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SURF and SIFT detect different features depending on how the application is run

I'm seeing strange behavior for cv::SurfFeatureDetector::detect and cv::SiftFeatureDetector::detect. I've got a camera pointing to a monitor, and I need to know how the camera image coordinates correspond to the screen coordinates. My approach is to put an image on the screen, and grab a camera shot. I then detect features of the screen image and the camera image, find matching feature pairs, and use their coordinates to extract a homography matrix.

The problem occurs in feature detection. Depending on how the application is run, I get different sets of features for the camera image. Visually, the camera images are indistinguishable but I get different feature sets if I run the app by double-clicking on it, or running it from the debugger, or if I run it from a different account. This is all on the same computer with the same camera/monitor, running the exact same code; not a copy.

I've tried relaxing/tightening various parameters of SIFT and SURF, but always results differ depending on how the application is run.

I'm using OpenCV 2.4.5, building with Visual Studio 2010 and running on a Windows 7 Pro, 64-bit, although I'm building the OpenCV application as a 32-bit.

Has anybody else seen this behavior?

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updated 2013-08-10 05:12:48 -0500

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SURF and SIFT detect different features depending on how the application is run

I'm seeing strange behavior for cv::SurfFeatureDetector::detect and cv::SiftFeatureDetector::detect. I've got a camera pointing to a monitor, and I need to know how the camera image coordinates correspond to the screen coordinates. My approach is to put an image on the screen, and grab a camera shot. I then detect features of the screen image and the camera image, find matching feature pairs, and use their coordinates to extract a homography matrix.

The problem occurs in feature detection. Depending on how the application is run, I get different sets of features for the camera image. Visually, the camera images are indistinguishable but I get different feature sets if I run the app by double-clicking on it, or running it from the debugger, or if I run it from a different account. This is all on the same computer with the same camera/monitor, running the exact same code; not a copy.

I've tried relaxing/tightening various parameters of SIFT and SURF, but always results differ depending on how the application is run.

I'm using OpenCV 2.4.5, building with Visual Studio 2010 and running on a Windows 7 Pro, 64-bit, although I'm building the OpenCV application as a 32-bit.

Has anybody else seen this behavior?