How is the position and color information implemented in the code, and the position information (0, 0, 0) refers to which point in the world coordinate system?

[PetershengWUHU]

How is the position and color information implemented in the code, and the position information (0, 0, 0) refers to which point in the world coordinate system?

[wesleyliwei]

COLOR: comes from original images, which provided in command line, or selected path in gui.
Detail:

colmap/src/controllers/incremental_mapper.cc

Line 133 in 9949d4e

void ExtractColors(const std::string& image_path, const image_t image_id,

POSITION: directly use the image without really prior info, such as GPS/ECEF info, the position is meaningless. it's just a normalized center of entire model.

[PetershengWUHU]

colmap/src/base/reconstruction.cc

This code shows how colors and world coordinates are calculated, but the exact formation process cannot be known.

[tsattler]

but the exact formation process cannot be known

Given that the source code is available, it can be known. Of course, it requires a bit of effort to go through the code.
I suggest starting here: https://github.com/colmap/colmap/blob/dev/src/exe/colmap.cc#L211

[PetershengWUHU]

I have read the code, but I have a question, why the parameters in the code are multiplied by 3, not other numbers.
Detail:colmap/src/util/bitmap.cc

[tsattler]

Because this part of the code is accessing an RGB image, where three subsequent values correspond to the color of a single pixel.

[PetershengWUHU]

The color is extracted twice in the colmap.cc file. Why is the color extracted for the second time?
Detail:colmap/src/exe/colmap.cc
The color is extracted for the first time.

The color is extracted for the second time.

[tsattler]

These are two different functionalities of Colmap (corresponding to the color_extractor and point_triangulator arguments for the command line interface).