TextureTools: assert that DistanceField output is framebuffer-drawable.

If it's not, it's a programmer error (i.e., don't use Luminance or
packed formats, won't work), and since there's no way for the API to
report a failure in a programmatic way, this was causing hard-to-track
errors.

doc/changelog.dox

@@ -1654,6 +1654,9 @@ See also:
     -   The @ref Text::AbstractGlyphCache::image() query now returns a
         @ref MutableImageView3D instead of an @ref Image2D in order to support
         incremental population and texture arrays
+-   @ref TextureTools::DistanceField now asserts that the output texture has a
+    framebuffer-drawable format. Before it only printed an error message and
+    silently did nothing, causing a hard-to-track error.
 -   The @ref Trade::AnimationTrackTarget enum was extended from 8 to 16 bits to
     provide more room for custom targets, consistently with
     @ref Trade::MeshAttribute.

src/Magnum/TextureTools/DistanceField.cpp

@@ -205,12 +205,8 @@ void DistanceField::operator()(GL::Texture2D& input, GL::Framebuffer& output, co
     Vector2i imageSize = input.imageSize(0);
     #endif
 
-    const GL::Framebuffer::Status status = output.checkStatus(GL::FramebufferTarget::Draw);
-    if(status != GL::Framebuffer::Status::Complete) {
-        Error() << "TextureTools::DistanceField: cannot render to given output texture, unexpected framebuffer status"
-                << status;
-        return;
-    }
+    CORRADE_ASSERT(output.checkStatus(GL::FramebufferTarget::Draw) == GL::Framebuffer::Status::Complete,
+        "TextureTools::DistanceField: output texture format not framebuffer-drawable:" << output.checkStatus(GL::FramebufferTarget::Draw), );
 
     output
         .clear(GL::FramebufferClear::Color)

src/Magnum/TextureTools/DistanceField.h

@@ -81,15 +81,6 @@ http://www.valvesoftware.com/publications/2007/SIGGRAPH2007_AlphaTestedMagnifica
 @attention This is a GPU-only implementation, so it expects an active GL
     context.
 
-@note If internal format of @p output texture is not renderable, this function
-    prints a message to error output and does nothing. On desktop OpenGL and
-    OpenGL ES 3.0 it's common to render to @ref GL::TextureFormat::R8. On
-    OpenGL ES 2.0 you can use @ref GL::TextureFormat::Red if
-    @gl_extension{EXT,texture_rg} is available; if not, the smallest but still
-    inefficient supported format is in most cases @ref GL::TextureFormat::RGB,
-    rendering to @ref GL::TextureFormat::Luminance is not supported in most
-    cases.
-
 @note This function is available only if Magnum is compiled with
     @ref MAGNUM_TARGET_GL enabled (done by default). See @ref building-features
     for more information.
@@ -136,6 +127,15 @@ class MAGNUM_TEXTURETOOLS_EXPORT DistanceField {
          *      on desktop GL the information is gathered automatically using
          *      @ref GL::Texture2D::imageSize().
          * @m_since_latest
+         *
+         * The @p output texture is expected to have a framebuffer-drawable
+         * @ref GL::TextureFormat. On desktop OpenGL and
+         * OpenGL ES 3.0 it's common to render to @ref GL::TextureFormat::R8.
+         * On OpenGL ES 2.0 you can use @ref GL::TextureFormat::Red if
+         * @gl_extension{EXT,texture_rg} is available; if not, the smallest but
+         * still inefficient supported format is in most cases
+         * @ref GL::TextureFormat::RGB. The @ref GL::TextureFormat::Luminance
+         * format usually isn't renderable.
          */
         void operator()(GL::Texture2D& input, GL::Framebuffer& output, const Range2Di& rectangle, const Vector2i& imageSize
             #ifndef MAGNUM_TARGET_GLES

src/Magnum/TextureTools/Test/CMakeLists.txt

@@ -99,7 +99,12 @@ if(MAGNUM_BUILD_GL_TESTS)
         list(APPEND TextureToolsDistanceFieldGLTest_SRCS DistanceFieldGLTestFiles)
     endif()
     corrade_add_test(TextureToolsDistanceFieldGLTest ${TextureToolsDistanceFieldGLTest_SRCS}
-        LIBRARIES MagnumTextureTools MagnumGL MagnumTrade MagnumDebugTools MagnumOpenGLTester
+        LIBRARIES
+            MagnumDebugTools
+            MagnumGL
+            MagnumOpenGLTester
+            MagnumTextureToolsTestLib
+            MagnumTrade
         FILES
             DistanceFieldGLTestFiles/input.tga
             DistanceFieldGLTestFiles/output.tga)

src/Magnum/TextureTools/Test/DistanceFieldGLTest.cpp

@@ -67,6 +67,9 @@ struct DistanceFieldGLTest: GL::OpenGLTester {
     /* This tests the GL::Texture overload, which itself calls into the
        GL::Framebuffer overload so both are covered */
     void run();
+
+    void formatNotDrawable();
+
     #ifndef MAGNUM_TARGET_WEBGL
     void benchmark();
     #endif
@@ -93,6 +96,8 @@ DistanceFieldGLTest::DistanceFieldGLTest() {
     addInstancedTests({&DistanceFieldGLTest::run},
         Containers::arraySize(RunData));
 
+    addTests({&DistanceFieldGLTest::formatNotDrawable});
+
     #ifndef MAGNUM_TARGET_WEBGL
     addBenchmarks({&DistanceFieldGLTest::benchmark}, 5, BenchmarkType::GpuTime);
     #endif
@@ -255,6 +260,42 @@ void DistanceFieldGLTest::run() {
         (DebugTools::CompareImageToFile{_manager, 1.0f, 0.178f}));
 }
 
+void DistanceFieldGLTest::formatNotDrawable() {
+    CORRADE_SKIP_IF_NO_ASSERT();
+
+    #ifndef MAGNUM_TARGET_GLES
+    if(!GL::Context::current().isExtensionSupported<GL::Extensions::EXT::texture_shared_exponent>())
+        CORRADE_SKIP(GL::Extensions::EXT::texture_shared_exponent::string() << "not supported, can't test");
+    #endif
+
+    GL::Texture2D input;
+    input.setMinificationFilter(GL::SamplerFilter::Nearest, GL::SamplerMipmap::Base)
+        .setMagnificationFilter(GL::SamplerFilter::Nearest)
+        .setStorage(1, GL::textureFormat(PixelFormat::R8Unorm), {64, 64});
+
+    GL::Texture2D output;
+    #ifdef MAGNUM_TARGET_GLES2
+    output.setImage(0, GL::TextureFormat::Luminance, ImageView2D{GL::PixelFormat::Luminance, GL::PixelType::UnsignedByte, Vector2i{4}});
+    #else
+    output.setImage(0, GL::TextureFormat::RGB9E5, ImageView2D{GL::PixelFormat::RGB, GL::PixelType::UnsignedInt5999Rev, Vector2i{4}});
+    #endif
+
+    DistanceField distanceField{4};
+
+    std::ostringstream out;
+    Error redirectError{&out};
+    distanceField(input, output, {{}, Vector2i{4}}
+        #ifdef MAGNUM_TARGET_GLES
+        , Vector2i{64}
+        #endif
+        );
+    #ifndef MAGNUM_TARGET_GLES
+    CORRADE_COMPARE(out.str(), "TextureTools::DistanceField: output texture format not framebuffer-drawable: GL::Framebuffer::Status::Unsupported\n");
+    #else
+    CORRADE_COMPARE(out.str(), "TextureTools::DistanceField: output texture format not framebuffer-drawable: GL::Framebuffer::Status::IncompleteAttachment\n");
+    #endif
+}
+
 #ifndef MAGNUM_TARGET_WEBGL
 void DistanceFieldGLTest::benchmark() {
     #ifdef MAGNUM_TARGET_GLES