Question about position encoding #52
Description
This is going to be embarrassing. Maybe for me, but that's a risk that I'm willing to take.
I encountered some issues with SPZ-encoded data versus the original PLY data. The following is an archive that contains the original data as ASCII- and BinaryLE PLY data, as well as in SPZ format, once created with my own tools, and once created with the SPZ library:
It's not tremendously complex. In fact, this is the ASCII version of the PLY data:
ply
format ascii 1.0
element vertex 8
property float x
property float y
property float z
property float f_dc_0
property float f_dc_1
property float f_dc_2
property float opacity
property float scale_0
property float scale_1
property float scale_2
property float rot_0
property float rot_1
property float rot_2
property float rot_3
end_header
-2050.0 -0.0 -0.0 1.7724539 1.7724539 1.7724539 20.0 1.0 1.0 1.0 1.0 0.0 -0.0 -0.0
-2000.0 -0.0 -0.0 1.7724539 1.7724539 1.7724539 20.0 1.0 1.0 1.0 1.0 0.0 -0.0 -0.0
-2050.0 -50.0 -0.0 1.7724539 1.7724539 1.7724539 20.0 1.0 1.0 1.0 1.0 0.0 -0.0 -0.0
-2000.0 -50.0 -0.0 1.7724539 1.7724539 1.7724539 20.0 1.0 1.0 1.0 1.0 0.0 -0.0 -0.0
-2050.0 -0.0 -50.0 1.7724539 1.7724539 1.7724539 20.0 1.0 1.0 1.0 1.0 0.0 -0.0 -0.0
-2000.0 -0.0 -50.0 1.7724539 1.7724539 1.7724539 20.0 1.0 1.0 1.0 1.0 0.0 -0.0 -0.0
-2050.0 -50.0 -50.0 1.7724539 1.7724539 1.7724539 20.0 1.0 1.0 1.0 1.0 0.0 -0.0 -0.0
-2000.0 -50.0 -50.0 1.7724539 1.7724539 1.7724539 20.0 1.0 1.0 1.0 1.0 0.0 -0.0 -0.0
Yeah, it's a "unit cube" (rather one with an edge length of 50), somewhere at x=-2000.
Dragging-and-dropping the BinaryLE-PLY file into the BabylonJS sandbox shows the expected result:
When converting this data into SPZ, using the SPZ library, and dragging-and-dropping the resulting file into BabylonJS, then this is the result:
That doesn't look right.
I've inserted some debug statements after this line in packGaussians
// XXX TEST
{
// Decode 24-bit fixed point coordinates
float scaled = 1.0 / (1 << packed.fractionalBits);
int32_t fixed32d = packed.positions[i * 3 + 0];
fixed32d |= packed.positions[i * 3 + 1] << 8;
fixed32d |= packed.positions[i * 3 + 2] << 16;
fixed32d |= (fixed32d & 0x800000) ? 0xff000000 : 0; // sign extension
float resultPosition = static_cast<float>(fixed32d) * scaled;
std::cout << "Position " << i << " was " << g.positions[i] << " fixed32 is " << fixed32 << " converted to " << (int)packed.positions[i * 3 + 0] << " " << (int)packed.positions[i * 3 + 1] << " " << (int)packed.positions[i * 3 + 2] << " back to " << fixed32d << " sign check " << (fixed32d & 0x800000) << " back to result " << resultPosition << std::endl;
}This is doing what is done during the decoding, to print what the actual encoded information will become.
The output of this debug part is
Position 0 was -2050 fixed32 is -8396800 converted to 0 224 127 back to 8380416 sign check 0 back to result 2046
Position 1 was -0 fixed32 is 0 converted to 0 0 0 back to 0 sign check 0 back to result 0
Position 2 was -0 fixed32 is 0 converted to 0 0 0 back to 0 sign check 0 back to result 0
Position 3 was -2000 fixed32 is -8192000 converted to 0 0 131 back to -8192000 sign check 8388608 back to result -2000
Position 4 was -0 fixed32 is 0 converted to 0 0 0 back to 0 sign check 0 back to result 0
Position 5 was -0 fixed32 is 0 converted to 0 0 0 back to 0 sign check 0 back to result 0
Position 6 was -2050 fixed32 is -8396800 converted to 0 224 127 back to 8380416 sign check 0 back to result 2046
Position 7 was -50 fixed32 is -204800 converted to 0 224 252 back to -204800 sign check 8388608 back to result -50
Position 8 was -0 fixed32 is 0 converted to 0 0 0 back to 0 sign check 0 back to result 0
Position 9 was -2000 fixed32 is -8192000 converted to 0 0 131 back to -8192000 sign check 8388608 back to result -2000
Position 10 was -50 fixed32 is -204800 converted to 0 224 252 back to -204800 sign check 8388608 back to result -50
Position 11 was -0 fixed32 is 0 converted to 0 0 0 back to 0 sign check 0 back to result 0
Position 12 was -2050 fixed32 is -8396800 converted to 0 224 127 back to 8380416 sign check 0 back to result 2046
Position 13 was -0 fixed32 is 0 converted to 0 0 0 back to 0 sign check 0 back to result 0
Position 14 was -50 fixed32 is -204800 converted to 0 224 252 back to -204800 sign check 8388608 back to result -50
Position 15 was -2000 fixed32 is -8192000 converted to 0 0 131 back to -8192000 sign check 8388608 back to result -2000
Position 16 was -0 fixed32 is 0 converted to 0 0 0 back to 0 sign check 0 back to result 0
Position 17 was -50 fixed32 is -204800 converted to 0 224 252 back to -204800 sign check 8388608 back to result -50
Position 18 was -2050 fixed32 is -8396800 converted to 0 224 127 back to 8380416 sign check 0 back to result 2046
Position 19 was -50 fixed32 is -204800 converted to 0 224 252 back to -204800 sign check 8388608 back to result -50
Position 20 was -50 fixed32 is -204800 converted to 0 224 252 back to -204800 sign check 8388608 back to result -50
Position 21 was -2000 fixed32 is -8192000 converted to 0 0 131 back to -8192000 sign check 8388608 back to result -2000
Position 22 was -50 fixed32 is -204800 converted to 0 224 252 back to -204800 sign check 8388608 back to result -50
Position 23 was -50 fixed32 is -204800 converted to 0 224 252 back to -204800 sign check 8388608 back to result -50
The negative x-coordinate (for the first splat) is encoded in a way that when decoding it, the coordinate becomes positive. Given that this seems like a pretty fundamental issue, I wanted to confirm that I didn't do something embarassingly wrong here. I'm prepared for a "🤦♂️", but right now, I'm pretty stumped.