Please, note the following important items concerning proper camera selection and its usage in the analyzed domain:

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     1. Resolution-value: the most high quality cameras support high resolutions of 6 mega pixels or better, which is acceptable for achieving satisfying             results of perspective restitution procedures. 

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     2. Lense-types: it is recommended to choose high quality wide-angle non-zoom lenses (i.e. fixed lenses). Avoid using the automatic digital zoom               settings because digital zoom feature manipulates the photo actually without adjusting the focal length. However, bearing in mind the fact that               almost all digital consumer cameras are sold with zoom lenses, it is so often very difficult to avoid this feature. In this case, zoom lenses must be           used only if extra care is taken. Namely, if you have zoom lenses, the optical zoom should be set to one setting for all photographs taken in one             input set (note that it is important you use the same focal length for all pictures taken!). Because some cameras do not store/show actual focal               length information, one has to fix the zoom at a known focal length (such as the widest angle for the most telephoto-shootings, or the power-on-             default zoom for common shooting-procedures). So, be careful not to change the zoom feature during the actual photo-shooting procedure in                 order not to affect the requested accuracy of perspective restitution process results. In that sense, bear in mind that Digital SLRs cameras have             options that allow choosing custom lenses manually. On the other hand, some consumer cameras have more stable zoom than other ones (the             focal length does not change during camera handling) – because of that, these cameras are preferred in analyzed context. 

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     3. Possible manual control over settings: if the camera used has automatic features such as: image stabilization or orientation detection, make sure           that all controls for turning these features are switched off before shooting.

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                                                                                                                  -ooo-

If the choice is an analog (film-based) camera along with a film scanner, one has to know that, during photo-digitalization process, real image frames might be cropped unintentionally resulting in decreased accuracy of perspective restitution process results. In order to prevent this decrease (i.e. to accurately identify optical characteristics of the actual lens used), it is necessary to insert a special “film-plane” between film and optical system of the analog  camera (this insert is a small piece of transparent plastic film with two visual (“fiducial”) marks on it that appear on each photograph). 

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When higher accuracy of the perspective restitution procedures is required, before performing any shooting activity it is recommended to make a contact here (in order your (chosen) camera to be professionally calibrated in time).

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If you decide to use your own camera, also bear in mind the following important facts. These facts are associated with how the shots have to be taken and with the kind/nature of the surfaces needed for achieving acceptable results of perspective restitution procedures.

 

BASIC SHOOTING GUIDELINES

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     1. Shooting direction should be perpendicular to the targeted surface.

     2. Shooting angles between each two successive shots should be the right angles (90°) if it is possible (“usually” convergent). should this not be               the case, the acceptable range is from 30° to 100°.

     3. Shots should be taken from both above and below the object of interest, if it is possible.

     4. It is necessary as many shots as possible to be taken - at least three ones.

     5. Each two successive images should be mutually overlapped as much as possible (min. 60%), so that they to be easily and accurately matched,             and

     6. Previously adjusted zoom characteristics must not to be changed during concrete shooting procedure (if an adjustable zoom lens camera is                   used).

 

Note that mutually “highly convergent”  ("high-angle") consecutive images (taken for shooting angles larger then 100°) are not proper for our perspective restitution procedures, since the same targeted element (present on each of them) might not be interpreted as a unique one (because of the fact that it does not “appear” identically on each of such images). 

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On the other hand, in some cases (related to concrete situation requests), using smaller angles between consecutive shooting directions (less then 30° - “low-angle” shots) can also be acceptable:  it is possible to match and restitute i.e mutually translated shots as well (obtained for parallel consecutive shooting directions), but only if they have been taken with special extra care - by using technical support offered here.

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Bear in mind that successive camera positions should mutually be approximately one quarter the distance apart, compared to the distance that they are from the concrete shot Target. This measure is called a 0.25 “base to distance (height) ratio”. “Base to distance (height) ratios” of 0.2 to 0.5 are proper for achieving  accurate results of applied perspective restitution procedures. 

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REQUIRED NATURE OF TARGET'S SURFACE

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To be successfully restituted, the shot surfaces should have a visible “random” texture/pattern (examples of materials with “random” textures are: stone/rock, dirt, skin (even face), wooden corps, brick-tile, trees, etc. and examples of materials that are not textured are: plaster, plastic, metal or glass, etc.). 

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Fortunately, it is also possible to restitute accurately,  in a standard way,  matte surfaces with no texture at all - by applying special additional procedure. Note that reflective (not matte) surfaces (whether or not having texture) may produce the trouble due to characteristics of reflection in the sense of ”hot-spot-light” visual distribution (light reflections/”hot spots” move on the surface as the camera position changes). Fortunately, this problem can often be solved as well - by applying another special procedure that successfully removes the most of existing reflections.

 

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IN-DOOR AND OUT-DOOR SHOOTING CONDITION RESTRICTIONS

 

Bear in mind that there are a number of limitation factors, presented during shooting, that can cause decreasing accuracy of the perspective restitution processes significantly:

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     1. Light: Usually, usage of the flash - mounted on the camera is not recommended, because by changing camera positions, generated shadows              also “moves” on the targeted objects (i.e. they appear on different locations between shoots). With regard to that, it is better to use light sources            that do not produce previously mentioned “moving” shadows (like fixed artificial ones and sunlight). In the case of sunshine (i.e. outdoor-                        scene/daily shooting), photos should not be taken hours or days apart, so as to avoid the previously described situation. In addition, it is                        important not to use light in the scene that causes glares from the targeted surfaces, because the same described effects will appear. This is                especially true of shiny/reflective targeted surfaces. Contrary to that, matte surfaces (that do not show highlights from directed light sources) are            less prone to this influence and some side/indirect lighting. Generally, recommendation is either the usage of flat/diffuse light such as sunlight -              but on a cloudy day only or the usage of artificial light with similar characteristics.

    2. Air particles and pollution effects: turbulence in the air between the targeted objects and the camera as well as present pollution particles could              bend the optical light rays that generate images.

    3. Lens distortion: imperfect camera lenses could distort the light ray directions. 

    4. Imperfect analog (film) imaging sensors/digital imaging sensors:  imperfect imaging sensors cannot generate sharp images, because they shift              optical light rays.

    5. Unknown optical/sensor system characteristics: unknown location of the imaging (analog (film) or CCD) sensor of the camera used at the time of          the exposure (i.e. internal camera orientation), causes imprecise geometric and dimensional perspective restitution procedures.

    6. Shooting parameters changing: if  shooting characteristics (such as focal length) are not constant during the concrete shooting procedure, the                quality of the achieved perspective restitution results becomes drastically increased. 

 

Should there be any misunderstandings about the facts explained above or should there be any additional questions or comments related to the mentioned tasks, please, make a contact here.