MagnumFontConverter: gracefully fail with glyph caches on OpenGL ES.

src/MagnumPlugins/MagnumFontConverter/MagnumFontConverter.cpp

@@ -47,6 +47,11 @@ auto MagnumFontConverter::doFeatures() const -> Features {
 }
 
 std::vector<std::pair<std::string, Containers::Array<char>>> MagnumFontConverter::doExportFontToData(AbstractFont& font, AbstractGlyphCache& cache, const std::string& filename, const std::u32string& characters) const {
+    if(!(cache.features() & GlyphCacheFeature::ImageDownload)) {
+        Error{} << "Text::MagnumFontConverter::exportFontToData(): passed glyph cache doesn't support image download";
+        return {};
+    }
+
     Utility::Configuration configuration;
 
     configuration.setValue("version", 1);

src/MagnumPlugins/MagnumFontConverter/Test/MagnumFontConverterTest.cpp

@@ -46,6 +46,7 @@ struct MagnumFontConverterTest: TestSuite::Tester {
     explicit MagnumFontConverterTest();
 
     void exportFont();
+    void exportFontNoGlyphCacheImageDownload();
 
     /* Explicitly forbid system-wide plugin dependencies */
     PluginManager::Manager<Trade::AbstractImageConverter> _imageConverterManager{"nonexistent"};
@@ -54,7 +55,8 @@ struct MagnumFontConverterTest: TestSuite::Tester {
 };
 
 MagnumFontConverterTest::MagnumFontConverterTest() {
-    addTests({&MagnumFontConverterTest::exportFont});
+    addTests({&MagnumFontConverterTest::exportFont,
+              &MagnumFontConverterTest::exportFontNoGlyphCacheImageDownload});
 
     /* Load the plugins directly from the build tree. Otherwise they are static
        and already loaded. */
@@ -147,6 +149,35 @@ void MagnumFontConverterTest::exportFont() {
     CORRADE_COMPARE(image->format(), PixelFormat::R8Unorm);
 }
 
+void MagnumFontConverterTest::exportFontNoGlyphCacheImageDownload() {
+    struct MyFont: AbstractFont {
+        /* Supports neither file nor data opening */
+        Features doFeatures() const override { return {}; }
+        bool doIsOpened() const override { return false; }
+        void doClose() override {}
+
+        UnsignedInt doGlyphId(char32_t) override { return {}; }
+        Vector2 doGlyphAdvance(UnsignedInt) override { return {}; }
+        Containers::Pointer<AbstractLayouter> doLayout(const AbstractGlyphCache&, Float, const std::string&) override {
+            return nullptr;
+        }
+    } font;
+
+    struct DummyGlyphCache: AbstractGlyphCache {
+        using AbstractGlyphCache::AbstractGlyphCache;
+
+        GlyphCacheFeatures doFeatures() const override { return {}; }
+        void doSetImage(const Vector2i&, const ImageView2D&) override {}
+    } cache{{100, 100}};
+
+    Containers::Pointer<AbstractFontConverter> converter = _fontConverterManager.instantiate("MagnumFontConverter");
+
+    std::ostringstream out;
+    Error redirectError{&out};
+    CORRADE_VERIFY(!converter->exportFontToFile(font, cache, Utility::Directory::join(MAGNUMFONTCONVERTER_TEST_WRITE_DIR, "font"), "Wave"));
+    CORRADE_COMPARE(out.str(), "Text::MagnumFontConverter::exportFontToData(): passed glyph cache doesn't support image download\n");
+}
+
 }}}}
 
 CORRADE_TEST_MAIN(Magnum::Text::Test::MagnumFontConverterTest)