From 4b1f655a8391d8b806c8fe9a4c6503c728d02d72 Mon Sep 17 00:00:00 2001
From: Behdad Esfahbod <behdad@behdad.org>
Date: Tue, 4 Mar 2025 23:56:01 -0700
Subject: [PATCH] [fontations] Implement LinearGradients

---
 src/fontations/lib.rs | 81 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 78 insertions(+), 3 deletions(-)

diff --git a/src/fontations/lib.rs b/src/fontations/lib.rs
index de245ba2e..5ef8a94a6 100644
--- a/src/fontations/lib.rs
+++ b/src/fontations/lib.rs
@@ -10,7 +10,7 @@ use read_fonts::tables::cpal::ColorRecord;
 use read_fonts::TableProvider;
 use skrifa::charmap::Charmap;
 use skrifa::charmap::MapVariant::Variant;
-use skrifa::color::{Brush, ColorGlyphCollection, ColorPainter, CompositeMode, Transform};
+use skrifa::color::{Brush, ColorGlyphCollection, ColorPainter, ColorStop, CompositeMode, Extend, Transform};
 use skrifa::font::FontRef;
 use skrifa::instance::{Location, NormalizedCoord, Size};
 use skrifa::metrics::BoundingBox;
@@ -346,6 +346,53 @@ impl HbColorPainter {
     }
 }
 
+struct ColorLineData<'a> {
+    color_stops: &'a [ColorStop],
+    extend: Extend,
+}
+extern "C" fn _hb_fontations_get_color_stops (
+    _color_line: *mut hb_color_line_t,
+    color_line_data: *mut ::std::os::raw::c_void,
+    start: ::std::os::raw::c_uint,
+    count_out: *mut ::std::os::raw::c_uint,
+    color_stops_out: *mut hb_color_stop_t,
+    user_data: *mut ::std::os::raw::c_void,
+) -> ::std::os::raw::c_uint
+{
+    let color_painter = unsafe { &*(color_line_data as *const HbColorPainter) };
+    let color_line_data = unsafe { &*(user_data as *const ColorLineData) };
+    let color_stops = &color_line_data.color_stops;
+    if count_out.is_null() {
+        return color_stops.len() as u32;
+    }
+    let count = unsafe { *count_out };
+    for i in 0..count {
+        let stop = color_stops.get(start as usize + i as usize);
+        if stop.is_none() {
+            unsafe { *count_out = i; };
+            return color_stops.len() as u32;
+        }
+        let stop = stop.unwrap();
+        unsafe {
+            *(color_stops_out.offset(i as isize)) = hb_color_stop_t {
+                    offset: stop.offset,
+                    color: color_painter.lookup_color(stop.palette_index, stop.alpha),
+                    is_foreground: (stop.palette_index == 0xFFFF) as hb_bool_t,
+                };
+        }
+    }
+    return color_stops.len() as u32;
+}
+extern "C" fn _hb_fontations_get_extend (
+    _color_line: *mut hb_color_line_t,
+    color_line_data: *mut ::std::os::raw::c_void,
+    _user_data: *mut ::std::os::raw::c_void,
+) -> hb_paint_extend_t
+{
+    let color_line_data = unsafe { &*(color_line_data as *const ColorLineData) };
+    color_line_data.extend as hb_paint_extend_t // They are the same
+}
+
 impl ColorPainter for HbColorPainter {
     fn push_transform(&mut self, transform: Transform) {
         unsafe {
@@ -421,6 +468,34 @@ impl ColorPainter for HbColorPainter {
                     );
                 }
             }
+            Brush::LinearGradient {
+                color_stops,
+                extend,
+                p0,
+                p1,
+            } => {
+                let color_stops = ColorLineData { color_stops: &color_stops, extend };
+                unsafe {
+                    let mut cl = std::mem::zeroed::<hb_color_line_t>();
+                    cl.data = self as *mut HbColorPainter as *mut ::std::os::raw::c_void;
+                    cl.get_color_stops = Some(_hb_fontations_get_color_stops);
+                    cl.get_color_stops_user_data = &color_stops as *const ColorLineData as *mut ::std::os::raw::c_void;
+                    cl.get_extend = Some(_hb_fontations_get_extend);
+                    cl.get_extend_user_data = &extend as *const Extend as *mut ::std::os::raw::c_void;
+
+                    hb_paint_linear_gradient(
+                        self.paint_funcs,
+                        self.paint_data,
+                        &mut cl,
+                        p0.x,
+                        p0.y,
+                        p1.x,
+                        p1.y,
+                        p1.x, // TODO: Where's p2?
+                        p1.y, // TODO: Where's p2?
+                    );
+                }
+            }
             _ => {}
         }
     }
@@ -435,7 +510,7 @@ impl ColorPainter for HbColorPainter {
         if mode.is_none() {
             return;
         }
-        let mode = mode.unwrap() as hb_paint_composite_mode_t;
+        let mode = mode.unwrap() as hb_paint_composite_mode_t; // They are the same
         unsafe {
             hb_paint_pop_group(self.paint_funcs, self.paint_data, mode);
         }
@@ -469,7 +544,7 @@ extern "C" fn _hb_fontations_paint_glyph(
     let mut color_records = Box::<[ColorRecord]>::default();
     if cpal.is_ok() {
         let cpal = cpal.unwrap();
-        // fontations doesn't seem to provide a way to access palettes
+        // TODO fontations doesn't seem to provide a way to access palettes
         // really. Just assume the first palette starts at the beginning
         // of the color records array.
         let color_records_array = cpal.color_records_array();