag = op.GetTag(c4d.Tuvw)
    if uvwtag == None:
        print "Active object has no UVW tag"
        return

    readUV(uvwtag)

    # write UVs
    uvwA = c4d.Vector(0.0, 0.0, 0.0)
    uvwB = c4d.Vector(1.0, 0.0, 0.0)
    uvwC = c4d.Vector(1.0, 72.0 / 1080.0, 0.0)
    uvwD = c4d.Vector(0.0, 72.0 / 1080.0, 0.0)
    print "write UV"
    print " A: %.15f %.15f" % (uvwA.x, uvwA.y)
    print " B: %.15f %.15f" % (uvwB.x, uvwB.y)
    print " C: %.15f %.15f" % (uvwC.x, uvwC.y)
    print " D: %.15f %.15f" % (uvwD.x, uvwD.y)
    uvwtag.SetSlow(0, uvwA, uvwB, uvwC, uvwD)

    readUV(uvwtag)


# Execute main()
if __name__=='__main__':
    main()

This is the result I get:

read UV 0 :
 A: 0.000000000000000 1.000000000000000
 B: 0.000000000000000 0.000000000000000
 C: 1.000000000000000 0.000000000000000
 D: 1.000000000000000 1.000000000000000
write UV
 A: 0.000000000000000 0.000000000000000
 B: 1.000000000000000 0.000000000000000
 C: 1.000000000000000 0.066666666666667
 D: 0.000000000000000 0.066666666666667
read UV 0 :
 A: 0.000000000000000 0.000000000000000
 B: 1.000000000000000 0.000000000000000
 C: 1.000000000000000 0.066666662693024
 D: 0.000000000000000 0.066666662693024

The values read after writing are close, but not what they are supposed to be. Why?
I know the problem with floating point representation.
But since the wanted value can be hold in a variable and printed out, I would expect this value to be able to be written into the UVWtag ... and read back. But apparently this is not the case.

I had encountered this issue within a C++ plugin, and tried this simple script just to point out the issue.

Is this a bug, a limitation, ... something else?
I know that for most users a UV pixel coordinate of 71.999999 or 72.0 would not make a big difference, but for some users sending this information to a LED processor it apparently is.

ferdinand

This post is deleted!

r_gigante

Hi @C4DS, thanks for reaching out us.

With regard to your issue, I confirm that, by design, the values stored in UVWTag are rounded to 8 significant digits.

Best, R

C4DS

@r_gigante
Bummer ... But thanks for confirming!

C4DS

@r_gigante
Just a quick question about this. Is the truncation the result of using a Float32 type, or is there another technique used to limit to the 8 significant digits?

r_gigante

It's actually due to a lower number of bits used to store the floating value.