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.

If you're rendering fractional frames, the integer frame number ($F) for both frame 5.0 and 5.5 will 5. To have unique filenames, use $N or $FF instead. The disadvantage of $FF is possible binary-to-decimal issues making $FF look like 31.99999. The disadvantage of $N is that it’s not related to frame numbers. You especially have to watch out that if you use $N and render frames 1-10, and then render 11-20, the second batch would overwrite the first.

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.

VEX has no padzero function, instead you would use sprintf to format a string containing the frame number.

When writing out files for fractional frame numbers, the best way to generate the filenames is:

• Generate the frame numbers based on $T to avoid precision issues. (See the note under global playback variables).

• Borrow Python’s formatting function through pythonexprs.

For example:

$HIP/geo/$HIPNAME.$OS.`pythonexprs("'%.2f'%"+($T*$FPS+1))`.bgeo.sc

The %.2f part of the expression formats the frame number with two decimal places.

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

MyImage`$F+12`.pic.

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.