Grabbing an Image

Table of Contents

• Opening and Configuring the Video Capture Device
• Setting up the Sink and Data Stream
• Grabbing an Image
• Stopping the Data Stream

This article shows in detail how to set up a data stream from a video capture device and grab a single image.

Opening and Configuring the Video Capture Device

First, the library is initialized, and the first available video capture device is opened:

ic4_init_library(NULL);
 
// Create a grabber object
struct IC4_GRABBER* grabber = NULL;
ic4_grabber_create(&grabber);
 
// Find the first available video capture device
struct IC4_DEVICE_ENUM* enumerator = NULL;
ic4_devenum_create(&enumerator);
ic4_devenum_update_device_list(enumerator);
struct IC4_DEVICE_INFO* info = NULL;
ic4_devenum_get_devinfo(enumerator, 0, &info);
ic4_devenum_unref(enumerator);
 
// Open it
if( !ic4_grabber_device_open(grabber, info) )
{
    print_last_error();
    ic4_devinfo_unref(info);
    goto cleanup_grabber;
}
ic4_devinfo_unref(info);

Opening the device can fail, so we check for the return value of the function. In case of an error the last error is printed. For details on error handling, see Error Handling.

Then, the device has to be configured. This step is important because in most situations, programs want the camera to be in a defined state before starting operation. In this example, the resolution is configured using the device's IC4_PROPID_WIDTH and IC4_PROPID_HEIGHT properties:

IC4_PROPERTY_MAP* map = NULL;
ic4_grabber_device_get_property_map(grabber, &map);
 
// Set the resolution to 640x480
ic4_propmap_set_value_int64(map, IC4_PROPID_WIDTH, 640);
ic4_propmap_set_value_int64(map, IC4_PROPID_HEIGHT, 480);
 
ic4_propmap_unref(map);

At this point, an application could also load a prepared device configuration file (using ic4_grabber_device_open_from_state_file or apply a serialized property configuration using ic4_propmap_deserialize).

Setting up the Sink and Data Stream

After the device has been configured, it is time to setup a data stream. To receive image data from the video capture device, a Sink object has to be created. The Snap Sink is the sink most suitable to grab images on demand.

struct IC4_SINK* sink = NULL;
ic4_snapsink_create(&sink);
 
if( !ic4_grabber_stream_setup(grabber, sink, NULL, true) )
{
    print_last_error();
    goto cleanup_sink;
}

The data stream is established by calling ic4_grabber_stream_setup, passing the sink as a parameter. We also set the start_acquisition parameter to true, so that the device is instructed to start image acquisition immediately after the data stream was created.

After the streamSetup call returned successfully, the device is continuously sending images to the host computer.

Grabbing an Image

By calling ic4_snapsink_snap_single, the sink is instructed to wait for the next image to arrive at the sink and, if an image is received during the specified timeout period, return it:

struct IC4_IMAGE_BUFFER* buffer = NULL;
if( !ic4_snapsink_snap_single(sink, &buffer, 1000) )
{
    printf("Failed to snap image\n");
    print_last_error();
    goto cleanup_sink;
}
 
if( !ic4_imagebuffer_save_as_bmp(buffer, "test.bmp") )
{
    printf("Failed to save image buffer\n");
    print_last_error();
    goto cleanup_sink;
}

In this example, we print information about the received image and save it in a bitmap file.

Stopping the Data Stream

A call to ic4_grabber_stream_stop stops the data stream:

    // Stop the data stream.
    ic4_grabber_stream_stop(grabber);
 
    // Clean up grabber and sink
cleanup_sink:
    ic4_sink_unref(sink);
cleanup_grabber:
    ic4_grabber_unref(grabber);

Stopping acquisition and data stream is important, because keeping the acquisition active would waste CPU and memory resources as well as bandwidth on the transmission medium.