Question

What are the differences between the SD (Standard Definition) and HD (High Definition) camera modes?

Answer

There are a number of differences between SD and HD camera modes in the uScopeMXII including:

• Image Resolution
• Image Contrast
• Scan Speed
• Image Size
• Whole Slide Image Processing

We have attempted to explain, as completely as possible, each of these in the following sections.

Image Resolution

First and foremost is image size.

• HD images are 1920×1080.
• SD images are 960×540.

HD images are exactly double the resolution of SD images and have four (4) times as many pixels (double horizontally and vertically).

Image Contrast

SD images have higher contrast than HD images. This is due, in part, to the way in which the SD images are derived from the original 1920×1080 image. HD images are captured and sent directly to the PC from the uScope. SD images are created from the original HD image using a technique known as 2×2 binning which combines four pixels into a single pixel. The combining process has several effects including:

• Reduced image noise.
• Reduced image size.
• Increased transmission speed.
• Increased camera frame rate.
• Increased image contrast.

While there are few arguments for more image noise, slower speed, or slower camera frame rates. Larger image size might be useful if more resolution is required. Likewise, increased contrast may not always be desirable.

Scan Speed (Performance)

As mentioned above, HD and SD have very different camera data throughput rates as shown in the following table.

As the table shows, the uScope can capture more SD images per second than HD images (about 3 times faster). But, that's not the only statistic that feeds into performance.

While scanning, the uScope must overlap adjacent images so they can be stitched together for the final, deep zoom image. The overlap required is measured in pixels and is the same for SD or HD images. Therefore, HD images are more "pixel-efficient" than SD images. When scanning large regions of interest, you may notice that the number of HD images is about 20% less than the number of SD images required to scan the same area.

So, taking the pixel efficiency of HD images into account, SD scans are about 2.5 times faster than HD scans.

Note
The actual frame rate of the camera is limited by the speed at which the USB2 interface can send data to the PC and the speed at which the PC can actually process the USB data. The Realistic Frame Rate is what is calculated (and generally observed) to be the maximum frame rate for high-end PCs with no USB interface problems.

Image Size

It should come as no surprise that an HD image (or scan) is larger and requires more disk space than an SD image (or scan) of the same field or region.

• HD images are 1920×1080 or about 2 megapixels.
• SD images are 960×540 or about 0.5 megapixels.

So, HD images are about four (4) times larger than SD images. Comparing an HD scan to an SD scan, we would expect the HD scan to require about four times as much disk space. This ratio is based on saving and storing uncompressed images.

By default, uScope Navigator saves images in JPEG format with a quality setting of 85. Images saved with JPEG(85) will achieve a typical compression ratio of 10-15:1.

Whole Slide Image Processing

When creating a whole slide image, more time is required to process and output the whole slide image when it is created from HD scanned images. Generally, the performance is about four times faster for SD images than for HD scans.