Shading Correction (Flat-Field Correction) with the Baumer GAPI SDK [AN201803]
Baumer cameras, Baumer GAPI SDK from v2.9.2
The Baumer GAPI shading correction (flat-field correction) helps calibrating your vision system to drastically reduce effects of brightness differences introduced by a camera/lens-system or non-even light sources.
概述
It is not uncommon for vision systems to have variations in brightness over the full image frame for various reasons. These variations will negatively affect algorithms, like object detection or code reading, developed to enable the various tasks of a vision system.
“Flat-field correction is a technique used to improve quality in digital imaging. The goal is to remove artifacts from 2-D images that are caused by variations in the pixel-to-pixel sensitivity of the detector and/or by distortions in the optical path. It is a standard calibration procedure in everything from pocket digital cameras to giant telescopes.
Flat fielding refers to the process of compensating for different gains and dark currents in a detector. Once a detector has been appropriately flat-fielded, a uniform signal will create a uniform output (hence flat-field). This then means any further signal is due to the phenomenon being detected and not a systematic error.”
Wikipedia: https://en.wikipedia.org/wiki/Flat-field_correction [05.06.2018]
了解算法的局限性非常重要。如果一張圖像中的亮度差異超過25%到30%，可能是選擇的組件不合適，因此可以考慮更換系統組件，因為通過算法可能已經無法達到預期的效果。
步驟1：視覺系統校準
注意
為了獲得理想的校準效果，應使用完整的視覺系統設置在真實環境中進行校準。
需要注意的是，校準過程中只能減少靜態影響。因此，如果在校準期間更換了鏡頭、光圈或光源，則必須重新校準視覺系統。
在設置完視覺系統并選擇所需的光圈和曝光時間設置后，本示例將介紹如何生成2個數據集：其中一個用于最暗的圖像（暗場幀），另一個用于最亮的圖像（明場幀）。
本示例將為每個數據集采集多張圖像，然后采用圖像平均法降低傳感器噪音的影響。然后，這些通過平均法得到的圖像將用于計算出校正視覺系統采集的圖像所需的數據。
準備一個干凈的白色目標物以獲取明場參考幀，在校準完成后，任何偽影（如污垢，甚至是紙張紋理）都可能出現在圖像中。白色目標物必須覆蓋需要校準的整個表面。最好選擇經過校準的目標物，但這并非硬性要求。
Set-up of the vision system
- Set-up your vision system including camera, lens and light sources for your intended use.
- Configure exposure time, aperture, gain and intensity of the light sources as required for your application.
- Configure a fixed exposure-time so that the image is not over-saturated. This can be done with the help of the Baumer Camera Explorer (Histogram-View, F7). Also ensure that the gain is set to the lowest possible value to reduce any noise in the image.
Acquire the dark field reference
This is an optional step to achieve full flat-field correction. If you require just a shading correction you can skip this step. The measurement is only necessary once for a specific camera.
- Compile and start the example 016_FlatField_Calculation.cpp at a convenient location.
- To create the dark-field reference it is necessary to completely block out any light from the camera sensor. This is best done by covering the lens with the lens-cap or by removing the lens and attaching the protective cap which came with your camera.
- Now press “D” to acquire the Dark Field Frame.
- The example will take some images and store the calculated reference data as a Baumer Raw Image (dark_field_reference.brw) in the same folder as the example. Brw-Files can be viewed with the Baumer Camera Explorer.
- The example will notify you when the measurement and calculations are successful. In case the example notifies you of issues please check your Vision System setup again and repeat the measurement.
Acquire the light reference
- Now you need to set-up the system for the light-frame. So please go and remove the lens-cap or attach the lens again and switch on all configured light sources.
- If your systems lighting shines towards the camera (e.g. to take the 2D-shape of an object), remove any object out of the view of the Vision System. Otherwise place a completely white sheet where the object will reside. Ensure the full view range is covered by the sheet!
- Start the compiled example again and press “L” to acquire the Light Frame.
- Again, the calculated reference data will be stored as a Baumer Raw Image (light_reference.brw) inside the folder of your example.
- The example will notify you when the measurement and calculations are successful. In case the example notifies you of issues please check your setup again and repeat the measurement.
- You now have everything ready to correct images taken by your system.
Notice
If a non-ideal target is used (e.g. Paper) it is possible that structures or dirt on the target are visible in the corrected image. In this case the Baumer GAPI box- and median-filters can help reduce the unwanted artefacts. The filters should only be used as a last resort as they will effect the shading-corection negatively.
Example how to use the filters:
bo_uint r = m_pShading->GetFilter(BGAPI2::Ext::Sc::Shading::BoxFilter, true);
bo_uint rMin = m_pShading->GetFilterMin(BGAPI2::Ext::Sc::Shading::BoxFilter, true);
bo_uint rMax = m_pShading->GetFilterMax(BGAPI2::Ext::Sc::Shading::BoxFilter, true);
// Set median-filter radius for light reference
m_pShading->SetFilter(BGAPI2::Ext::Sc::Shading::MedianFilter, true, 1);
// Set box-filter radius for light reference
m_pShading->SetFilter(BGAPI2::Ext::Sc::Shading::BoxFilter, true, 2);
步驟2：圖像校正
在步驟1中獲得的參考幀現在可用于校正經校準的視覺系統采集的圖像。通常情況下，圖像校正集成到視覺應用中，本示例將介紹具體的操作。
本示例中，通過計算得到的數據可實現兩種不同的圖像校正。如果提供暗場和明場參考幀，則可計算出平場校正數據。如果僅提供明場參考幀，則只能計算出陰影校正數據。
- 按照應用要求設置視覺系統，并放置一個符合要求的物體。
- 在合適的文件夾下編譯并啟動示例017_FlatField_Correction.cpp。
- 確保示例文件夾下確實有dark_field_reference.brw和light_reference.brw兩個參考幀文件。
- 按下任意一個鍵即可開始采集圖像。
- 本示例將采集一張圖像，將原始圖像和校正后的圖像存儲在示例文件夾下，并命名為imageSample.brw。該文件可以通過堡盟Camera Explorer打開并進行轉換。
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