#$sel

[andrehitsoy-blog]

05: Assigning hold out material on $sel

Often we want to have certain objects act as a hold out object (for matting purposes). Basicly, what it means is assigning a shader that is black in both rgb and alpha. However, when you do this on several objects it can become a bit of a hassle, because just can't just assign, say, a black surface shader. Well, you can, but it probably means that it wont line up with the beauty pass, since the hold out shader doesn't have the corresponding displacement shader and as such, you'll end up with mismatching edges.

So what we'll do here is take the "holdOut_mat" (a black surface shader with matte opacity 0), or create it if it doesn't allready exist, find the existing displacement shader on the selected objedt, and for each selected object create and assign a shading group with the "holdOut_mat" as a surface material and connect the original displacement shader to the shading group's displacement mat attribute:

Here's what the scene looks like:

We'll assign a hold out material on the 3 balls on the back row:

// --------------- assign holdout material --------------------- global proc assignHOMat(){ string $sel[] = `ls -sl`; if (!`size($sel)`){ print "nothing is selected!\n"; }else{ string $shader = "holdOut_mat"; if (!`objExists $shader`){ $shader = `shadingNode -asShader surfaceShader -n "holdOut_mat"`; setAttr ($shader + ".outMatteOpacity") -type double3 0 0 0 ; } for ( $i=0; $i < size($sel) ; $i++ ){ string $shape[] = `listRelatives -s $sel[$i]`; string $exSG[] = `listConnections -type shadingEngine $shape[0]`; for ( $sg=0; $sg<size($exSG); $sg++){ string $exDispShader[] = `listConnections -type displacementShader $exSG[$sg]`; if (`size($exDispShader)` > 0){ string $newSG = `sets -renderable true -noSurfaceShader true -empty -name ($sel[$i] + "_holdOut_SG")`; connectAttr -f ($shader + ".outColor") ($newSG + ".surfaceShader"); connectAttr -f ($exDispShader[0] + ".displacement") ($newSG + ".displacementShader"); select -r $sel[$i]; sets -e -forceElement $newSG; print ($sel[$i] + " was assigned the holdOut material\n"); }else{ string $newSG = "holdOut_SG"; if (!`objExists $newSG`){ $newSG = `sets -r 1 -nss 1 -em -n ( "holdOut_SG")`; connectAttr -f ($shader + ".outColor") ($newSG + ".surfaceShader"); } select -r $sel[$i]; sets -e -forceElement $newSG; print ($sel[$i] + " was assigned the holdOut material\n"); } } } } select -r $sel; // reselect the original selection. } assignHOMat();

We now end up with this:

Note the preserved displacement on the middle ball. The alpha channel looks like this:

The holdOut shading network now looks like this:

The two non-displaced spheres are connected to the general "holdOut_SG" while the displaced one (sphere2) has its own shading group called "sphere2_holdOut_SG". They all share the same hold out surface shader.

This is a nice way of applying other shaders that require per object displacement textures as well, like other "utility" shaders (if you can't use passes) i.e. depth, lighting passes shaders, shadow catchers' shaders, ambient occlusion, etc.

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[andrehitsoy-blog]

01: meet the for loop, my best friend

string $sel[] = `ls -sl`; for ( $i=0; $i<size($sel); $i++ ){     setAttr ($sel[$i] + ".someAttribute") $someValue }

The most useful thing about MEL, in my opinion, is that it makes working in Maya soo much quicker. Especially mass editing and automating repetitive tasks.

How many times have you gone through objects, one by one to change an attribute that doesn't show up in the channel box (where mass editing is possible)?

Next time, check the script Editor and notice the output from changing an attribute, say, the 'visible' attribute of an object (effectively hiding it). The output says:

setAttr "someNode.visibility" 0;

Now, in the Script Editor, write:

string $sel[] = `ls -sl`; for ( $i=0; $i<size($sel); $i++ ){     setAttr ($sel[$i] + ".visibility") 0; }

and select multiple objects in the view port. Now, select what you wrote in the Script Editor and press the 'enter' button on the numeric pad, which will execute the script.

The script will 'ls -sl' (list the selected objects) and store them in a data collection called $sel (note the brackets, defining $sel as an array). The backward ticks tells maya to store the RESULT of the 'ls' command, rather than the words "ls -sl". The backward tick is produces by pressing the button left of '1'. Again, you want ` not '.

Then it will say "for each item in the list, do:...". Actually, it say "for (start at 0, the first item, and as long as the item number is less than the number of items in the list, do whatever I'll say and after each time I've done it, I'll increase the item index and do it again)".

The curly brackets define a "scope". Every thing within the scope will be executed for each time the loop runs.

There are other ways of writing loops as well. Look it up in the documentation, or on the web.

Examples are: while loop, if loop, case/switch loop and "for ($element in $list)" loop (only works on strings)

So, here is a good example:

Say you have a lot of objects that you use in you render layer for casting reflections (shaded or textured with a projection of the shot plate. You either need to go through them one by one and setting primary visibility to '0' (or if they are textured with the projected shot plate also set 'casts shadow' and receive shadow' to '0'), or select all in on go and run:

string $sel[] = `ls -sl`; for ($i=0; $i<size($sel); $i++){     string $shape[] = `listRelatives -s $sel[$i]`;     setAttr ($shape[0] + ".castsShadows") 0;     setAttr ($shape[0] + ".receiveShadows") 0;     setAttr ($shape[0] + ".motionBlur") 0;     setAttr ($shape[0] + ".primaryVisibility") 0; }