MikeOxon Posted August 18, 2014 Share Posted August 18, 2014 This topic relates to an item raised by drduncan in his blog at http://www.rmweb.co.uk/community/index.php?/blog/1569/entry-14584-gwr-tenders-and-all-that/ : I've posted in this forum, so that I can include pictures. Now if anyone out there can remove the effect of being viewed at an angle I'd love to hear from them so that measurements can be accurately and easily taken I'd love to hear from them! I've used the following method. It's a bit subjective in some of the steps but it can give a fair indication: I use a photo editor that can adjust perspective, such as Photoshop Elements. First step is to use the 'Image | Transform | Perspective' tools to bring the horizontal lines parallel, rather than converging towards a vanishing point. Once the image looks rectangular, then you use 'Free Transform' to stretch the image horizontally until the wheels are round. this is tricky! I use an additional layer on which I draw a circle. I make this layer partially transparent, and stretch the image until a wheel matches the circle. Next, I measure the diameter of the wheel in pixels and the wheelbase, also in pixels (px). From the photo in drduncan's blog, after transformation as described, the wheel diameter was 65 px and the wheelbase 203 px. Since, in this case, we know the wheel diameter was 4' 1½" (49.5"), the wheelbase becomes 49.5 * (203/65) inches or 154.6" or 12.9', which is pretty close to 6' 6" + 6' 6", pointing to a 2500 gal tender! This method depends on having an object in the image of known shape - in this case a circular wheel, but it could be a square or a brick of known width/height ratio. Mike Link to post Share on other sites More sharing options...
Boatman Posted August 18, 2014 Share Posted August 18, 2014 You seem to have that one taped well enough! All gobbledigook to me. Boatman Link to post Share on other sites More sharing options...
kandc_au Posted August 19, 2014 Share Posted August 19, 2014 Thanks for this Mike. It is something I will have to try when I am in the right frame of mind and have the inclination to persevere with. Khris Link to post Share on other sites More sharing options...
RMweb Gold Mikkel Posted August 19, 2014 RMweb Gold Share Posted August 19, 2014 Very clever, Mike. Looks like your gut feeling about the tender type was right, too! Link to post Share on other sites More sharing options...
drduncan Posted August 19, 2014 Share Posted August 19, 2014 Fantastic work, Mike. Duncan Link to post Share on other sites More sharing options...
MikeOxon Posted August 19, 2014 Author Share Posted August 19, 2014 Thanks for the comments. I should emphasise that this is not a high-precision method, as it relies very much on subjective judgements about what is 'parallel' or what is 'round' In addition, I'm not sure how well the Photoshop perspective correction takes account of geometric size reduction with distance but it should work fairly well, providing that the perspective is not too extreme. Mike Link to post Share on other sites More sharing options...
rd84 Posted August 19, 2014 Share Posted August 19, 2014 If you draw lines from corner to corner you can figure the half way point on a perspective image of a regular shaped building you can then use the same technique to work out the 1/4 , 1/8 and so on points - using this method you can work out the dimensions of a building from nearby bricks, doors , windows etc. You can project perspective lines to a vanishing point - you can then project back from this vanishing point across the tops of windows and doors etc to the nearest vertical to figure out the true vertical position of everything. The attached image will hopefully make it clear what I'm waffling on about ... Cheers Paul Link to post Share on other sites More sharing options...
MikeOxon Posted August 19, 2014 Author Share Posted August 19, 2014 If you draw lines from corner to corner you can figure the half way point on a perspective image of a regular shaped building you can then use the same technique to work out the 1/4 , 1/8 and so on points. thanks for that, Paul - I used your drawing to check that Photoshop does, indeed, correct the geometrical perspective pretty well- after applying the corrections as I described before, the halves and quarters come out evenly spaced! Mike Link to post Share on other sites More sharing options...
rd84 Posted August 20, 2014 Share Posted August 20, 2014 Hi Mike, (Some of the following is for others to help them understand - I can see you have it figured out) Now you need to correct for the building being squashed - You can re-size images - in this case you want the height to remain the same but the width needs increasing - can you see that the lady in the picture is also looking squashed ? - You would correct the figure with known dimensions - it might be a door or window next to the nearest vertical for example. Say a door was twice as high as it was wide - say 6ft x 3ft it would have a ratio of 6/3 = 2 . If you were to measure the number of pixels high and wide you would be able to correct how squashed the image was. So for example if you measure 24 pixels high and 6 pixels wide this would be 24/6 = 4 - so the correction to correct the image would be to double the width of the door to give 24/12 = 2 - ie the same as the real door. So what we would do in this case is double the length of the entire perspective adjusted image so that the door and everything else had the correct height/width ratio. So easy to do but so hard to explain in words. The description in the above thread by you above is the same technique - but in this case a wheel's width is being changed to make it round - ie to convert it from an oval to a circle it must have equal diameters vertically and horizontally. - by correcting the wheel you are correcting the entire image. be warned though these techniques work in one plane only - So the wheels on the above tender would appear in the wrong place relative to the tender sides. Regular shaped buildings are much easier to deal with. By halving and quartering a building you might be able to count the near bricks for one quarter of the image and be able to estimate the dimensions of a building better - usually bricks in the distance are too small ? If you wish I'll make you another image with a door on it so the above concept can be practiced ? Google Earth satellite view can be helpful for present day buildings since you can actually measure the length and width of a building and you can count bricks, windows etc to estimate the height. The NMR RAF vertical photo archive is useful for older buildings in the UK. BTW if you use CAD programs Google Sketchup has a feature for turning photographs of regular buildings into 3D CAD drawings. Hope this helps ? Cheers Paul Link to post Share on other sites More sharing options...
allan downes Posted August 20, 2014 Share Posted August 20, 2014 " Eh?" Link to post Share on other sites More sharing options...
MikeOxon Posted August 20, 2014 Author Share Posted August 20, 2014 " Eh?" As rd84 wrote "easy to do but so hard to explain in words." I think this is one of those things that can be useful, if you need it, but you seem to do pretty well without it, Allan Link to post Share on other sites More sharing options...
Boatman Posted September 4, 2014 Share Posted September 4, 2014 There are,or used to be, correct measured perspective grids which include correct foreshortening, which seems to be what confounds most people. If one fits the projection of a loco/building in a photo, just lay it over to "measure" the subject. My technical illustration training taught me how to construct my own, but that was a bit long ago, I'm afraid. Alas it's all done by pootahs now. Another of my trades done for! Regards, Boatman Link to post Share on other sites More sharing options...
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