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Town Y value reported by var 80 is inconsistent on different map sizes #7149

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PikkaBird opened this issue Jan 31, 2019 · 4 comments
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@PikkaBird
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PikkaBird commented Jan 31, 2019

Town var 80 is the XY of the town. On a 256x256 map, the variable returns the expected values. On all map sizes, the X value in the low byte appears to work correctly (town X position truncated to a byte).

On larger or smaller maps, however, the Y value becomes erratic. On larger map sizes, it multiplies, and can take an overflow from the X coordinate. On smaller map sizes, it halves, with odd values underflowing into the X coordinate. The behaviour seems to be the same whether the variable is accessed as the high byte of var 80, or as var 81.

A test newgrf is attached. The reported values from var 80 are stored in the persistent town registers - to view them, inspect a test house then click the "parent" button.

image3
Doubled Y value on 512x map
image5
Halved Y value (with underflow) on 128x map
testhouse.zip

@Eddi-z
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Eddi-z commented Jan 31, 2019

Var 80 is not part of the specs, and the meaning has diverged a bit from the original TTD meaning. So i don't think this is technically a bug, but the behavior might need better documentation.

@PikkaBird
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Okay, I think I understand how it works now. I'll experiment.

@Eddi-z
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Eddi-z commented Jan 31, 2019

Basically, you need to read out the "X" bits from "Patch Var 13" and then recreate the TileX/TileY functions, this will be correct for all map sizes (smaller or bigger)

@PeterN
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PeterN commented Jan 31, 2019

This is an internal data variable which requires masking to get the data as required, and is working as expected.

@PeterN PeterN closed this as completed Jan 31, 2019
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