The art of The in­vis­i­ble ef­fects shot

Bound­ary Vis­ual Ef­fects (bound­aryvfx.com) co-founder Nick Lam­bert re­veals the se­crets be­hind in­vis­i­ble ob­ject re­moval and ad­di­tions

3D World - - FEATURE -

01 PRE­PAR­ING THE FOOTAGE

Our story re­quires a build­ing with two floors. The di­rec­tor finds the car parked on the left of the shot dis­tract­ing, and the back­ground build­ing too busy with de­tail. As al­ways, there are a number of ways to ac­com­plish the re­quired re­sults, but we’ll use a 3D ap­proach in Nukex. The first step is to de-noise the footage and take out the lens dis­tor­tion. If you have a lens checker­board you can un-dis­tort the footage eas­ily. If not, then a man­ual method can be used. Nukex’s lens node has a fa­cil­ity to al­low you to draw on straight lines and use this to equate the lens perime­ters.

02 RO­TO­SCOP­ING

In or­der to get a clean 3D track we will need to mask the car so as to tell the tracker to ig­nore this area when look­ing for points to track. A ba­sic garbage matte is all that is nec­es­sary here. Roto the car and its shadow through­out the shot. This does not need to be ex­act, it just needs to cover the of­fend­ing ar­eas of al­ter­na­tive mo­tion. With the orig­i­nal footage and our garbage mask we can now do our 3D track. This will still re­quire some man­ual in­put, such as delet­ing er­ro­neous tracks and ori­en­tat­ing the scene.

03 TRACK­ING

At this point, if you have the film back and lens data you can in­put this to aid the solve, if not, the node will try and es­ti­mate these. Af­ter run­ning an auto track it’s good prac­tice to delete any bad tracks be­fore solv­ing. Al­though a solve can be made with rel­a­tively few tracks, they need to show par­al­lax to work. Once solved we need to tell our 3D world which way is up. This is done by man­u­ally se­lect­ing solved points that are on the ground and set­ting these as our ground plane within the vir­tual world.

04 SOLV­ING THE CAM­ERA

Now we can cre­ate a cam­era, point set and scene for mod­el­ling from the ori­en­tated solve. Us­ing the point cloud as ref­er­ence, add a Model­builder node to the scene and use a cube prim­i­tive to po­si­tion and scale into the back­ground build­ing. Re­peat for the fore­ground build­ing ex­ten­sion and use a card for the ground plane. We are now set up to start paint­ing and com­posit­ing. As you have seen, even a quick and sim­ple-look­ing shot can re­quire the use of many dis­ci­plines, but in a smaller fa­cil­ity this can all be done by one artist in a sin­gle ap­pli­ca­tion (in this case, in Nukex).

05 FI­NAL COM­POSIT­ING

Us­ing a com­bi­na­tion of frame holds and pro­jec­tors it’s now pos­si­ble to paint out the of­fend­ing car left of frame and re­move the busy back­ground on the build­ing. The set ex­ten­sion uses the same setup, but with more work to build the tex­ture. Com­bin­ing these with the orig­i­nal footage is done by us­ing the Scan­lineren­der node to out­put a 2D ver­sion of the scene we can merge with it. Now we dis­tort the out­put of the ren­derer to match the orig­i­nal footage and the grain. Us­ing the in­put footage pre de-grain we can now merge the CGI with it seam­lessly.

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