co-maps/docs/feature_structure.md

[!NOTE] This info might be outdated.

First byte:

• 0 - Amount of types (1-8, write as 0-7, 3 bits)
• 3 - Name
• 4 - Layer
• 5, 6 - Geometry type (point = 00, line = 01, area = 10)
• 7 - Bit indicating the presence of additional information:
  • Point - rank (1 byte as the logarithm of population base 1.1);
  • Line - road number (string);
  • Area - house number (string, optimized for storing a two-digit number);

• Write types, name, layer, additional information, and point (for point type)

1 or 2 bytes of the next header (only for line and/or area objects):

• 4 bits for the number of internal points for a line object:

  • 0 - geometry is extracted; read the offset mask and offsets;
  • 2 - 0 bytes for the simplification mask;
  • 3-6 - 1 byte for the simplification mask;
  • 7-10 - 2 simplification mask bytes;
  • 11-14 - 3 simplification mask bytes;

• 4 bits for the number of internal triangles for an area object:

  • 0 - geometry is extracted; read the offset mask and offsets;
  • >0 - number of triangles in one strip (for multiple strips, geometry is extracted);

• 4 bits for the offset mask for line and area objects. The offset mask determines the presence of extracted geometry for the i-th scale row (out of 4, according to the corresponding bit).

These 2 bytes may be located in one byte when the object is of one type or the geometry is not extracted. In reality, this will be 2 bytes when the object is both line and area and has extracted geometry.

Following bytes:

• Write geometry ...
  • Simplification mask for a line object (1-3 bytes): The 1-byte simplification mask encodes the visibility of 4 points in 4 scale rows (2 bits per point), i.e. equal to the scale row value from which the point is already visible.
  • Array of geometry points (triangle strip) according to the known amount VarInt64
• ... or write the array of offsets to the extracted geometry (number taken from the offset mask)

Extracted geometry for a scale is representing a block:

• Size of the geometry in bytes
• Serialized VarInt64s by the number of bytes

For a line object, they represent an array of points. For an area object, they represent the following sequences:

• Number of points on the strip
• The strip itself (array of points)