PIPE­LINE TECH­NIQUES: Cre­ate a light­ning ex­plo­sion in Houdini

3D Artist - - CONTENTS -

Shape a pyro sim­u­la­tion and mas­ter RBD sim­u­la­tions

This tu­to­rial shows you my ba­sic work­flow in creat­ing the light­ning ex­plo­sion. i will mostly be talk­ing about the idea and my thought process, show­ing how i lay out and plan the whole shot be­fore start­ing a project. This is ab­so­lutely key in or­der to fin­ish the shot on time, try to keep it sim­ple and layer the com­plex­ity later if time al­lows. i will also go through some tips on shad­ing and shap­ing the pyro sim­u­la­tion, as well as the cre­ation of the light­ning. hope­fully this tu­to­rial will give you a bet­ter un­der­stand­ing of creat­ing a VFX shot from start to end.

Pre­pare Geo and An­i­ma­tion Plan­ning ahead be­fore start­ing a project is cru­cial. in this case, i know i’m creat­ing a naval mine ex­plod­ing in the air, there­fore the mine has to be hol­low on the in­side for me to fill in fuel and tem­per­a­ture. i started the model in Maya, and made sure that i mod­elled each panel of the mine to cre­ate an in­ter­est­ing look. Frac­tur­ing in houdini will give me a voronoi look, and that’s not what i’m go­ing for. i also made sure that each panel i model for the shell are not in­ter­sect­ing. next, nail­ing down the cam­era an­gle and mo­tion is im­por­tant so we can see and sim­u­late our ex­plo­sion at a fixed an­gle for a bet­ter com­po­si­tion.

Cre­ate RBD sim­u­la­tion i wanted to cre­ate some in­ter­est­ing force field ef­fects right be­fore the mine ex­plodes. There­fore, i cre­ated two forces: one with softer strength just to push the parts of the mine out­wards, and the other stronger one that trig­gers later on in the frame when the ex­plo­sion det­o­nates. The RBD wasn’t the main point of this project, so i an­i­mated the time scale to get the time freeze ef­fect. Then, by in­tro­duc­ing grav­ity and bring­ing the time scale back to nor­mal and with a stronger force, i was able to achieve the de­sired look.

Cre­ate the light­ning After nail­ing down the RBD sim­u­la­tion, i’ve pro­ceeded to creat­ing the light­ning. The idea is to have light­ing around the outer shell of the mine and a stronger light­ning core creat­ing a force field around the bomb right be­fore it det­o­nates. i’ve scat­tered the same amount of points on both the rods and outer shell, and paired them up with an Add sop which con­nects them to­gether. Then, i de­fined an at­tribute to trig­ger the light­ning in the frames it ac­ti­vates, as well as defin­ing a life­span for each light­ning. After that, i used cd to blast away non-ac­tive light­ning, and the light­ning turn on at a ran­dom frame range de­fined with a fit node. To add de­tail to the light­ning, i’ve

cre­ated and blended high-fre­quency noise for the big elec­tric waves and an­other lower-fre­quency noise for the de­tails. Fi­nally, to mesh each light­ning, i’ve de­fined a width at­tribute based on the uvs of each line to get a ta­per­ing thick­ness at the tip of the light­ning.

Add the light­ning core i’ve also made an­other stronger force field-like light­ning around the mine right be­fore it ex­plodes. To achieve that, i’ve used a sim­i­lar tech­nique for the light­ning on the outer shell by scat­ter­ing points at the core where the ex­plo­sion will hap­pen and an outer sphere that i’ve cre­ated. i’ve also used a Moun­tain sop on the outer sphere to avoid a smooth, round sil­hou­ette that the light­ning hits. The light­ning strik­ing out to the outer core doesn’t re­ally give me the force field ef­fect i’m go­ing for, so i’ve made an­other layer of light­ning along the outer sphere that we cre­ated. i scat­tered some points on the sur­face of the outer sphere and con­nected them by us­ing Add sop – just skip ev­ery nth point. With that, we get a ran­dom con­nec­tion of lines along the sur­face. To get a nice arc, we need to add a third point in-be­tween the straight lines. We can do this by first re­sam­pling the lines to get a third point in the mid­dle, and then use neigh­bour count node to find the mid­dle points by fil­ter­ing only points with more than one neigh­bour. Then, take those points and dis­place them along the nor­mals of the sphere. Fi­nally, con­vert it into a bezier curve to get a nice, smooth arc and then we can add the light­ning de­tail we de­sire.

Pyro sim­u­la­tion source An in­ter­est­ing-look­ing pyro sim­u­la­tion al­ways comes down to the pyro source. i want to cre­ate an in­ter­est­ing-look­ing pat­tern, so i’ve used a sim­ple sphere as the emis­sion source for my par­ti­cles. To layer the com­plex­ity even fur­ther, i’ve used a noise on the sphere to get a ran­domised emis­sion source. Also, i’ve emit­ted a layer of thin smoke first, and when the in­ner core – which is filled with tem­per­a­ture and fuel – ig­nites, it will push out the smoke. Lastly, i’ve emit­ted some par­ti­cles shoot­ing out­wards to get some fiery streaks shoot­ing out from the core.

Pyro sim­u­la­tion shape The shape of the ex­plo­sion is cru­cial to catch the viewer’s eye. us­ing the mi­cro-solvers, we can achieve very in­ter­est­ing shapes by lay­er­ing them one by one. First, we want to start with tur­bu­lence, as it’s the main force that drives an in­ter­est­ing shape. iso­late the tur­bu­lence by turn­ing the other dis­tur­bance forces off. After we get the big shape that we want, we then cre­ate an­other tur­bu­lence force with a higher fre­quency for the smaller de­tails. For ex­plo­sions, we want the tur­bu­lence to be strong at the be­gin­ning and fade off af­ter­wards. One thing i like to do is to keyframe the timescale; mak­ing the sim­u­la­tion twice as fast for the first few frames from det­o­na­tion gives the sim­u­la­tion more ex­plo­sive look.

Pyro shad­ing As for shad­ing the pyro, adding dark ar­eas within the fire ramp, as well as us­ing a sharper or spiky graph for the fire graph, will give you nice hot lay­ers within the fire. it will also give the fire a re­ally nice tex­ture and also a darker area within the pyro, adding an­other layer of re­al­ism. On the other hand, adding the white val­ues above 1.0 adds the blown-out hot spots at the very core of the ex­plo­sion. To achieve a py­ro­clas­tic smoke shape, i like to use a gra­di­ent graph to get the round plume-like smoke shapes of an ex­plo­sion. shad­ing the pyro can be time-con­sum­ing, but al­ways try to get a wide range of val­ues within your fire so you can use it to get the de­sired look in com­posit­ing. it’s much faster to tweak the val­ues when com­posit­ing than get­ting the de­sired ex­plo­sion look in the ren­der it­self.

Cre­ate cloud­scape and light­ing hav­ing the cam­era an­gle nailed at the very be­gin­ning helps with de­sign­ing the cloud­scape. i only cre­ated six dif­fer­ent cu­mu­lus cloud shapes – you might think that it is far too few for a cloud scape, but by ro­tat­ing and even merg­ing them to­gether, we can get a to­tally dif­fer­ent-look­ing cloud. Then, i placed them around the scene for a nice com­po­si­tion, mak­ing sure that i had a clear fore­ground, midground and back­ground. Then i’ve lit the scene and made sure that the clouds do not look too flat.

Dy­namic clouds and scene in­ter­ac­tion

In­ter­ac­tions among el­e­ments in a shot is im­por­tant to avoid hav­ing a shot look­ing like it is from a dif­fer­ent world. As for this shot, the mine needed some in­ter­ac­tion with the sur­round­ings. But to make the whole cloud­scape dy­namic is way too ex­pen­sive and heavy to sim­u­late. There­fore, by strate­gi­cally plac­ing only one cloud in the area where the mine flies through and sim­u­late, that one cloud gives us the in­ter­ac­tion we need. To fur­ther blend all the dif­fer­ent el­e­ments to­gether, hav­ing the light from the ex­plo­sion af­fect­ing the clouds is cru­cial. These sim­ple ad­di­tions tie all the el­e­ments to­gether, and thus gives us a more com­plete shot.

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