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Develop-Geometry-Perspective, Please add X/Y scaling

(3 posts)

  • noam
    Focus Group

    Hi,

    As I said in the past, the new Perspective module in Develop mode is a vast improvement over the old clunky module in 2.5 (which oddly remained in Edit mode. Please update it there as well!).

    However, there is one missing degree of freedom in the controls! In addition to H/V perspective and H/V shear transformation, there should also be H/V scaling transformation sliders! Whenever I use the perspective sliders to straighten a building shot from the ground (the main purpose of this feature, I presume), The result is a stretched or squashed building (i.e., if it has square windows, they now appear as elongated or squashed rectangles). I need the ability to compensate for that and scale the image (vertically in this example) accordingly.

    Best,

    Noam

    EDIT: I'm no expert on perspective correction, but it appears to me as if the strech/squash effect depends on whether the object is in the upper or lower part of the image (i.e. closer to the longer or shorter base of the transform trapezoid). It's too late in the night for me to do the math at the moment, but I'm guessing that the best thing would be a non-linear scaling compensation (2nd order I'm guessing) so that all parts look right. But at least please add the simple linear scaling. It will do wonders in the common simple cases.

    Posted On August 3, 2009 - 06:54 PM (7 months ago) (Permalink to this post)

  • John Radcliffe
    Member

    Whenever I use the perspective sliders to straighten a building shot from the ground ... the result is a stretched or squashed building.

    Can I ask at what settings you notice this kind of distortion?

    Generally I've been vary pleased with how this features deals with X/Y scaling, as it offers very similar results to PTLens, which I really like. At least that is the case for settings of up to around "Vertical" = 20. If I go much beyond that the image may start to appear noticeably squashed vertically. How noticeable it is depends on the nature of the image (including such things as whether it contains anything obviously circular or square). Perhaps one reason I don't see this very often is that I never totally remove converging verticals, and tend to leave more convergence the more there was in the original.

    Adding a Vertical/Horizontal scaling adjustment would offer a quick fix, but ideally I'd like to see the adjustment look OK without me having to make any further adjustments. As the calculations involved are complex, I'd much prefer to have the program do them for me rather than my having to adjust the adjustments.

    * * * * * * * *

    When I first started using PTLens I tried to check its accuracy by drawing a square "building" and then distorting it in accordance with my calculation of how it would look when "photographed" from varying distances, resulting in varying degrees of convergence, etc. PTLens performed very well and returned the "building" to a nearly square except when the "photographer" was very near and the convergence extreme. I've just repeated the test with Pro3, and it is spot on in every case (i.e. the adjusted image is square to well within 1%).

    However, Pro3 (like PTLens) doesn't do as well with real-world images, and this suggests to me that Pro3 and I are both failing to take into account some additional factor, but I've no idea what that might be.

    Posted On August 5, 2009 - 05:20 PM (7 months ago) (Permalink to this post)

  • John Radcliffe
    Member

    ... it appears to me as if the stretch/squash effect depends on whether the object is in the upper or lower part of the image (i.e. closer to the longer or shorter base of the transform trapezoid).

    I can't say that I've noticed this. As I see it, for that to be the case a diagonal line across image would appear curved after adjustment. I've just tested this by drawing diagonal lines on an unadjusted image, and after adjustment the lines appear perfectly straight.

    Posted On August 5, 2009 - 05:20 PM (7 months ago) (Permalink to this post)

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