TI’s DLP® WVGA (848×480) and WXGA (1280×800) microdisplays use what are commonly known as “diamond” pixels. This article will explain what these pixels look like, their effect on image quality, and the reason behind diamond pixel.
As a side note: all the pictures of projected images were taken on a Qumi 1280×800 projector using the HDMI (digital) input driven at the “native” 1280×800 resolution to try and give the best possible source image.
Diamond Pixel Organization
Fig. 2 Show how the “diamond pixels” are organized. The pixels themselves are square, but they are rotated 45 degrees and fit like tiles in a zig-zag arrangement. The Columns and row numbering are based on how the memory bit (signified by the red dot in Fig. 2) for each pixel is address. You should notice that the columns numbering is more spread out than the row numbering. For a WVGA (848×480) there are 608 columns [roughly 848/sqrt(2)] and 684 rows [roughly 480xsqrt(2)]. Note that if you multiple 608 x 864 you will get slightly more than 848×480 so there are roughly the same number “pixels.” Details on this organization for the WVGA can be found in the DLP® 0.3 WVGA Series 220 DMD data sheet. For the WXGA (1280×800) there are about 910 columns by 1136 rows (the datasheet is not publicly available).
Image Re-sampling Effect of Diamond Pixels
The image quality issues with diamond pixels comes from trying to “re-sample” (remap) the pixels from a normal square grid to the “diamond grid. Fig. 3 Show shows a simplified example of the problem. There are two black pixels labeled “A” and “B” shown. On the left side of the figure, the green grid shows the normal/square array of pixels and it is overlaid with the the diamond grid in red. As can be seen pixel A straddles/touches pixels 1 through 5 in the diamond grid and pixel B straddles pixels 5 though 11. There is no really good way to map the pixels. If you you just mapped to the nearest pixel on the diamond grid, it would cause severe jaggies, so it is best to re-sample/filter the pixels onto the diamond grid.
On the right hand side of Fig. 3, I did a simple weighting of the areas of overlap to remap the pixels. Notice how the pixels have blurred out. Also notice that the two black pixels end up mapping into different shapes in on the diamond grid because their relative alignment between the square and diamond grid is different.
TI’s DLP uses more complex algorithms than those used in Fig 3 that attempt to reduce the blurring by sharping (particularly in the horizontal direction) or allowing more jagged artifacts (in the vertical direction). Any algorithm/filtering/re-sampling by necessity will be a compromise between jagged pixel artifacts and blurring.
Fig. 4 below shows a close-up of a series of simple test patterns of a checkerboard, horizontal lines, and vertical lines with a picture of the projected image from a test pattern. The checkerboard is pretty well obliterated in the DLP’s re-sampling process and you see what is known as a classic under-sampling problem where you see the “difference frequency” or a low resolution version of the repeating pattern. It is also interesting is that the artifacts are different in the horizontal and vertical direction because they use different re-sampling algorithms horizontally and vertically.
Fig. 5 below shows more more of the test pattern (click on image to see all the detail – at the end of this article, I have included the whole the test image used so you can duplicate this test). The longer lines in the test pattern are suppose to be 4 alternating black and white lines. Where the 4 line pairs cross there are some strange effects (one of which is circled). On vertical lines or groups of lines there is the occasional “ghost line” (some of which are pointed to with red arrows); these are as a result of the horizontal filtering/re-sampling process. Vertical lines generally look a little better but they have that ghosts lines (“sharpening halos”) as a result of trying to keep from loosing sharpness.
The vertical re-sampling process (which shows up in horizontal lines) is much simpler and results in more jagged artifacts and less effective resolution. You can also see in the dot in the letter “i” in the “8 Point Arial” how a single pixel blurs out over multiple pixels in the diamond grid of pixels. Notice how all the dots in the “i’s” vary (circled in red) and in the Arial 10 point font the dots in the letters “i” even point in different directions.
Why DLP Uses Diamond Pixel
The “marketing spin” I have heard for the diamond pixel organization is that it can give perceptively better resolution. But this could only be true if the source image is much higher resolution than the displayed image. As the pictures above demonstrate, about the worst case image is to drive the Diamond Pixel with its supposed “native” resolution.
At least one real reason that the TI DLP® uses the diamond arrangement is to try and reduce the projector’s thickness. DLP’s require “off axis illumination” which means the light comes into the mirror array non-perpendicular to the surface. When the mirror is tilted “on,” the angle of the mirror directs the light toward the projection lens. The current DLP mirrors tilt approximately +/- 12 degrees and they tilt diagonally (at 45 degrees).
With normal/square mirror arrays (left half of FIG. 6) this would mean that for light to go out of the projector the light must come from above or below projection lens. But in order to have the illumination light come from above or below the projection lens would make the projector much thicker. This was not an issue for DLP in RPTVs or larger data projectors, but with Pico Projectors, customers care a lot about thickness. The “Diamond Mirror Solution” was to rotate the mirrors so that the light could be in the same plane (right half of Fig. 6) as the projected image.
While the diamond pixels address the thickness issue, they definitely hurt resolution and cause artifacts and a loss of resolution. The obvious problem is that a single pixel on a rectangular grid will cover part of at least 4 or more pixels on the diamond grid. So there has to be some scaling/filtering to map the pixels from the rectangular grid onto the diamond grid. The DLP ASIC tries to combat this blurring in the horizontal direction by filtering and sharpening. The vertical processing is simpler and results in more jaggies.
Many of the problems with DLP’s diamond pixel are closely related to the resolution problems of Laser beam scanning that I wrote about in a previous article (see: http://www.kguttag.com/2012/01/09/cynics-guild-to-ces-measuring-resolution/). Namely, it is the problem of trying to re-sample the original image to fit a grid or scanning process that does not match the original image. Even if the number of “pixels” is the same, the re-sampling process hurts resolution and causes unwanted artifacts (for those engineers in the audience, it is a classic Nyquist sampling issue).
In optical terms DLP optics are “off-axis” which means the illumination of the display is not perpendicular to the imager. Off-Axis optics are always bigger and their light path takes up more volume. The diamond rotation keeps this volume from increasing thickness, but still the volume of a DLP optical engine tends to be bigger for the same F-number and Focal Length optics. In general, DLP “pico projectors tend to have longer throw ratios (longer focal length lenses) and this may be due to trying to keep the optics from becoming larger.
Below is the test pattern used to create the images above on a DLP 1280×800 projector. To use this correctly, you need to drive the HDMI input of the projector at 1280×800.
I have included a similar test pattern for WVGA (848×480) that can be used with the WVGA DLP projectors like the PK201 or Microvision’s ShowWX/+/HDMI projectors. Additionally, I have included a 720p (1280×720) version of the test patter in case you come across a 720p projector.
I would like to thank Paul Anderson for taking the picture of the test pattern on his 1280×800 DLP Qumi Projector that I used in this article.
1280×800 Test Pattern:
848×480 Test Pattern:
720P Test Pattern: