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fix: set default pixel format for Aravis cameras when unsupported format #28086 Fixes #26523 This PR fixes an issue where Aravis VideoCapture returns empty frames when the camera's pixel format is not explicitly set via CAP_PROP_FOURCC. Problem: The Aravis backend's retrieveFrame() function only processes four specific pixel formats: - ARV_PIXEL_FORMAT_MONO_8 - ARV_PIXEL_FORMAT_BAYER_GR_8 - ARV_PIXEL_FORMAT_MONO_12 - ARV_PIXEL_FORMAT_MONO_16 If the camera has a different pixel format configured (or the camera's default format is unsupported), retrieveFrame() returns false on line 333 and all frames appear empty. This happens even though: - The camera opens successfully (isOpened() returns true) - The camera is capturing data - The user has set up autotrigger and auto-exposure correctly Root Cause: In open() at line 271, the code retrieves the camera's current pixel format with arv_camera_get_pixel_format(). But if this format doesn't match one of the four supported formats, there's no fallback, causing all subsequent frames to fail retrieval. Solution: Added a check after retrieving the camera's pixel format. If the format is not one of the four supported formats, the code now: 1. Sets pixelFormat to a sensible default (MONO_8) 2. Applies this format to the camera via arv_camera_set_pixel_format() This ensures: - Cameras work out-of-the-box without requiring explicit CAP_PROP_FOURCC setup - Users can still override the format with CAP_PROP_FOURCC if needed - Behavior matches user expectations from other camera backends (V4L2, MSMF, etc.) The default of MONO_8 was chosen because it's the most universally supported format across USB3 Vision and GigE Vision cameras. Changes: modules/videoio/src/cap_aravis.cpp: Added pixel format validation and default setting (10 lines) Testing: With this fix: - Cameras with unsupported default formats will automatically switch to MONO_8 - The sample code from the issue works without uncommenting the CAP_PROP_FOURCC line - Users can still explicitly set their preferred format via CAP_PROP_FOURCC Pull Request Readiness Checklist: - [x] I agree to contribute to the project under Apache 2 License. - [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV - [x] The PR is proposed to the proper branch (4.x) - [x] There is a reference to the original bug report and related work (issue #26523) - [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable N/A - This requires specialized USB3 Vision / GigE Vision hardware not available in CI - [x] The feature is well documented and sample code can be built with the project CMake The fix maintains existing API behavior and makes the backend work as expected
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