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First pass at defining an updated encoder configuration scheme that does not rely on IFTB.
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‎ift/encoder/encoder_config.proto‎

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edition = "2023";
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// This message is used to configure and IFT encoder to generate and IFT encoding of some input font.
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//
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// The produced encoding will be a "mixed mode" encoding that makes use of both table keyed (https://w3c.github.io/IFT/Overview.html#table-keyed)
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// patches to extend non-glyph data (everything other than glyf, gvar, CFF, and CFF2) and glyph keyed patches to extend
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// the glyph data tables (glyf, gvar, CFF, CFF2).
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//
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// The configuration has two distinct parts, the first "Glyph Data Extension Configuration" section configures how the font
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// will be split into a set of glyph keyed patches. The generated encoding will contain one glyph keyed patch per specified
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// glyph segment and unique configuration of the design space reachable via the non glyph table keyed patches.
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//
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// By default each glyph segment will be reachable by matching any code points that map to any glyph in that segment.
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// However, this default mapping can be overriden use the glyph_patch_dependecies map to specify a set of glyph segments
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// and features that when matched will trigger that glyph segment. For example if there were three input segments (1, 2, 3)
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// where segment 1 contained the f glyph, segment 2 contained the i glyph and segment 3 contains the fi ligature glyph
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// then segment 3 should be configured to be dependent on segments 1 and 2 which would result in segment 3 to only be
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// selected by the client if segments 1 and 2 were also selected.
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//
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// The second section of the configuration is the "Non Glyph Extension Configuration" which describes how to segment the
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// data in all non-glyph tables (everything but glyf, gvar, CFF, and CFF2). These tables will be extended by table keyed
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// patches. Since table keyed patches are invalidating (see: https://w3c.github.io/IFT/Overview.html#font-patch-invalidations)
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// a graph of patches will be produced such that the font can be extended to include data for any of the specified segments.
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//
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// In the simplest case a graph is generated by the following process:
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// - each node will contain one patch per code point, feature, and design space segment that has not yet been included.
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// Where each patch will extend the font to add data only for the associated segment.
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//
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// Additional controls are provided which can modify how the graph is formed. For example to reduce potential client
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// round trips or reduce the number of patches in the graph. The following options are provided:
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//
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// - jump_ahead: defaults to 1, normally each node would contain one patch per outstanding segment. If jump ahead is greater
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// than 1 then additional patches will be included at each node which simultaneously extends the font to
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// all combinations of 2 through jump_ahead segments. For example if a node has three segments a, b, and c
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// and jump ahead is set to 2 then the node will include patches for a, b, c, a+b, a+c, and b+c.
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//
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// - max_depth: max depth limits the depth of the produced graph. In the second last level of the graph all nodes will
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// contain only a single patch which adds all remaining segments. This setting is useful to limit the total
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// size of the graph.
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//
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// - include_all_segment_patches: if true in the generated table keyed patch graph every node (in addition to the usual patches)
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// will have a patch available which adds all remaining segments to the font. This makes
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// it possible for the client to reach any combination of coverage in at most one round
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// trip from any intermediate (or initial) state.
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message EncoderConfig {
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// TODO validation criteria for the configuration:
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// - check that fully expanded font meets closure requirement.
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// ### Glyph Data Extension Configuration ###
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// A list of segments. In the generated encoding there will be one glyph keyed patch (containing all
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// data for all of the glyphs in the segment) per segment and unique design space configuration.
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//
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// The key of the mapping is an integer id which other parts of the configuration can use to reference the segment.
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map<uint32, GlyphPatches> glyph_segments = 1;
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// Encodes a depencies between other glyph patches and layout features.
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//
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// An entry in this map implies that the patch mapping entry for the glyph patch identified by the key
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// will be set up such that it will only be matched for loading iff:
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// - All of GlyphPatchDependency.required_patches are matched, AND
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// - All of GlyphPatchDependency.required_features are matched.
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map<uin32, GlyphPatchDependency> glyph_patch_dependencies = 2;
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// ### Non Glyph Extension Configuration ###
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// For table keyed patches the patch graph will include patches that can add up to this many
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// segments in a single patch. Defaults to 1.
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uint32 jump_ahead = 3;
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// For table keyed patches the patch graph will be at most this many levels deep, or in other
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// words at most max depth round trips will ever be needed to reach any content in the font.
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//
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// This is implemented in the graph by having the nodes in the second last level contain
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// only a single patch which adds everything remaining.
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//
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// Defaults to unlimited depth.
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uint32 max_depth = 4;
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// In the generated table keyed patch graph every node (in addition to the usual patches) will
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// have a patch available which adds everything remaining to the font. This makes it possible
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// for the client to reach any combination of coverage in at most one round trip from any
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// intermediate (or initial) state.
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bool include_all_segment_patches = 5;
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// The initial_* fields defines all of the data that the initial IFT font will include. If these are all
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// left empty then the initial font will be a desiccated font which contains the minimum possible amount of
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// data.
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//
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// Any glyph patches listed in initial_glyph_patches will be added to the initial font and patch files will
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// not be generated for them. Glyph dependencies will be automatically updated to reflect the inclusion of
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// these patches in the initial font. Any glyph patches whose dependencies are satisfied as a result will also be
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// included in the initial font (this is done recursively).
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Codepoints initial_codepoints = 6;
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GlyphPatches initial_glyph_patches = 7;
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Features initial_features = 8;
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DesignSpace initial_design_space = 9;
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// Groups of codepoints which the non glyph data in the font can be extended in a single
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// jump. These can be specified as either groups of codepoints
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// (via non_glyph_codepoint_segmentation), or by grouping together glyph keyed patches defined
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// above (via glyph_patch_groupings).
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repeated Codepoints non_glyph_codepoint_segmentation = 10;
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// An alternative way to specify codepoint segmentations. One segment will be formed from
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// the union of all of the codepoints associated with each referenced glyph patch. Glyph
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// patches are defined above. This field can be used in addition to non_glyph_codepoint_segmentation.
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repeated GlyphPatches glyph_patch_groupings = 11;
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// Groups of open type layout features which the non glyph data in the font can be extended
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// for in a single jump.
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//
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// Note: all segments (codepoints, features, design space) implicitly include the features
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// in the "default feature list" (https://w3c.github.io/IFT/Overview.html#feature-tag-list)
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// so this segmentation should be used to provide optional access to features not already
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// on that list.
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repeated Features non_glyph_feature_segmentation = 12;
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// Parts of the fonts overall design space which the non glyph data in the font can be extended
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// for in a single jump.
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repeated DesignSpace non_glyph_design_space_segmentation = 13;
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// ### Glyph and Non Glyph Configuration ###
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// If true, the for each of glyph, codepoint, feature, and design space segmentations an additional segment
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// will be automatically added (if needed) which includes anything not covered by the specified segments
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//
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// Note: this applies to both the glyph and non-glyph segmentations.
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//
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// Setting this ensures that all data in the original font is always reachable.
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bool add_everything_else_segments = 14;
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}
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message GlyphPatchDependency {
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GlyphPatches required_patches = 1;
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Features required_features = 2;
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}
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// A list of glyph patch ids
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message GlyphPatches {
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repeated uint32 values = 1;
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}
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// A list of glyph ids
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message Glyphs {
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repeated uint32 values = 1;
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}
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// A list of unicode code points.
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message Codepoints {
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repeated uint32 values = 1;
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}
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// A list of open type layout feature tags.
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message Features {
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repeated string values = 1;
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}
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// A variable font design space.
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message DesignSpace {
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// TODO
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}

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