diff --git a/include/plutovg.h b/include/plutovg.h
index f95eecc..e79d33a 100644
--- a/include/plutovg.h
+++ b/include/plutovg.h
@@ -847,7 +847,7 @@ PLUTOVG_API void plutovg_font_face_get_metrics(const plutovg_font_face_t* face,
  * @param left_side_bearing Pointer to store the left side bearing of the glyph.
  * @param extents Pointer to a `plutovg_rect_t` object to store the glyph bounding box.
  */
-PLUTOVG_API void plutovg_font_face_get_glyph_metrics(const plutovg_font_face_t* face, float size, plutovg_codepoint_t codepoint, float* advance_width, float* left_side_bearing, plutovg_rect_t* extents);
+PLUTOVG_API void plutovg_font_face_get_glyph_metrics(plutovg_font_face_t* face, float size, plutovg_codepoint_t codepoint, float* advance_width, float* left_side_bearing, plutovg_rect_t* extents);
 
 /**
  * @brief Retrieves the path of a glyph and its advance width.
@@ -860,7 +860,7 @@ PLUTOVG_API void plutovg_font_face_get_glyph_metrics(const plutovg_font_face_t*
  * @param path Pointer to a `plutovg_path_t` object to store the glyph path.
  * @return The advance width of the glyph.
  */
-PLUTOVG_API float plutovg_font_face_get_glyph_path(const plutovg_font_face_t* face, float size, float x, float y, plutovg_codepoint_t codepoint, plutovg_path_t* path);
+PLUTOVG_API float plutovg_font_face_get_glyph_path(plutovg_font_face_t* face, float size, float x, float y, plutovg_codepoint_t codepoint, plutovg_path_t* path);
 
 /**
  * @brief Traverses the path of a glyph and calls a callback for each path element.
@@ -874,7 +874,7 @@ PLUTOVG_API float plutovg_font_face_get_glyph_path(const plutovg_font_face_t* fa
  * @param closure User-defined data passed to the callback function.
  * @return The advance width of the glyph.
  */
-PLUTOVG_API float plutovg_font_face_traverse_glyph_path(const plutovg_font_face_t* face, float size, float x, float y, plutovg_codepoint_t codepoint, plutovg_path_traverse_func_t traverse_func, void* closure);
+PLUTOVG_API float plutovg_font_face_traverse_glyph_path(plutovg_font_face_t* face, float size, float x, float y, plutovg_codepoint_t codepoint, plutovg_path_traverse_func_t traverse_func, void* closure);
 
 /**
  * @brief Computes the bounding box of a text string and its advance width.
@@ -887,7 +887,7 @@ PLUTOVG_API float plutovg_font_face_traverse_glyph_path(const plutovg_font_face_
  * @param extents Pointer to a `plutovg_rect_t` object to store the bounding box of the text.
  * @return The total advance width of the text.
  */
-PLUTOVG_API float plutovg_font_face_text_extents(const plutovg_font_face_t* face, float size, const void* text, int length, plutovg_text_encoding_t encoding, plutovg_rect_t* extents);
+PLUTOVG_API float plutovg_font_face_text_extents(plutovg_font_face_t* face, float size, const void* text, int length, plutovg_text_encoding_t encoding, plutovg_rect_t* extents);
 
 /**
  * @brief Represents a color with red, green, blue, and alpha components.
@@ -2169,7 +2169,7 @@ PLUTOVG_API float plutovg_canvas_clip_text(plutovg_canvas_t* canvas, const void*
  * @param line_gap The line gap of the font.
  * @param extents The bounding box of the font.
  */
-PLUTOVG_API void plutovg_canvas_font_metrics(plutovg_canvas_t* canvas, float* ascent, float* descent, float* line_gap, plutovg_rect_t* extents);
+PLUTOVG_API void plutovg_canvas_font_metrics(const plutovg_canvas_t* canvas, float* ascent, float* descent, float* line_gap, plutovg_rect_t* extents);
 
 /**
  * @brief Retrieves metrics for a specific glyph.
diff --git a/source/plutovg-canvas.c b/source/plutovg-canvas.c
index a82cd46..6c2a698 100644
--- a/source/plutovg-canvas.c
+++ b/source/plutovg-canvas.c
@@ -666,7 +666,7 @@ float plutovg_canvas_clip_text(plutovg_canvas_t* canvas, const void* text, int l
     return advance_width;
 }
 
-void plutovg_canvas_font_metrics(plutovg_canvas_t* canvas, float* ascent, float* descent, float* line_gap, plutovg_rect_t* extents)
+void plutovg_canvas_font_metrics(const plutovg_canvas_t* canvas, float* ascent, float* descent, float* line_gap, plutovg_rect_t* extents)
 {
     plutovg_state_t* state = canvas->state;
     if(state->font_face && state->font_size > 0.f) {
diff --git a/source/plutovg-font.c b/source/plutovg-font.c
index 17a363f..5d0f667 100644
--- a/source/plutovg-font.c
+++ b/source/plutovg-font.c
@@ -251,31 +251,31 @@ void plutovg_font_face_get_metrics(const plutovg_font_face_t* face, float size,
     }
 }
 
-static glyph_t* get_glyph(const plutovg_font_face_t* face, plutovg_codepoint_t codepoint)
+static glyph_t* plutovg_font_face_get_glyph(plutovg_font_face_t* face, plutovg_codepoint_t codepoint)
 {
     unsigned int msb = (codepoint >> 8) & 0xFF;
     if(face->glyphs[msb] == NULL) {
-        ((plutovg_font_face_t*)face)->glyphs[msb] = calloc(GLYPH_CACHE_SIZE, sizeof(glyph_t*));
+        face->glyphs[msb] = calloc(GLYPH_CACHE_SIZE, sizeof(glyph_t*));
     }
 
     unsigned int lsb = codepoint & 0xFF;
-    if(face->glyphs[msb][lsb]) {
-        return face->glyphs[msb][lsb];
+    if(face->glyphs[msb][lsb] == NULL) {
+        glyph_t* glyph = malloc(sizeof(glyph_t));
+        glyph->index = stbtt_FindGlyphIndex(&face->info, codepoint);
+        glyph->nvertices = stbtt_GetGlyphShape(&face->info, glyph->index, &glyph->vertices);
+        stbtt_GetGlyphHMetrics(&face->info, glyph->index, &glyph->advance_width, &glyph->left_side_bearing);
+        if(!stbtt_GetGlyphBox(&face->info, glyph->index, &glyph->x1, &glyph->y1, &glyph->x2, &glyph->y2))
+            glyph->x1 = glyph->y1 = glyph->x2 = glyph->y2 = 0;
+        face->glyphs[msb][lsb] = glyph;
     }
 
-    glyph_t* glyph = malloc(sizeof(glyph_t));
-    glyph->index = stbtt_FindGlyphIndex(&face->info, codepoint);
-    glyph->nvertices = stbtt_GetGlyphShape(&face->info, glyph->index, &glyph->vertices);
-    stbtt_GetGlyphHMetrics(&face->info, glyph->index, &glyph->advance_width, &glyph->left_side_bearing);
-    if(!stbtt_GetGlyphBox(&face->info, glyph->index, &glyph->x1, &glyph->y1, &glyph->x2, &glyph->y2))
-        glyph->x1 = glyph->y1 = glyph->x2 = glyph->y2 = 0;
-    return (face->glyphs[msb][lsb] = glyph);
+    return face->glyphs[msb][lsb];
 }
 
-void plutovg_font_face_get_glyph_metrics(const plutovg_font_face_t* face, float size, plutovg_codepoint_t codepoint, float* advance_width, float* left_side_bearing, plutovg_rect_t* extents)
+void plutovg_font_face_get_glyph_metrics(plutovg_font_face_t* face, float size, plutovg_codepoint_t codepoint, float* advance_width, float* left_side_bearing, plutovg_rect_t* extents)
 {
     float scale = plutovg_font_face_get_scale(face, size);
-    glyph_t* glyph = get_glyph(face, codepoint);
+    glyph_t* glyph = plutovg_font_face_get_glyph(face, codepoint);
     if(advance_width) *advance_width = glyph->advance_width * scale;
     if(left_side_bearing) *left_side_bearing = glyph->left_side_bearing * scale;
     if(extents) {
@@ -304,12 +304,12 @@ static void glyph_traverse_func(void* closure, plutovg_path_command_t command, c
     }
 }
 
-float plutovg_font_face_get_glyph_path(const plutovg_font_face_t* face, float size, float x, float y, plutovg_codepoint_t codepoint, plutovg_path_t* path)
+float plutovg_font_face_get_glyph_path(plutovg_font_face_t* face, float size, float x, float y, plutovg_codepoint_t codepoint, plutovg_path_t* path)
 {
     return plutovg_font_face_traverse_glyph_path(face, size, x, y, codepoint, glyph_traverse_func, path);
 }
 
-float plutovg_font_face_traverse_glyph_path(const plutovg_font_face_t* face, float size, float x, float y, plutovg_codepoint_t codepoint, plutovg_path_traverse_func_t traverse_func, void* closure)
+float plutovg_font_face_traverse_glyph_path(plutovg_font_face_t* face, float size, float x, float y, plutovg_codepoint_t codepoint, plutovg_path_traverse_func_t traverse_func, void* closure)
 {
     float scale = plutovg_font_face_get_scale(face, size);
     plutovg_matrix_t matrix;
@@ -318,7 +318,7 @@ float plutovg_font_face_traverse_glyph_path(const plutovg_font_face_t* face, flo
 
     plutovg_point_t points[3];
     plutovg_point_t current_point = {0, 0};
-    glyph_t* glyph = get_glyph(face, codepoint);
+    glyph_t* glyph = plutovg_font_face_get_glyph(face, codepoint);
     for(int i = 0; i < glyph->nvertices; i++) {
         switch(glyph->vertices[i].type) {
         case STBTT_vmove:
@@ -365,7 +365,7 @@ float plutovg_font_face_traverse_glyph_path(const plutovg_font_face_t* face, flo
     return glyph->advance_width * scale;
 }
 
-float plutovg_font_face_text_extents(const plutovg_font_face_t* face, float size, const void* text, int length, plutovg_text_encoding_t encoding, plutovg_rect_t* extents)
+float plutovg_font_face_text_extents(plutovg_font_face_t* face, float size, const void* text, int length, plutovg_text_encoding_t encoding, plutovg_rect_t* extents)
 {
     plutovg_text_iterator_t it;
     plutovg_text_iterator_init(&it, text, length, encoding);