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MPD - MultiPart Document

A MultiPart Document, in short MPD, is a project containing multiple models instead of only one, where the word „model“ can be extended to parts or the even smaller LDraw primitives. You could obviously build the above minifig vignette without the usage of this feature by just adding the parts needed for the Sandbox or the Tree to the Mini Dumper Truck. But, packing the different models as submodels into a MPD has multiple benefits:

A submodel can be added several times to a larger layout behaving as a simple LDraw part. Any modification (such as changing color or substituting parts) made to the submodel will be extended to all copies in the layout. But there is more: In case the submodel has been build using the LDraw Main color 16, the single copies in the layout might be colored differently.
The submodel in the layout can be rotated at odd angles, while the original is kept in its default orientation maintaining modification simple. If you have ever tried modifying a model that is not laid out on the standard x, y or z axes you might grasp what I'm talking about.
An MPD allows the inclusion of unofficial or custom parts which are not part of the original LDraw Parts Library. Basically speaking a LDraw model is nothing more than a collection of part number and the figures that determine the color/position/rotation of the part in relationship to each other. If an editor cannot find the part number in the LDraw Parts Library it has no idea how this parts looks like and will show: nothing! An editor supporting MPD will check the project file for the missing data and if all geometries, lines, triangles and quads have been added to the MPD extract these figures and use it to draw the missing part.

The LDraw System of Tools uses different file extensions to ease identification. Today ".dat" is used exclusively for parts and primitives though you might still find some very old models on the internet using this extension. Single model files get the extension ".ldr" while multipart projects use ".mpd".

NOTE! Some very LDraw nerds might argue that you could also use the “.ldr” extension or even “.dat” for a multipart document and that an LDraw editor worth this name has to load such a project without complaints. True(!), but to keep a some sort of order try get used to the following hierarchy:

.dat	LDraw part made of lines, triangles, quads and LDraw primitives, which also use the .dat extension but are stored in a different folder named “p”
.ldr	Single model made of LDraw parts – mostly, as you have seen with the triangles and quads building an arrow everything is exchangeable in LDraw.
.mpd	Multipart document made of single submodels. As with .ldr nothing hinders you to include any of the above.

MLCad automatically distinguishes between the above formats when loading a file and suggests the right format when saving them. Furthermore a MPD project gets automatically created when the first submodel has been added so you don't have to push special buttons.

	(The screenshot shows all menus and dialogs in one composed image).

	22.	There are two ways to compose the final vignette. With the Mini Dumper Truck with Buffer-Exchange.ldr file still open, you could just add an empty submodel via Multipart > New Model..., which would work as container for the final scene naming it Main Model. The other submodels would then be added/rearranged later ...

	(The screenshot shows all menus and dialogs in one composed image).

23..

… I prefer you creating an empty file instead. This will save us a bit of moving things around and be better organized. The empty file will become our container, where all the submodels are placed and arranged. Click the New button

in the Mainbar (File > New)

24.

Select Multipart > Import Model... for every submodel we need for the vignette.

25.

It is good advice importing the models with a some sort of hierarchy in mind: What will become the central piece, say the baseplate of my project? Does it make sense grouping all the minifigs at the end of the submodel list to spot them easier or I'd better group my minifig with the car it is supposed to drive? Do I list all the furnish of my modular building detached from the building or do they belong to the single floors?
Be aware, we are not talking about how the submodels are positioned in the vignette or layout but how they are arranged in the list of the models. To spot them in a wink of an eye if we need making some corrections to them. For the vignette import the submodels in the following order:

Sandbox.ldr
Tree.ldr
Mini Dumper Truck with Buffer-Exchange.ldr
Minifig Boy.ldr
Minifig Supergirl.ldr

Note! You might have noticed that MLCad offers solely *.ldr and *.dat as accepted filetypes. In fact you cannot import or unify two MPD projects in a new MPD project. And there is more! Read what MLCad's help file says about recursive inclusions:

"The program will check for recursive inclusions and display an error if that would happen. Example:
- The project consists of 3 models A, B and C. Model A includes model B and model B includes model C.
- Valid operations are: Including model B again into A or including model C into model A
- Invalid would be to include model B into model C, or to include model A into model C"

	26.	The model imported last becomes automatically the so-called active model and as such it is shown in the Building Area. To access/modify the other submodels you have to select them from the Active Model: drop down list atop of the Parts Project List or pick them from the dialog accessible via Multipart > Activate Model... NOTE! In case you're not happy with the order the submodels are displayed in the list, select Multipart > Model Sequence... and use the Move up, Move down buttons in the popping-up dialog to reorder your list. To delete a submodel, which might has been imported wrongly or because it's parts have been integrated into another model select Multipart > Remove Model... Since almost all submodels are fit to be added to the final vignette and no further changes are required select the empty file Untitled.ldr. Don't be surprised if the file is still empty. You haven't place any submodel so far, you have only told MLCad which submodels will be part of the final vignette.

	(The screenshot shows all menus and dialogs in one composed image).

27.

Untitled.ldr isn't a proper name for our container model. Multipart > Change Model Info... and …

28.

… enter Main Model for Submodel name: as well as Submodel description:.

NOTE! MLCad will automatically add the .ldr file extension to the Submodel name:.

Dialog Wrong Name

The same as for the Group feature applies to the name chosen for the submodel: The name has to be unique, while all description might be the same. Try renaming the Tree by giving it the same name as the Sandbox and you'll get the above warning. This might has given you some other clues. The description is not connected to the submodel's name. Some people use this for a clever numbering scheme when they reproduce original LEGO instructions. The submodel's names is based on the page and step number the sub assembly appears in the instruction booklet, while the description is used for extensive descriptions of the content. So they do not get lost in a list with many, many submodels when building big.

29.

Save (File > Save) your project and name it Playground. If anything goes wrong from here on you've got a solid base where to start from again. As mentioned earlier MLCad automatically picks the right file extension and proposes only Multipart files (*.mpd) in the Files of type: drop-down menu of the Save Multipart File dialog.

NOTE! Nothing hinders you substituting the .mpd file extension in the File Name: entry box with .ldr and save your MPD project as a multipart document with an .ldr extension – a good LDraw editor will handle it. Since you don't gain anything there is no good reason left to do it. Simpler as that?

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