General camera communication concepts are described on the corresponding page
NI IMAQ frame grabbers interface¶
NI IMAQ is the interface from National Instruments, which is used in a variety of frame grabbers. It has been tested with NI PCI-1430 and PCI-1433 frame grabbers together with PhotonFocus MV-D1024E camera.
This interfaces requires
imaq.dll, which is installed with the freely available Vision Acquisition Software, which also includes all the necessary drivers. After installation, the DLL is automatically added to the
System32 folder, where pylablib looks for it by default. If the DLL is located elsewhere, the path can be specified using the library parameter
import pylablib as pll pll.par["devices/dlls/niimaq"] = "path/to/dlls" from pylablib.devices import IMAQ cam = IMAQ.IMAQCamera()
The cameras are identified by their name, which usually looks like
"img0". To get the list of all cameras, you can use NI MAX (Measurement and Automation Explorer), or
>> from pylablib.devices import IMAQ >> IMAQ.list_cameras() ['img0', 'img1'] >> cam1 = IMAQ.IMAQCamera('img0') >> cam2 = IMAQ.IMAQCamera('img1') >> cam1.close() >> cam2.close()
Unlike most camera classes, the frame grabber interface only deals with the frame transfer between the camera and the PC over the CameraLink interface. Therefore, in can not directly control camera parameters such as exposure, frame rate, triggering, ROI, etc. Some similar-looking parameters are still present, but they have a different meaning:
- External trigger controls frame transfer, not frame acquisition, which is defined by the camera. By default, when the internal frame grabber trigger is used, the frame grabber transfer rate is synchronized to the camera, so every frame gets transferred. However, if the external transfer trigger is used and it is out of sync with the camera, it can result in duplicate or missing frames.
- ROI is defined within the transferred image, whose size itself is determined by the camera ROI. Hence, e.g., if the camera chip is 1024x1024px and its roi is 512x512, then the frame grabber ROI can go only up to 512x512. Any attempts to set it higher result in the frozen acquisition, as the frame grabber expects a larger frame than it receives, and waits forever to get the rest.
The SDK also provides a universal interface for getting and setting various camera attributes using their name. You can use
IMAQCamera.set_grabber_attribute_value() for that:
>> cam = IMAQ.IMAQCamera() >> cam.get_grabber_attribute_value("FRAMEWAIT_MSEC") # frame read request timeout 1000
To get a all available attributes as a dictionary, you can call
IMAQCamera.get_all_grabber_attribute_values(). Their meaning, as well as descriptions of trigger modes and other settings, is explained in the manual supplied with the Vision Acquisition Software.
Fast buffer readout mode¶
At high frame rates (above ~10kFPS) dealing with each frame individually becomes too slow for Python. Hence, there is an option to read out and process frames in larger ‘chunks’, which are 3D numpy arrays with the first axis enumerating the frame index. This approach leverages the ability to store several frame buffers in the contiguous memory locations (resulting in a single 3D array), and it essentially eliminates the overhead for dealing with multiple frames at high frame rates, as long as the total data rate is manageable (typically below 600Mb/s).
This option can be accessed by calling using
IMAQCamera.set_frame_format() method to set frames format to
"chunks" (former way of supplying
IMAQCamera.read_multiple_images() is now deprecated). In this case, instead of a list of individual frames (which is the standard behavior), the method returns list of chunks about 1Mb in size, which contain several consecutive frames.
Communication with the camera and camera files¶
The frame grabber needs some basic information about the camera: sensor size, bit depth, data transfer format, timeouts, aux lines mapping, etc. In NI MAQ this information is contained in the so-called camera files. These files can be assigned to cameras in the NI MAX, and are usually supplied by NI or by the camera manufacturer. In addition, NI MAX allows one to adjust some settings within these files, which are read-only within the NI IMAQ software. These include frame timeout and camera bit depth.
The communication with the camera itself greatly varies between different cameras. Some will have additional connection to control the parameters. Others use serial communication built into the CameraLink interface. This communication can be set up with
IMAQCamera.setup_serial_params() and used via
IMAQCamera.serial_write(). The communication protocols are camera-dependent, and are frequently described in the camera manual. However, some other cameras (e.g., Photon Focus) use proprietary communication protocol. In this case, they provide their own DLLs, which independently use NI-provided DLLs for serial communication (most notably,
clallserial.dll) to communicate with the camera. In this case, one needs to maintain two independent connections: one directly to the NI frame grabber to obtain the frame data, and one to the manufacturer library to control the camera. This is the way it is implemented in PhotonFocus camera interface.
- Sometimes when the acquisition is stopped and restarted without being cleared, the acquired frame counter does not refresh. This might show up as the software not reporting any new frames. It has been tracked down to a very low (~1ms) frame read timeout. Hence, it is recommended to keep this timeout at least at 500ms.
- If you are unable to access full camera sensor size, check the camera file (it can be opened in the text editor).
MaxImageSizeparameter defines the maximal allowed image size, and it should be equal to the camera sensor size.
- Same goes for bitness. If the camera bitness is higher than set up in the frame grabber, a single camera pixel gets treated as several pixels by the frame grabber, typically resulting in 1px-wide vertical stripes on the image. In the opposite case, the frame grabber expects more bytes than the camera sends, it never receives the full frame, and the acquisition times out.
- Keep in mind that as long as the frame grabber is accessed in NI MAX, it is blocked from use in any other software. Hence, you need to close NI MAX before running your code.
- As mentioned above, ROI is defined within a frame transferred by the camera. Hence, if it includes pixels with positions outside of the transferred frame, the acquisition will time out. For example, suppose the camera sensor is 1024x1024px, and the camera ROI is selected to be central 512x512 region. As far as the frame grabber is concerned, now the camera sensor size is 512x512px. Hence, if you try to set the same frame grabber ROI (i.e., 512x512 starting at 256,256), it will expect at least 768x768px frame. Since the frame is, actually, 512x512px, the acquisition will time out. The correct solution is to set frame grabber ROI from 0 to 512px on both axes. In general, it is a good idea to always follow this pattern: control ROI only on camera, and always set frame grabber ROI to cover the whole transfer frame.
- Some frame grabbers have a limit on the data transfer rate (for one model observed to be about 200 Mb/s). If the camera data generation rate exceeds it (e.g., it produces 1024x1024px 16-bit frames at >100FPS), then the camera will raise
IMG_ERR_FIFOerror shortly after the acquisition start. In this case, you will need to reduce the data rate by reducing the frame rate or frame size (through ROI, binning, or bitness).