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Point Grey White Paper Series
Point Grey White Paper Series
Enhancing USB
3.0 Cameras
Reliability with
Frame Buffer
What’s inside:
• Reasons for image corruption and image loss
• Preventing image corruption and image loss
• Frame buffer explained
• Frame buffer performance
Point Grey White Paper Series
Image transmission reliability is often a key factor to consider when building a machine
vision system. Corrupted or lost image data often hinders system performance and
decreases efficiency. A frame buffer on board the camera can be used to address these
reliability issues.
What causes image corruption or data loss?
Image data can get corrupted or lost for several reasons. A poorly constructed cable or
an electrically noisy environment may cause signal degradation leading to image
corruption. A host system that is busy doing image processing may be unable to request
additional images from the camera, causing the data to overflow on the camera and
corrupt the data.
USB 3.0 utilizes Dynamic Memory Access (DMA), where the image data can be
transferred from the host controller to the system memory with minimal management
from the CPU. Instead of waiting for the transfer to complete, the CPU can now go on
with other operations and be interrupted by the DMA controller when the transfer has
been completed. However if the image processing software does not process the image
data, the host controller runs out of memory buffers to store the new image data. At
this point, the host system stops requesting new images from the camera, and the
image data start to accumulate on the camera buffer and eventually overflows. When
the overflow happens, an existing image may be partially overwritten with the new
image, causing an image frame to be corrupt and appear “torn”.
Data overflow causing two images to appear in the same frame and the image to appear “torn”
© 2015 Point Grey
5/4/2015
Point Grey White Paper Series
How do I prevent image corruption or data loss?
USB 3.0 supports several transmission mechanisms including isochronous transfer and
bulk transfer. The USB3 Vision committee has chosen to use bulk transfer which
guarantees data delivery and has key reliability features such as retransmission and
error checking capabilities. Point Grey is further leveraging these improvements in its
camera design by equipping each USB 3.0 camera with a frame buffer.
A frame buffer is memory onboard the camera allocated to temporarily store image
data before transmission. The image data is organized using a FIFO (first-in, first-out).
The buffer typically sits at the end of the imaging processing pipeline, right before the
interface controller. With a large frame buffer that stores several images, the camera
can retransmit the whole image if necessary or drop the image cleanly without
introducing torn images. A frame buffer large enough to store at least one frame can
always guarantee the arrival of a single triggered image. The retransmission is managed
by the host controller and does not need to be managed by the user.
Model Number
CM3-U3-13S2M-CS
BFLY-U3-03S2M-CS
GS3-U3-23S6M-C
Frame Buffer
Capacity
16 MB
16 MB
128 MB
Image Size
(8 bits/pixel)
1.3 MB
0.3 MB
2.3 MB
Maximum Number of
Image Frame Stored
12 Image Frames
53 Image Frames
55 Image Frames
Frame buffer size for Point Grey USB 3.0 cameras
Can the frame buffer impact transmission latency?
Even though the frame buffer improves camera reliability, some users have concerns
that data transmission will take longer due to having the extra buffer in the transmission
pipeline. To maximize camera reliability without sacrficing camera performance, the
data transmission process starts as soon as partial data is received from the image
sensor. As long as the host is avaliable to receive the data, the transmission delay is
minimized.
In summary, the frame buffer is a safe guard against unexpected events such as the host
system being busy and dropping the data. The camera feature is a must have for
© 2015 Point Grey
5/4/2015
Point Grey White Paper Series
machine vision cameras being used in mission critical systems. All Point Grey USB 3.0
cameras ship with frame buffers to provide maximum camera reliability.
© 2015 Point Grey
5/4/2015