Support loading images with weird urls and improve error message (#5431)

* Closes #5341
* [x] I have followed the instructions in the PR template

crates/egui/src/context.rs

@@ -3455,15 +3455,23 @@ impl Context {
             return Err(load::LoadError::NoImageLoaders);
         }
 
+        let mut format = None;
+
         // Try most recently added loaders first (hence `.rev()`)
         for loader in image_loaders.iter().rev() {
             match loader.load(self, uri, size_hint) {
                 Err(load::LoadError::NotSupported) => continue,
+                Err(load::LoadError::FormatNotSupported { detected_format }) => {
+                    format = format.or(detected_format);
+                    continue;
+                }
                 result => return result,
             }
         }
 
-        Err(load::LoadError::NoMatchingImageLoader)
+        Err(load::LoadError::NoMatchingImageLoader {
+            detected_format: format,
+        })
     }
 
     /// Try loading the texture from the given uri using any available texture loaders.

crates/egui/src/load.rs

@@ -77,16 +77,19 @@ pub enum LoadError {
     /// Programmer error: There are no image loaders installed.
     NoImageLoaders,
 
-    /// A specific loader does not support this scheme, protocol or image format.
+    /// A specific loader does not support this scheme or protocol.
     NotSupported,
 
+    /// A specific loader does not support the format of the image.
+    FormatNotSupported { detected_format: Option<String> },
+
     /// Programmer error: Failed to find the bytes for this image because
     /// there was no [`BytesLoader`] supporting the scheme.
     NoMatchingBytesLoader,
 
     /// Programmer error: Failed to parse the bytes as an image because
-    /// there was no [`ImageLoader`] supporting the scheme.
-    NoMatchingImageLoader,
+    /// there was no [`ImageLoader`] supporting the format.
+    NoMatchingImageLoader { detected_format: Option<String> },
 
     /// Programmer error: no matching [`TextureLoader`].
     /// Because of the [`DefaultTextureLoader`], this error should never happen.
@@ -96,6 +99,20 @@ pub enum LoadError {
     Loading(String),
 }
 
+impl LoadError {
+    /// Returns the (approximate) size of the error message in bytes.
+    pub fn byte_size(&self) -> usize {
+        match self {
+            Self::FormatNotSupported { detected_format }
+            | Self::NoMatchingImageLoader { detected_format } => {
+                detected_format.as_ref().map_or(0, |s| s.len())
+            }
+            Self::Loading(message) => message.len(),
+            _ => std::mem::size_of::<Self>(),
+        }
+    }
+}
+
 impl Display for LoadError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
@@ -105,12 +122,15 @@ impl Display for LoadError {
 
             Self::NoMatchingBytesLoader => f.write_str("No matching BytesLoader. Either you need to call Context::include_bytes, or install some more bytes loaders, e.g. using egui_extras."),
 
-            Self::NoMatchingImageLoader => f.write_str("No matching ImageLoader. Either you need to call Context::include_bytes, or install some more bytes loaders, e.g. using egui_extras."),
+            Self::NoMatchingImageLoader { detected_format: None } => f.write_str("No matching ImageLoader. Either no ImageLoader is installed or the image is corrupted / has an unsupported format."),
+            Self::NoMatchingImageLoader { detected_format: Some(detected_format) } => write!(f, "No matching ImageLoader for format: {detected_format:?}. Make sure you enabled the necessary features on the image crate."),
 
             Self::NoMatchingTextureLoader => f.write_str("No matching TextureLoader. Did you remove the default one?"),
 
             Self::NotSupported => f.write_str("Image scheme or URI not supported by this loader"),
 
+            Self::FormatNotSupported { detected_format } => write!(f, "Image format not supported by this loader: {detected_format:?}"),
+
             Self::Loading(message) => f.write_str(message),
         }
     }

crates/egui_extras/src/image.rs

@@ -28,7 +28,7 @@ pub struct RetainedImage {
 }
 
 impl RetainedImage {
-    pub fn from_color_image(debug_name: impl Into<String>, image: ColorImage) -> Self {
+    pub fn from_color_image(debug_name: impl Into<String>, image: egui::ColorImage) -> Self {
         Self {
             debug_name: debug_name.into(),
             size: image.size,
@@ -54,7 +54,7 @@ impl RetainedImage {
     ) -> Result<Self, String> {
         Ok(Self::from_color_image(
             debug_name,
-            load_image_bytes(image_bytes)?,
+            load_image_bytes(image_bytes).map_err(|err| err.to_string())?,
         ))
     }
 
@@ -154,7 +154,7 @@ impl RetainedImage {
         self.texture
             .lock()
             .get_or_insert_with(|| {
-                let image: &mut ColorImage = &mut self.image.lock();
+                let image: &mut egui::ColorImage = &mut self.image.lock();
                 let image = std::mem::take(image);
                 ctx.load_texture(&self.debug_name, image, self.options)
             })
@@ -190,8 +190,6 @@ impl RetainedImage {
 
 // ----------------------------------------------------------------------------
 
-use egui::ColorImage;
-
 /// Load a (non-svg) image.
 ///
 /// Requires the "image" feature. You must also opt-in to the image formats you need
@@ -200,9 +198,19 @@ use egui::ColorImage;
 /// # Errors
 /// On invalid image or unsupported image format.
 #[cfg(feature = "image")]
-pub fn load_image_bytes(image_bytes: &[u8]) -> Result<egui::ColorImage, String> {
+pub fn load_image_bytes(image_bytes: &[u8]) -> Result<egui::ColorImage, egui::load::LoadError> {
     crate::profile_function!();
-    let image = image::load_from_memory(image_bytes).map_err(|err| err.to_string())?;
+    let image = image::load_from_memory(image_bytes).map_err(|err| match err {
+        image::ImageError::Unsupported(err) => match err.kind() {
+            image::error::UnsupportedErrorKind::Format(format) => {
+                egui::load::LoadError::FormatNotSupported {
+                    detected_format: Some(format.to_string()),
+                }
+            }
+            _ => egui::load::LoadError::Loading(err.to_string()),
+        },
+        err => egui::load::LoadError::Loading(err.to_string()),
+    })?;
     let size = [image.width() as _, image.height() as _];
     let image_buffer = image.to_rgba8();
     let pixels = image_buffer.as_flat_samples();

crates/egui_extras/src/loaders/image_loader.rs

@@ -7,7 +7,7 @@ use egui::{
 use image::ImageFormat;
 use std::{mem::size_of, path::Path, sync::Arc};
 
-type Entry = Result<Arc<ColorImage>, String>;
+type Entry = Result<Arc<ColorImage>, LoadError>;
 
 #[derive(Default)]
 pub struct ImageCrateLoader {
@@ -31,9 +31,14 @@ fn is_supported_uri(uri: &str) -> bool {
         .any(|format_ext| ext == *format_ext)
 }
 
-fn is_unsupported_mime(mime: &str) -> bool {
+fn is_supported_mime(mime: &str) -> bool {
+    // This is the default mime type for binary files, so this might actually be a valid image,
+    // let's relay on image's format guessing
+    if mime == "application/octet-stream" {
+        return true;
+    }
     // Uses only the enabled image crate features
-    !ImageFormat::all()
+    ImageFormat::all()
         .filter(ImageFormat::reading_enabled)
         .map(|fmt| fmt.to_mime_type())
         .any(|format_mime| mime == format_mime)
@@ -46,39 +51,39 @@ impl ImageLoader for ImageCrateLoader {
 
     fn load(&self, ctx: &egui::Context, uri: &str, _: SizeHint) -> ImageLoadResult {
         // three stages of guessing if we support loading the image:
-        // 1. URI extension
+        // 1. URI extension (only done for files)
         // 2. Mime from `BytesPoll::Ready`
-        // 3. image::guess_format
+        // 3. image::guess_format (used internally by image::load_from_memory)
 
         // (1)
-        if !is_supported_uri(uri) {
+        if uri.starts_with("file://") && !is_supported_uri(uri) {
             return Err(LoadError::NotSupported);
         }
 
         let mut cache = self.cache.lock();
         if let Some(entry) = cache.get(uri).cloned() {
             match entry {
                 Ok(image) => Ok(ImagePoll::Ready { image }),
-                Err(err) => Err(LoadError::Loading(err)),
+                Err(err) => Err(err),
             }
         } else {
             match ctx.try_load_bytes(uri) {
                 Ok(BytesPoll::Ready { bytes, mime, .. }) => {
-                    // (2 and 3)
-                    if mime.as_deref().is_some_and(is_unsupported_mime)
-                        || image::guess_format(&bytes).is_err()
-                    {
-                        return Err(LoadError::NotSupported);
+                    // (2)
+                    if let Some(mime) = mime {
+                        if !is_supported_mime(&mime) {
+                            return Err(LoadError::FormatNotSupported {
+                                detected_format: Some(mime),
+                            });
+                        }
                     }
 
+                    // (3)
                     log::trace!("started loading {uri:?}");
                     let result = crate::image::load_image_bytes(&bytes).map(Arc::new);
                     log::trace!("finished loading {uri:?}");
                     cache.insert(uri.into(), result.clone());
-                    match result {
-                        Ok(image) => Ok(ImagePoll::Ready { image }),
-                        Err(err) => Err(LoadError::Loading(err)),
-                    }
+                    result.map(|image| ImagePoll::Ready { image })
                 }
                 Ok(BytesPoll::Pending { size }) => Ok(ImagePoll::Pending { size }),
                 Err(err) => Err(err),
@@ -100,7 +105,7 @@ impl ImageLoader for ImageCrateLoader {
             .values()
             .map(|result| match result {
                 Ok(image) => image.pixels.len() * size_of::<egui::Color32>(),
-                Err(err) => err.len(),
+                Err(err) => err.byte_size(),
             })
             .sum()
     }