Add post-processing page (#16)

* add post-processing page

* Formatting and the like

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Co-authored-by: LightArrowsEXE <[email protected]>

src/wobbly/postprocessing.md

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+# Post-processing fieldmatched frames
+
+Fundamentally, the point of IVTC is to paste two fields together and get a full
+frame out of them. However, even if you do the pasting correctly, the output
+frame might still have combing artifacts. This can happen if one field is
+brighter than another due to a fade to/from white applied to the telecined
+footage. It can also happen during a crossfade where one field is further
+progressed in the fade than the other.
+
+![Example of interlaced fade](imgs/example_of_interlaced_fade.png)
+
+![Example of a crossfade](imgs/example_of_crossfade.png)
+
+We can use vs-deinterlace's `fix_interlaced_fades` and `vinverse`,
+respectively, to solve these two problems.¹ Here are two image comparisons
+which show the respective fixes: [one](https://slow.pics/c/wVFUpLi5), [two](https://slow.pics/c/2lDZ2Jo3).
+You can see that the vinverse fix is imperfect
+(there is ringing and other gunk on the eyebrow).
+
+This guide will explain how to tell wobbly to apply these fixes to specific
+ranges of frames.
+
+## Setting up and using custom lists in wobbly
+
+We can use the custom lists feature in wobbly to mark ranges that should be
+handled with vinverse or fix_interlaced_fades. Setting up custom lists is
+complicated, but you only have to go through the process once.
+
+1. Set up presets
+
+   We need to set up "presets" in wobbly, which are basically vapoursynth
+   functions that we want wobbly to apply to certain ranges.
+
+   For example, I can open the preset window, click new, name the preset "vinverse,"
+   and put `clip = vinverse(clip)` in the body. Or I can name the preset "black"
+   and put `clip = fix_interlaced_fades.Darken(clip, colors=[0, 0, 0], planes=[0, 1, 2])`
+   in the body—this would be for fades to/from black. Note that the usage
+   of `clip` is important; you cannot use another variable name. Every
+   project probably needs a preset for white fades, for black fades, and for
+   vinverse (for completion's sake, the preset for fades to/from white could be
+   `clip = fix_interlaced_fades.Brighten(clip, colors=[1, 0, 0], planes=[0, 1, 2])`).
+
+   Obviously, something like `vinverse(clip)` will error out if you do not import
+   vinverse in the final encode script. You will have to handle this yourself. I
+   handle it by copying the script wobbly gives me into a preexisting template
+   with the relevant imports.
+
+2. Create custom lists
+
+   We need to create custom lists that correspond to each preset. You can do this
+   by opening the custom lists window and clicking "new." Each custom list needs
+   to be set as "Post field match" or "Post decimate."
+
+3. Match each preset to its corresponding custom list
+
+   In the custom lists window, there is a dropdown on the left that will contain
+   all your presets. Highlight each custom list and use the dropdown to assign the
+   relevant preset to each list.
+
+4. Select the appropriate custom list
+
+   There is a hotkey to cycle through the custom lists. It is `x` by default. You
+   can see the currently selected custom list in the bottom right.
+
+5. Set the range and save it to the custom list
+
+   The hotkey to start a custom list is `e` by default, and the hotkey to end the
+   range and save it to the list is `c`. You can set a range "backwards" (press `e`
+   on the last frame and then press `c` on the first frame).
+
+Steps 1-3 take a fair amount of time, but we can skip them. When you've used
+wibbly to make a list of wobbly projects, you can do steps 1-3 on the first one
+and then immediately navigate to the rest of the projects and use
+`Project > Import from project` to copy the information over (you'll want to do
+this before you have set any specific frame ranges). Or, since the project files
+are in plaintext, you can use a text editor to copy the relevant information and
+transfer it to new project files. Do this at your own risk, since a mispaste
+will corrupt the project file.
+
+¹Note: Vinverse is *not* an adequate solution for crossfades where the fields
+in one part of the fade are not properly matched, like so:
+
+![Example of improperly matched crossfade](imgs/crossfade_improperly_matched.png)
+
+Using vinverse is dangerous in general. It can be destructive, and so turning
+it on in the middle of a scene can create a jarring change from one frame to
+the next.