Callback: Station slope check

Land slope check (149/157)

This callback is called for each tile where the new station part will be built. For OpenTTD since r22658 allowed return values are like callback 28, but with the difference that the meaning of bit 10 is inverted for backwards compatibility. This means that success is 0 instead of 0x400 while custom GRF texts start at 0x400 instead of 0. Return value 1 stands for "Land sloped in wrong direction" instead of "Site unsuitable". Earlier version of OpenTTD and TTDPatch only support 0 for success and 1 for error.

Since the station isn't built yet, no 8x variables can be accessed. Only variables 43 and 67 will work from the 4x and 6x variables. You get the following information, though:

• variable 18:

bit numbers	Meaning
0..7	offset of current tile on the platform, 0 is the northmost tile
8..15	bits 8..15: current platform number, 0 is the northmost platform
16..23	total length of station being built
24..31	total number of platforms being built

• variable 10:

bit numbers	Meaning
4..7	Slope info of the current tile: bit 7 - N corner is elevated bit 6 - E corner is elevated bit 5 - S corner is elevated bit 4 - W corner is elevated
0..3	the same information, but "mirrored" (bit 0 and 2 swapped) if the station is built in the NW-SE orientation. This allows you to check slopes without checking the orientation explicitly.
other bits	reserved for future use

The callback is called only after the normal checks TTD does for slopes, so it's not possible to allow a slope that isn't allowed by default; you can only narrow the set of allowed slopes. If the callback fails, the tile will be accepted. It is, however, called for flat tiles, so you can force your station to sloped land. You can't access the station info even if the platform is added to an existing rail station or is overbuilding station tiles. That's because TTD will decide this later in the code, after the slope is already accepted.

Please note that for Objects only the low byte of variable 18 is valid (Offset from north tile (origin), stored as YX) and that variable 10 does not have the reversed version, and bits 0 - 3 contain the actual raised corners (same order as bits 4 - 7).