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Old 12-01-2008, 12:26 PM   #11 (permalink)
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Old 12-01-2008, 12:55 PM   #12 (permalink)
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i think it's always worth t looking for some blueprints of the car, a site i recently discovered

The-Blueprints.com - 26351 blueprints online has a lot of usefull stuff they often include a frontal view
sometimes even with measures but if not these can be found and applied the the drawing

if you're working with pictures and you know the with and height of the car, you could set it to the correct width in photoshop and that stretch/squash the height untill this matches the scale of the width... this still leaves room for errors but i imagine if you match a crappy 3 view drawing with a picture and real measures and than do a pixel count your arrive pretty close to the real value
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Old 12-01-2008, 01:04 PM   #13 (permalink)
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here's a few of the best ones for the guys


Suzuki Swift Mk2 1600GLX 4WD Saloon

Suzuki Swift GS 3-Door
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aer·o·dy·nam·ics: the science of passing gass

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Old 02-18-2009, 02:46 PM   #14 (permalink)
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You've discussed the roll of perspective projection distortion, but no one seems to have considered optical (lens) distortion.
Quote:
The radial distortion can usually be classified as one of two main types:

* barrel distortion, in which image magnification decreases with distance from the optical axis. The apparent effect is that of an image which has been mapped around a sphere. Fisheye lenses, which take hemispherical views, utilize this type of distortion as a way to map an infinitely wide object plane into a finite image area.

* pincushion distortion, in which image magnification increases with the distance from the optical axis. The visible effect is that lines that do not go through the centre of the image are bowed inwards, towards the centre of the image. In photography, this aberration is often seen in older or low-end telephoto lenses.

A mixture of both types, sometimes referred to as moustache distortion, is less common but not rare. It starts out as barrel distortion close to the image center and gradually turns into pincushion distortion towards the image periphery. It is observed with certain retrofocus lenses, also more recently on large-range zooms such as the Nikon 18-200mm.
In the context of the above quoted material, it can be taken it as a given that your digital camera's integrated (not accessory) zoom lens would be considered low-end telephoto lenses. With virtual certainty -
  • Non-zoomed, you have some amount of barrel distortion.
  • Somewhere in the zoom range you have some mustache distortion.
  • At the far end of zoom you have some amount of pincushion distortion.
Recent versions of PaintShop Pro ( and I believe Photoshop, Photoshop Elements as well) have manual tools to correct for pincushion and barrel distortion. There are commercial plug-ins available for the same programs that provide more automatic correction based on camera/lens model (but probably don't include you consumer grade digital in their lists.)

It's still winter here and I'm waiting for warmer weather. When that happens, I'll find a wall to back the car up to such that there is a long, level, unobstructed expanse in front of the car to position my camera shot. Prop a 4x8 sheet of plywood against the wall - vertical but in landscape position orientation. Position my tripod mounted camera* such that the plywood sheet fills about 90% of a picture taken with camera set to maximum Safety Digital Zoom (no in camera pixel interpolation - only cropping) at 1600x1200 resolution. This picture will be used as a reference to determine the best numeric setting to use in the pincushion correction filter.

* Tripod mounted camera: To minimize vertical perspective distortion, height of camera lens center to be 1/4 (height of car + 48")

Moving on to the picture of the car. As controls
  • A small object (soup can?) will be positioned on the roof of the car at the point of it's maximum height.
  • Measured lengths of fluorescent flagging tap, will be taped to key points on the car's body.
  • The measured distance between outer tips of the side mirrors will also be used as a control.
These controls will be used to decide any application of perspective correction after application of lens correction.

When I actually do this, I'll post a thread with pictures.
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Old 02-18-2009, 04:30 PM   #15 (permalink)
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Or, the low-tech approach (Mechanical skills required: the ability to bang nails, or apply duct tape). Build a simple rectangular frame high enough and wide enough to fit around the widest/tallest points of your vehicle. Calculate the area of the enclosed rectangle. Take a series of measurements of the gap between your vehicle and the frame (the more measurements you take, the more accurate your result). From there it's relatively simple to calculate the difference by various means, either by 'filling in' the space with progressively smaller cubes, or by creating an analog in a drawing programme.
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Old 02-18-2009, 04:38 PM   #16 (permalink)
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Sounds like a variation of the "cardboard cutout" idea in post #7.

TestDrive: I'd be most interested in the amount of error in area calculations between corrected and uncorrected photos. Sounds like an interesting test.
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Old 02-18-2009, 04:44 PM   #17 (permalink)
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One drawback of the "cardboard cutout" method is that it doesn't allow you to measure the area under the car, which is a big ol' chunk of space.
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Old 02-18-2009, 06:51 PM   #18 (permalink)
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MetroMPG, no doubt well south of 5%. Think I'll refrain from any other predictions.
davidgrey50, due to ground effect and all the usual clutter underneath, the conventional wisdom is to include it when measuring frontal area.
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Old 02-18-2009, 07:42 PM   #19 (permalink)
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Hi,

I would use DataCAD to trace a polyline on the telephoto picture, and then measure the area with the handy Measures menu...report it in square meters or square feet; no problem. Here's two examples using the Aptera; the one on the left is a perspective distorted rendering, the one on the right is a pretty long shot of an actual vehicle:



Using the area from the long photo, the Aptera 2e has a CdA of 0.277 meters. (The hatch grid is 12" squares, btw.)
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Old 02-18-2009, 08:32 PM   #20 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
I would use DataCAD to trace a polyline on the telephoto picture, and then measure the area with the handy Measures menu...report it in square meters or square feet; no problem. Here's two examples using the Aptera; the one on the left is a perspective distorted rendering, the one on the right is a pretty long shot of an actual vehicle:

Using the area from the long photo, the Aptera 2e has a CdA of 0.277 meters. (The hatch grid is 12" squares, btw.)
What would the numbers be if you had corrected the picture for lens distortion prior to working in DataCAD?

I took the long shot and based on the pole (tree?? Note curve relative to the straight blue line I added) on the left and corner of the building on the right, I applied correction for pincushion distortion. Would the DataCAD numbers come out the same for the corrected shot?


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