On my quest to create an OpenCL enabled Particle System I realized that I need a way for the Blender Game Engine to know when to render an object as a particles as opposed to a regular mesh. A straight forward way to achieve this (with other benefits) would be to create a Particle modifier that can be added to an object which the game engine can then check for. I’d like to thank Moguri for this idea!

So I set out today to make a custom modifier that does nothing but printf its existence! I figured this out by copying the SoftBody modifier and scouring the source code for all references to it. I called my modifier RTPS, so just replace any instance of RTPS in this post with your own chosen name.

First we want to create our main modifier code file (all paths start in the source folder)

blender/modifiers/intern/MOD_rtps.c

cp blender/modifiers/intern/MOD_softbody.c blender/modifiers/intern/MOD_rtps.c

In here you will add functionality, initialize default values and of course replace all instances of SoftBody with RTPS. You can see my code MOD_rtps.c

The rest of the instructions are just modifying files (if you are confused about where things go, just look at Softbody!

At line 52 of blender/modifiers/CMakeLists.txt

	intern/MOD_rtps.c

At line 184 of blender/modifiers/intern/MOD_util.c

	INIT_TYPE(RTPS);

At line 70 of blender/modifiers/MOD_modifiertypes.h

extern ModifierTypeInfo modifierType_RTPS;

At line 564 of: blender/makesrna/RNA_access.h

extern StructRNA RNA_RTPSModifier;

At line 303 of: blender/blenkernel/BKE_modifier.h

int modifiers_isRTPSEnabled(struct Object *ob);

We need to edit blender/makesdna/DNA_modifier_types.h in several places:

on line 69, just after eModifierType_Screw and before NUM_MODIFIER_TYPES add

eModifierType_RTPS,

on line 441, after the SoftbodyModifierData struct, add

typedef struct RTPSModifierData {
    ModifierData modifier;
    int system;
} RTPSModifierData;

{eModifierType_RTPS, "RTPS", ICON_MOD_SOFT, "RTPS", ""},

case eModifierType_RTPS:
  	return &RNA_RTPSModifier;

static void rna_def_modifier_rtps(BlenderRNA *brna)
{

    StructRNA *srna;
    PropertyRNA *prop;

    srna= RNA_def_struct(brna, "RTPSModifier", "Modifier");
    RNA_def_struct_ui_text(srna, "RTPS Modifier", "Add a particle system");
    RNA_def_struct_sdna(srna, "RTPSModifierData");
    RNA_def_struct_ui_icon(srna, ICON_MOD_SOFT);

    prop= RNA_def_property(srna, "system", PROP_INT, PROP_NONE);
    RNA_def_property_int_sdna(prop, NULL, "system");
    // we should use an enum but this is hacked together for now
    // this range is to allow the user to select a different system
    RNA_def_property_ui_range(prop, 0, 2, 1, 0);
    RNA_def_property_ui_text(prop, "Systems", "Available particle systems");
    RNA_def_property_update(prop, 0, "rna_Modifier_update");

}

rna_def_modifier_rtps(brna);

def RTPS(self, layout, ob, md):
    layout.label(text="System:")
    layout.prop(md, "system")
    layout.label(text="0: lorentz 1: gravity")