Merge pull request #28086 from Ghazi-raad:fix/aravis-default-pixel-format-26523

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

modules/videoio/src/cap_aravis.cpp

@@ -127,6 +127,8 @@ protected:
 
     void autoExposureControl(const Mat &);
 
+    double getExpectedMidGrey(ArvPixelFormat fmt) const;
+
     ArvCamera       *camera;                // Camera to control.
     ArvStream       *stream;                // Object for video stream reception.
     void            *framebuffer;           //
@@ -269,6 +271,19 @@ bool CvCaptureCAM_Aravis::open( int index )
 
         // get initial values
         pixelFormat = arv_camera_get_pixel_format(camera, NULL);
+
+        // If camera's pixel format is not one of the supported formats, set a default
+        if (pixelFormat != ARV_PIXEL_FORMAT_MONO_8 &&
+            pixelFormat != ARV_PIXEL_FORMAT_BAYER_GR_8 &&
+            pixelFormat != ARV_PIXEL_FORMAT_MONO_12 &&
+            pixelFormat != ARV_PIXEL_FORMAT_MONO_16) {
+            pixelFormat = ARV_PIXEL_FORMAT_MONO_8;
+            arv_camera_set_pixel_format(camera, pixelFormat, NULL);
+            CV_LOG_WARNING(NULL, "Current camera pixel format is not supported. Failed back to MONO_8.");
+        }
+
+        midGrey = getExpectedMidGrey(pixelFormat);
+
         exposure = exposureAvailable ? arv_camera_get_exposure_time(camera, NULL) : 0;
         gain = gainAvailable ? arv_camera_get_gain(camera, NULL) : 0;
         fps = arv_camera_get_frame_rate(camera, NULL);
@@ -489,6 +504,26 @@ double CvCaptureCAM_Aravis::getProperty( int property_id ) const
     return -1.0;
 }
 
+double CvCaptureCAM_Aravis::getExpectedMidGrey(ArvPixelFormat fmt) const
+{
+    double grey = 0.;
+    switch(fmt)
+    {
+        case ARV_PIXEL_FORMAT_MONO_8:
+        case ARV_PIXEL_FORMAT_BAYER_GR_8:
+            grey = 128.;
+            break;
+        case ARV_PIXEL_FORMAT_MONO_12:
+            grey = 2048.;
+            break;
+        case ARV_PIXEL_FORMAT_MONO_16:
+            grey = 32768.;
+            break;
+    }
+
+    return grey;
+}
+
 bool CvCaptureCAM_Aravis::setProperty( int property_id, double value )
 {
     switch(property_id) {
@@ -535,24 +570,22 @@ bool CvCaptureCAM_Aravis::setProperty( int property_id, double value )
                     case MODE_GREY:
                     case MODE_Y800:
                         newFormat = ARV_PIXEL_FORMAT_MONO_8;
-                        targetGrey = 128;
                         break;
                     case MODE_Y12:
                         newFormat = ARV_PIXEL_FORMAT_MONO_12;
-                        targetGrey = 2048;
                         break;
                     case MODE_Y16:
                         newFormat = ARV_PIXEL_FORMAT_MONO_16;
-                        targetGrey = 32768;
                         break;
                     case MODE_GRBG:
                         newFormat = ARV_PIXEL_FORMAT_BAYER_GR_8;
-                        targetGrey = 128;
                         break;
                 }
+
                 if(newFormat != pixelFormat) {
                     stopCapture();
                     arv_camera_set_pixel_format(camera, pixelFormat = newFormat, NULL);
+                    midGrey = getExpectedMidGrey(newFormat);
                     startCapture();
                 }
             }