Improve rendering of italic fonts in linux

Now rather than down-size characters from italic fonts that do not fit
into the cell width, trim the left and right margins to make them fit,
instead.

kitty/fonts/freetype.py

@@ -100,27 +100,27 @@ def render_char(text, bold=False, italic=False, width=1):
                 face = alt_face_cache[font] = Face(font.face)
                 set_char_size(face, **cff_size)
         bitmap = render_to_bitmap(font, face, text)
-        if width == 1 and bitmap.width > cell_width * 1.1:
-            # rescale the font size so that the glyph is visible in a single
-            # cell and hope somebody updates libc's wcwidth
-            sz = cff_size.copy()
-            sz['width'] = int(sz['width'] * cell_width / bitmap.width)
-            # Preserve aspect ratio
-            sz['height'] = int(sz['height'] * cell_width / bitmap.width)
-            try:
-                set_char_size(face, **sz)
-                bitmap = render_to_bitmap(font, face, text)
-            finally:
-                set_char_size(face, **cff_size)
+        if width == 1 and bitmap.width > cell_width:
+            extra = bitmap.width - cell_width
+            if italic and extra < cell_width // 2:
+                bitmap = face.trim_to_width(bitmap, cell_width)
+            elif extra > max(2, 0.1 * cell_width):
+                # rescale the font size so that the glyph is visible in a single
+                # cell and hope somebody updates libc's wcwidth
+                sz = cff_size.copy()
+                sz['width'] = int(sz['width'] * cell_width / bitmap.width)
+                # Preserve aspect ratio
+                sz['height'] = int(sz['height'] * cell_width / bitmap.width)
+                try:
+                    set_char_size(face, **sz)
+                    bitmap = render_to_bitmap(font, face, text)
+                finally:
+                    set_char_size(face, **cff_size)
         m = face.glyph_metrics()
         return CharBitmap(bitmap.buffer, ceil_int(abs(m.horiBearingX) / 64),
                           ceil_int(abs(m.horiBearingY) / 64), ceil_int(m.horiAdvance / 64), bitmap.rows, bitmap.width)
 
 
-def is_wide_char(bitmap_char):
-    return min(bitmap_char.advance, bitmap_char.columns) > cell_width * 1.1
-
-
 def place_char_in_cell(bitmap_char):
     # We want the glyph to be positioned inside the cell based on the bearingX
     # and bearingY values, making sure that it does not overflow the cell.

kitty/freetype.c

@@ -184,6 +184,46 @@ bitmap(Face *self) {
     return ans;
 }
  
+static PyObject*
+trim_to_width(Face UNUSED *self, PyObject *args) {
+#define trim_to_width_doc "Trim edges from the supplied bitmap to make it fit in the specified cell-width"
+    PyObject *bitmap, *t;
+    unsigned long cell_width, rows, width, rtrim = 0, extra, ltrim;
+    unsigned char *src, *dest;
+    bool column_has_text = false;
+    if (!PyArg_ParseTuple(args, "O!k", &BitmapType, &bitmap, &cell_width)) return NULL;
+    rows = PyLong_AsUnsignedLong(PyStructSequence_GET_ITEM(bitmap, 0));
+    width = PyLong_AsUnsignedLong(PyStructSequence_GET_ITEM(bitmap, 1));
+    extra = width - cell_width;
+    if (extra >= cell_width) { PyErr_SetString(PyExc_ValueError, "Too large for trimming"); return NULL; }
+    PyObject *ans = PyStructSequence_New(&BitmapType);
+    if (ans == NULL) return PyErr_NoMemory();
+    src = (unsigned char*)PyByteArray_AS_STRING(PyStructSequence_GET_ITEM(bitmap, 3));
+    PyObject *abuf = PyByteArray_FromStringAndSize(NULL, cell_width * rows);
+    if (abuf == NULL) { Py_CLEAR(ans); return PyErr_NoMemory(); }
+    dest = (unsigned char*)PyByteArray_AS_STRING(abuf);
+    PyStructSequence_SET_ITEM(ans, 1, PyLong_FromUnsignedLong(cell_width));
+    PyStructSequence_SET_ITEM(ans, 2, PyLong_FromUnsignedLong(cell_width));
+    PyStructSequence_SET_ITEM(ans, 3, abuf);
+#define COPY(which) t = PyStructSequence_GET_ITEM(bitmap, which); Py_INCREF(t); PyStructSequence_SET_ITEM(ans, which, t);
+    COPY(0); COPY(4); COPY(5); COPY(6);
+#undef COPY
+
+    for (long x = width - 1; !column_has_text && x > -1 && rtrim < extra; x--) {
+        for (unsigned long y = 0; y < rows * width; y += width) {
+            if (src[x + y] > 200) { column_has_text = true; break; }
+        }
+        if (!column_has_text) rtrim++;
+    }
+    rtrim = MIN(extra, rtrim);
+    ltrim = extra - rtrim;
+    for (unsigned long y = 0; y < rows; y++) {
+        memcpy(dest + y*cell_width, src + ltrim + y*width, cell_width);
+    }
+
+    return ans;
+}
+
 // Boilerplate {{{
 
 static PyMemberDef members[] = {
@@ -205,6 +245,7 @@ static PyMethodDef methods[] = {
     METHOD(get_char_index, METH_VARARGS)
     METHOD(glyph_metrics, METH_NOARGS)
     METHOD(bitmap, METH_NOARGS)
+    METHOD(trim_to_width, METH_VARARGS)
     {NULL}  /* Sentinel */
 };
 
@@ -234,6 +275,7 @@ init_freetype_library(PyObject *m) {
     if (PyStructSequence_InitType2(&GlpyhMetricsType, &gm_desc) != 0) return false;
     if (PyStructSequence_InitType2(&BitmapType, &bm_desc) != 0) return false;
     PyModule_AddObject(m, "GlyphMetrics", (PyObject*)&GlpyhMetricsType);
+    PyModule_AddObject(m, "Bitmap", (PyObject*)&BitmapType);
     PyModule_AddIntMacro(m, FT_LOAD_RENDER);
     PyModule_AddIntMacro(m, FT_LOAD_TARGET_NORMAL);
     PyModule_AddIntMacro(m, FT_LOAD_TARGET_LIGHT);