Structured Light - How to do when the projector's resolution is lower than patterns?

Question

I try to build a structured light environment to do 3D scanning.

As far as I know, if I choose to use gray code to reconstruct a 3D model, I have to implement specific patterns that were encode in power 2(2^x, x = 0 ~ 10).

That is said, the patterns must be at least 1024 x 1024 in resolution.

What if my DLP projector only support resolution up to 800 x 480? It projects Moire pattern when the gray code pattern resolution becomes too high(I tried). What should I do?

My friends suggest that I create 1024 x 1024 patterns, and "crop" them into 800 x 480,

but I thought the gray code should follow specific sequence and patterns, my friends suggestion will create several image that is not symmetry.

Does anyone have the same experience like me?

----------2015.8.4 Update Question----------

I was thinking that if my projector can't perfectly projects high resolution patterns, can I just let it projects the patterns with low resolution, for instance, from 2^0 to 2^6?

Or the gray code strictly demands patterns from 2^0 to 2^10? Otherwise gray code is not available?

Beaufert

Crop to 800x480, do not rescale. I don't think that the missing area will be an issue (unless your software demands the 1024 stripes, which I doubt). – Encincture

Also check that your lens is good enough to sustain that spatial frequency (MTF curve). An what about your scanner/camera resolution ? – Encincture

Answer 1

you can not directly scale down to your resolution

because it would distort the pattern make it useless

instead you can:

1. crop it to your resolution

   but you need to handle that in the scanning part too because you would not have the full pattern available

2. use nearest usable power of 2 resolution

   like 512x256 and create pattern for it. The rest of the space is unused (wasting pixels)

3. use bullet #2 + scale up to fit your resolution better

   so create pattern 512x256 and linearly scale to fit to 800x480 as much as you can so:

   800/512 = 1.5625
   480/256 = 1.8750
   

   use the smaller scale (512x256 * 1.5625 -> 800x400) so scale the pattern by 1.5625 and use that as a pattern image

   this is scaled by nearest neighbor to avoid subpixel grayscale colors which are harder to detect. This will waste less pixels but it can lower the precision of 3D scan !!!

This is how I generate my pattern in C++ and VCL:

// [generate pattern xs*ys power of 2 resolution]
// clear buffer
bmp->Canvas->Brush->Color=clBlack;
bmp->Canvas->FillRect(TRect(0,0,xs,ys));
int x,y,a,da;
for (da=0;1<<da<xs;da++);                   // number of bits per x resolution
for (a=0,y=0;y<ys;y++,a=(y*da)/ys)
 for (x=0;x<xs;x++)
  if (int((x>>a)&1)==0) pyx[ys-1-y][x]=0x00FFFFFF;
bmp->SaveToFile("3D_scann_pattern0.bmp");

• bmp is VCL bitmap
• xs,ys is the resolution of a bitmap
• p[ys][xs] is direct 32bit pixel access to bitmap

This is slight differently encoded then your pattern !!!

[Notes]

• If you need precision use bullet #2
• If you need to cover bigger area use bullet #3
• You can also scale in y axis differently then in x axis as this is just 1D encoding

Mcelroy

I suppose you mean "projector's resolution" in the first line and bullet 1, right? or you mean my PC(NB) resolution? – Beaufert

@Beaufert yes of coarse projectors resolution ... visualization device has nothing to do with scanning. The only resolutions you have to mind are projectors and camera(s) ... – Mcelroy