Action 0 allows to change the feature properties of 'features', i.e. vehicles, stations, bridges, houses and more. Each feature has its own very specific properties. These are explained in detail in their separate pages.
You will also find the minimum version requirements for properites on the pages. Note, that due to the nature of Action 0 unknown properties cannot be ignored. Only a whole Action 0 can be skipped. OpenTTD and TTDPatch will disable GRFs once they encounter an unknown property. So, if you use some properties for optional features, which you want to disable for older versions of OpenTTD or TTDPatch, you have to skip the Action 0 using Action 9.
The data for Action 0 looks as follows:
<Sprite-number> * <Length> 00 <Feature> <Num-props> <Num-info> <Id> (<Property <New-info>)...
Here is a short overview of what every term means:
|<Sprite-number>||dec||A sequential sprite number|
|<Length>||dec||The total number of bytes in the action|
|00||B||Action type. In this case, 00|
|<Feature>||B||Which type of feature you would like to change|
|<Num-props>||B||How many properties you would like to change per vehicle or station|
|<Num-info>||B||How many vehicles/stations you would like to change|
|<Id>||B*||The ID of the first vehicle/station to change|
|<Property>||B||What property to change for each vehicle/station|
|<New-info>||V||The new value of the property|
You can put an Action 0 anywhere after Action 8 in the GRF file.
0.7 GRFv7 In OpenTTD since r13482, extended IDs (up to 65535) can be used for vehicles as well. However there is currently a caveat that articulated parts must be below 128. 1.2 GRFv8 In OpenTTD 1.2, extended IDs (up to 65535) can be used for vehicles as well. The ID for articulated vehicles must be below 8192.
Action 0 can appear anywhere in the GRF file, so set it to the sprite number you are currently at.
The total number of bytes in Action 0. Start counting from <Action>, the bit that sets the pseudo-sprite to act as the specified action.
The type of action this pseudo-sprites defines. It should be 00 here because we want this pseudo-sprite to act as Action 0.
This sets the type of feature that you wish to change. Set it to:
|00||Trains||Common vehicle properties|
|07||Houses (see defaults)|
|09||Industry Tiles (see defaults)|
|0A||Industries (see defaults)|
|0B||Cargos (see defaults)|
|11||1.1 Airport Tiles|
This is the number of properties that you wish to change per vehicle or station. Note: even if you wish to set the same properties to the same value for different vehicles then you must still repeat the properties and their values for each vehicle.
Simply the number of vehicles that you will change using this action 0.
The Vehicle ID of the first vehicle or station to change. If num-info is greater than one, this vehicle/station and the following vehicles/stations will be changed.
The number of the property that will be changed. This and the New-info section are repeated as many times as there are properties to set; in total, there are < num-props> property bytes.
See relevant sub-section (links at the bottom) for more details.
The new information that will replace the previous information for the specified property. This is a variable size; dependent upon the property. Each property byte is followed by <num-info> new-info sections. The appropriate \b, \w, and \d escape sequences can be quite useful for most < new-info>s. See the discussion of escape sequences for further information.