Screen-door effect
Encyclopedia
The screen-door effect or fixed-pattern noise (FPN) is a visual artifact
of the projection technology
used in digital projectors, where the fine lines separating the projector's pixel
s become visible in the projected image
. This appears because projector optics typically have significantly higher resolution than the image they project, allowing these fine lines, which are much smaller than the pixels themselves, to be seen. This results in an image that appears as if viewed through a fine screen or mesh such as those used on anti-insect screen door
s. It most commonly appears as a rectangular mesh
or sometimes as a hexagonal structure.
The screen door effect was noticed on the first digital projector: an LCD projector made in 1984 by Gene Dolgoff
, the inventor of the LCD projector. To eliminate this artifact, Dolgoff invented "depixelization", which used various optical methods to eliminate the visibility of the spaces between the pixels. The dominant method made use of a microlens
array, wherein each micro-lens caused a slightly magnified image of the pixel behind it, filling in the previously-visible spaces between pixels. In addition, when making a projector with a single, full-color LCD panel, an additional appearance of pixelation was visible due to the noticeability of green pixels (appearing bright) adjacent to red and blue pixels (appearing dark), forming a noticeable repeating light and dark pattern. Use of a micro-lens array at a slightly greater distance created new pixel images, with each "new" pixel being a summation of six neighboring sub-pixels (made up of two full color pixels, one above the other). Since there were as many micro-lenses as there were original pixels, no resolution was lost, which was confirmed with modulation transfer function (MTF) measurements.
The screen door effect on DLP projectors can be mitigated by deliberately setting the projected image slightly out of focus, which blurs the boundaries of each pixel to its neighbor. This minimizes the effect by filling the black pixel perimeters with adjacent light. Some older LCD projectors have a more noticeable screen door effect than first generation DLP projectors. Newer DLP chip designs promise closer spacing of the mirror elements which would reduce this effect; however, some space is still required along one edge of the mirror to provide a control circuit pathway. Use of Dolgoff's depixelization method could also produce a DLP projector without noticeable pixelation.
Some low resolution matte displays can also exhibit this effect.
Visual artifact
Visual artifacts are anomalies during visual representation of e.g. digital graphics and imagery.-Examples in digital graphics:* Image quality factors, different types of visual artifacts...
of the projection technology
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...
used in digital projectors, where the fine lines separating the projector's pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s become visible in the projected image
Image
An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...
. This appears because projector optics typically have significantly higher resolution than the image they project, allowing these fine lines, which are much smaller than the pixels themselves, to be seen. This results in an image that appears as if viewed through a fine screen or mesh such as those used on anti-insect screen door
Screen door
A screen door can refer to a hinged storm door or hinged screen door covering an exterior door; or a screened sliding door used with sliding glass doors. In any case, the screen door incorporates screen mesh to block flying insects from entering and pets and small children from exiting interior...
s. It most commonly appears as a rectangular mesh
Mesh
Mesh consists of semi-permeable barrier made of connected strands of metal, fiber, or other flexible/ductile material. Mesh is similar to web or net in that it has many attached or woven strands.-Types of mesh:...
or sometimes as a hexagonal structure.
The screen door effect was noticed on the first digital projector: an LCD projector made in 1984 by Gene Dolgoff
Gene Dolgoff
Eugene Dolgoff is the founder, CEO, and CTO of The 3D Source, Inc., involved in the printing of 3-D moving images for advertising and promotions, labels, financial payment and ID cards, and medical imaging. He holds the same position in 3-D Vision, Inc., which is involved with the development of...
, the inventor of the LCD projector. To eliminate this artifact, Dolgoff invented "depixelization", which used various optical methods to eliminate the visibility of the spaces between the pixels. The dominant method made use of a microlens
Microlens
A microlens is a small lens, generally with a diameter less than a millimetre and often as small as 10 micrometres . The small sizes of the lenses means that a simple design can give good optical quality but sometimes unwanted effects arise due to optical diffraction at the small features...
array, wherein each micro-lens caused a slightly magnified image of the pixel behind it, filling in the previously-visible spaces between pixels. In addition, when making a projector with a single, full-color LCD panel, an additional appearance of pixelation was visible due to the noticeability of green pixels (appearing bright) adjacent to red and blue pixels (appearing dark), forming a noticeable repeating light and dark pattern. Use of a micro-lens array at a slightly greater distance created new pixel images, with each "new" pixel being a summation of six neighboring sub-pixels (made up of two full color pixels, one above the other). Since there were as many micro-lenses as there were original pixels, no resolution was lost, which was confirmed with modulation transfer function (MTF) measurements.
The screen door effect on DLP projectors can be mitigated by deliberately setting the projected image slightly out of focus, which blurs the boundaries of each pixel to its neighbor. This minimizes the effect by filling the black pixel perimeters with adjacent light. Some older LCD projectors have a more noticeable screen door effect than first generation DLP projectors. Newer DLP chip designs promise closer spacing of the mirror elements which would reduce this effect; however, some space is still required along one edge of the mirror to provide a control circuit pathway. Use of Dolgoff's depixelization method could also produce a DLP projector without noticeable pixelation.
Some low resolution matte displays can also exhibit this effect.
See also
- Rainbow effect, an artifact associated with single-chip DLP projectors
- Silk screen effectSilk screen effectSilk Screen Effect is a visual phenomenon seen in rear-projection televisions. SSE is described by viewers as seeing the texture of the television screen in front of the image. SSE may be found on all rear-projection televisions including DLP and Liquid Crystal on Silicon . The effect is most...