Справочник Пользователя для National Instruments Image Acquisition Software

Chapter 3

Programming with NI-IMAQ

©

 National Instruments Corporation

3-15

for you when you acquire an image with it, you must perform the alignment 
yourself if you use window widths not aligned on a 32-bit boundary.

After the program sets the ROI, it locks the memory and acquires the 
image. If you choose to plot the image using the 

imgPlot

 function, 

you must align the image width on a 32-bit boundary as well.

The 

LLGrab.c

 example demonstrates how to perform a grab acquisition 

using low-level calls. The example sets up a continuous acquisition to a 
single user-allocated buffer.

As described in the low-level snap example, the program retrieves the 
acquisition window width of the selected camera and aligns it on a 32-bit 
boundary. The program creates a buffer list to describe the acquisition 
buffers. Next, the program sets the ROI to the acquisition window width. 
The program performs a calculation to determine the correct memory 
requirements of the user buffer. The program creates the buffer and 
configures buffer element 0 for a single continuous acquisition. The 
program then locks the memory and starts the image acquisition 
asynchronously. The main processing loop of the code shows how to wait 
for vertical blank and copy the buffer to an analysis buffer.

Keep your analysis code fast to minimize the number of missed frames 
during analysis. If you need more time to examine a buffer, set up a 
multiple-buffer ring and call 

imgSessionExamineBuffer

 to extract 

the desired buffer from the live sequence.

The 

LLSeq.c 

example demonstrates how to perform a sequence 

acquisition using low-level calls. The example sets up a sequence 
acquisition to multiple buffers allocated by NI-IMAQ. As described in the 
low-level snap example, the program retrieves the acquisition window 
width of the selected camera and aligns it on a 32-bit boundary. It creates a 
buffer list to describe the acquisition buffers. Next, the program sets the 
ROI to the acquisition window width. The program calculates the correct 
memory requirements of the frame buffer. However, this is not necessary 
if you choose to use the default acquisition window width, rowPixels, and 
ROI. In this case, NI-IMAQ will allocate the correct size buffer if you pass 
a NULL as the size parameter to 

imgCreateBuffer

. The program creates 

the buffer and configures the buffer list for each buffer element in the ring. 
The program locks the memory and starts the image acquisition 
asynchronously.

UM.book  Page 15  Monday, July 13, 1998  9:49 AM