337: Filesystem browser thumbnail fallback: offscreen viewport has no render handler for supplied ImageBufPtr

[pleprince]

Linked issues

Fixes #337

Summarize your change.

Add the missing render_viewport_to_image_atom message handler to OffscreenViewport that renders a supplied media_reader::ImageBufPtr down to a thumbnail:

(render_viewport_to_image_atom, int width, int height, media_reader::ImageBufPtr)
    -> thumbnail::ThumbnailBufferPtr  // TF_RGB24

The handler renders the supplied full-resolution image through the viewport's colour-managed offscreen pipeline (renderToImageBuffer → RGBA_16F) and converts the result to an RGB24 ThumbnailBufferPtr via the existing rgb96thumbFromHalfFloatImage, mirroring the regular renderToThumbnail path. Invalid input (null buffer or non-positive dimensions) returns a CAF error instead of dereferencing or attempting an invalid FBO size.

File changed: src/ui/qt/viewport_widget/src/offscreen_viewport.cpp (+33 lines).

Describe the reason for the change.

When a media reader cannot produce a thumbnail buffer directly (MediaDetailAndThumbnailReaderActor::get_thumbnail_from_reader_plugin, src/media_reader/src/media_detail_and_thumbnail_reader_actor.cpp), it falls back to reading the full image and asking the offscreen viewport to render it to a thumbnail, sending:

mail(ui::viewport::render_viewport_to_image_atom_v, thumb_width, thumb_height, buf)  // buf = ImageBufPtr

OffscreenViewport had no handler for this signature, so the CAF runtime reported it as an unexpected message:

unexpected message [id: 651, name: user.scheduled-actor]: message(xstudio::ui::viewport::render_viewport_to_image_atom(), 128, 72, xstudio::media_reader::ImageBufPtr)

The failed request propagated back as an error, which the QML thumbnail provider (ThumbnailReader in src/ui/qml/helper/src/thumbnail_provider_ui.cpp) reported as "Thumbnail does not exist 2." — surfaced at the filesystem browser delegate:

qrc:/FileSystemBrowser/FileSystemBrowser/xstudio/FSThumbItem.qml:74:21: QML QQuickImage*: Thumbnail does not exist 2.

So formats that took the full-image fallback showed blank thumbnails and spammed these two messages whenever the filesystem browser loaded thumbnails.

Describe what you have tested and on which operating system.

• Windows 10 (x64), MSVC 2022, Release.
• The reported messages are gone: loading thumbnails in the filesystem browser no longer produces the unexpected message CAF warning or the Thumbnail does not exist 2. QML error for media that exercises the full-image fallback.

Add a list of changes, and note any that might need special attention during the review.

• src/ui/qt/viewport_widget/src/offscreen_viewport.cpp: new (render_viewport_to_image_atom, int, int, media_reader::ImageBufPtr) handler returning ThumbnailBufferPtr (RGB24), with null/dimension validation.
• Note for review: the new handler is intentionally signature-distinct from the existing (atom, int, int, ImageFormat) → ImageBufPtr handler — its third element is the image buffer, not a pixel format — so CAF dispatch is unambiguous. It reuses the existing renderToImageBuffer / rgb96thumbFromHalfFloatImage colour-management path; no other code was touched.

If possible, provide screenshots.

N/A — the observable change is the absence of the two console/QML error messages above.

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[pleprince]

This is not the best fix: closing.