Houdini 16.5 Rendering

Expressions in file names

How to use variables and expressions in file path fields to generate numbered and unique filenames.

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You can insert expressions into file name parameters that Houdini will evaluate when it goes to read or write the file. The most common use for this is to include $F in the render output filename so each frame’s output file is numbered and the output files don’t overwrite each other.

For example, mine$F.pic produces filenames mine1.pic, mine2.pic, mine3.pic, and so on.

Note that if you need to "protect" the variable from surrounding letters, you can use the form ${F}. For example, if you want an F after the frame number, you can’t use frame$FF.pic, because $FF is a variable name. In this case, you would use frame${F}F.pic, which would give you frame1F.pic, frame2F.pic, and so on.

Useful variables


Current frame number.


Current fractional frame number. This comes into play when Houdini is calculating sub-frame motion for motion blur.


Number of input/output files counter. This is not related to the frame number. It’s the number of frames Houdini has read/written/rendered so far, when it processes multiple files.

For example, if you render frames 10-15, $N will be 1, 2, 3, 4, 5.


Current time.


Current simulation timestep number. For saving files in dynamics simulations, such as simulation state, use $SF instead of $F. For each value of $F, a simulation may calculate several substeps, so if you use $F you may have several sub-frames overwrite the same file. $SF is equal to $ST (simulation time) divided by $TIMESTEP (simulation time step size).


Current simulation time.


Current node name.

When rendering fields, the integer frame number ($F) for both frame 5.0 and 5.5 will 5. Therefore, to have unique filenames, use $N or $FF instead. The disadvantage of using $FF is possible binary-to-decimal arithmetic errors causing $FF to have a value like 31.99999. The disadvantage of using $N is that it not related to frame numbers, it only counts the number of frames rendered. So if you render frames 1-10, and then render 11-20, the second batch would overwrite the first.

Leading zeros

To generate leading zeros before the frame number, put a non-zero digit after $F. This will generate frame numbers with that many digits. For example, mine$F3.pic produces filenames mine001.pic, mine002.pic, mine003.pic, and so on.

You can also use the padzero expression function, see below.

Advanced expressions

For anything beyond simple variable substitution, you can enclose a full expression inside backticks.

For example, you may want the filename to increment with the frame number, but be offset by a certain number. You can use a filename with an embedded expression inside backticks like


This will number the files by the current frame number + 12. This produces filenames MyImage13.pic, MyImage14.pic, MyImage15.pic, …

You can use expression functions within the backquoted expression, such as padzero.

frame`padzero(5, $F)`.pic

…giving you filenames frame00001.pic, frame00002.pic, frame00003.pic, and so on.


  • To store images in directories based on image resolution, use a path like Pics${W}x${H}/$F.pic.

  • To include the name of the current operator in a filename (for example, in the name of a Z-depth map you want to include the name of the light in the filename), use $OS. For example, $OS_$F.pic produces filenames light1_1.pic, light1_2.pic, light1_3.pic, …

  • Do not use a hyphen to separate the frame number from the name (for example frame-$F.pic). MPlay will interpret the resulting filenames (frame-1.pic, frame-2.pic, frame-3.pic, and so on) as containing negative frame numbers (-1, -2, -3, …).

  • Avoid using spaces in filenames. Although most operating systems support them, many software packages (including Houdini!) sometimes assume that filenames don’t have spaces, and this can cause a lot of trouble. Use CamelCase or under_scores instead.


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