Add Projective Transforms #546

JimBobSquarePants · 2018-04-26T03:48:34Z

Prerequisites

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I have provided test coverage for my change (where applicable)

Description

Adds the ability to perform projective transforms (perspective, taper) on images

JimBobSquarePants · 2018-04-26T05:41:31Z

@antonfirsov Surprising comparison results here. 32bit passing 64 bit failing? WTF?

antonfirsov · 2018-04-26T10:43:12Z

@JimBobSquarePants the difference is not big however. I suspect the root cause is a difference between SIMD and software implementations of Vector4 operations.

Hope it's not CPU dependent & locally reproducible.

antonfirsov · 2018-04-28T00:11:25Z

tests/ImageSharp.Tests/Processing/Transforms/ProjectiveTransformTests.cs

+                image.Mutate(i => { i.Transform(m); });
+
+                string testOutputDetails = $"{taperSide}-{taperCorner}";
+                image.DebugSave(provider, testOutputDetails);


@JimBobSquarePants added some tests to cover CreateTaperMatrix() parameters. I may get it wrong but the XxxOrYyy stuff might be the inverse of the intended. Can you check the output?

I'll have to do some tests locally to ensure the output is expected. The class is based on a Skia example and and old Charles Petzold blog post.

antonfirsov

I think I managed to workaround the floating point issue. It is related to Vector2 again.

The API is perfect. If CreateTaperMatrix() parameters behave as you originally intended, we can add the reference images for Transform_WithTaperMatrix and merge the PR. (But we probably need to invert TaperCorner behavior.)

antonfirsov · 2018-04-28T00:12:37Z

src/ImageSharp/Processing/Transforms/ProjectiveTransformHelper.cs

+        /// <param name="taperCorner">An enumeration that indicates on which corners to taper the rectangle.</param>
+        /// <param name="taperFraction">The amount to taper.</param>
+        /// <returns>The <see cref="Matrix4x4"/></returns>
+        public static Matrix4x4 CreateTaperMatrix(Size size, TaperSide taperSide, TaperCorner taperCorner, float taperFraction)


Cool suff! Covers basic cases & good for testing without getting outside the scope of the library.

Thanks for adding the extra tests, they affirm the expected output well.

codecov · 2018-04-28T01:56:59Z

Codecov Report

Merging #546 into master will increase coverage by 0.41%.
The diff coverage is 98.19%.

@@            Coverage Diff             @@
##           master     #546      +/-   ##
==========================================
+ Coverage   88.28%   88.69%   +0.41%     
==========================================
  Files         838      840       +2     
  Lines       35186    35300     +114     
  Branches     2538     2544       +6     
==========================================
+ Hits        31064    31310     +246     
+ Misses       3377     3243     -134     
- Partials      745      747       +2

Impacted Files	Coverage Δ
...Sharp/Processing/Transforms/TransformExtensions.cs	`54.54% <ø> (+18.18%)`	⬆️
.../Transforms/Processors/AffineTransformProcessor.cs	`100% <100%> (ø)`	⬆️
...s/Processors/InterpolatedTransformProcessorBase.cs	`100% <100%> (ø)`	⬆️
...nsforms/Processors/ProjectiveTransformProcessor.cs	`100% <100%> (+100%)`	⬆️
...ests/Processing/Transforms/AffineTransformTests.cs	`98.52% <100%> (-0.02%)`	⬇️
.../Processing/Transforms/ProjectiveTransformTests.cs	`97.22% <97.22%> (ø)`
...Processing/Transforms/ProjectiveTransformHelper.cs	`97.22% <97.22%> (ø)`

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JimBobSquarePants · 2018-04-30T03:32:53Z

@antonfirsov Back to you; do your magic please.

antonfirsov · 2018-04-30T15:33:58Z

Bottom-LeftOrTop:

Bottom-RightOrBottom:

@JimBobSquarePants you sure TaperCorner works fine? Isn't it the inverse of the intended? Or I'm getting it wrong?

JimBobSquarePants · 2018-05-01T01:31:04Z

Bottom-LeftOrTop:

Bottom-RightOrBottom:

@JimBobSquarePants you sure TaperCorner works fine? Isn't it the inverse of the intended? Or I'm getting it wrong?

@antonfirsov I think it's correct.

In the first example we are choosing to taper the "Bottom" side on either the "Left" or "Top" corner. Since "Left" is possible regardless of whether "Top" is impossible "Left" is chosen.

In the seconds example we are choosing to taper the "Bottom" side on either the "Right" or "Bottom" corner. Since "Right" is possible regardless of whether "Bottom" is possible "Right" is chosen.

The original article with source code for the class this is based on can be found here:

http://www.charlespetzold.com/blog/2009/07/Taper-Transforms-with-Matrix3DProjection-An-Analytical-Approach.html

SkiaSharp have an example here:
https://docs.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/graphics/skiasharp/transforms/non-affine

I can attempt to compare against the demos. (Feel free to have a look first though) https://developer.xamarin.com/samples/xamarin-forms/SkiaSharpForms/Demos/

JimBobSquarePants · 2018-05-02T05:19:18Z

Behaviour confirmed 😄

antonfirsov · 2018-05-02T10:35:07Z

LOL .. that image!

But isn't the first example tapering towards the right corner? Or should I start over my English courses? :D

JimBobSquarePants · 2018-05-02T10:44:54Z

Yeah I felt a little guilty posting that 😄

Here's the comparative images side by side and scaled.

Bottom-LeftOrTop:

Bottom-RightOrBottom:

antonfirsov · 2018-05-02T10:54:56Z

It seems to me that the original definition is wrong, because the "tapered" edge is not on the left (but on the right!) corner of the image.

But again, it might be just my wrong understanding of the term "tapering".

JimBobSquarePants · 2018-05-02T11:04:08Z

Ah no... I can see how this is confusing but the language is correct.

A classic definition of taper would be

to become progressively smaller toward one end.

You're tapering towards the bottom edge and choosing which corner to taper with.

antonfirsov · 2018-05-02T21:43:26Z

I understand now.
The PR is in a perfect state, sorry for the long linguistic session! :)

antonfirsov · 2018-05-02T21:47:18Z

Well .. almost. Haven't noticed, the tests are failing again, gonna check it out now.

JimBobSquarePants · 2018-05-03T00:35:56Z

Haha! 🤣

It's amazing really the trouble we've had with this. Will be happy when MS push all their Vector2 fixes.

JimBobSquarePants added 10 commits March 23, 2018 10:41

Get transforms working

f79024e

Merge branch 'master' into js/projective-transforms

5f9aff4

Init projective transform testing

815264d

Merge branch 'master' into js/projective-transforms

2d48d57

Optimize transforms and reduce struct copying.

c6cce4d

Begin adding tests.

7f7c513

Revert ref extensions.

16183a4

Add another test and make some methods public

75d6729

Rename helper

c45fb1a

Enable tests

a3dcedc

JimBobSquarePants requested a review from antonfirsov April 26, 2018 03:48

workaround Vector2 CLR bug + cover taper parameters with tests

5d9dd8f

antonfirsov reviewed Apr 28, 2018

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antonfirsov requested changes Apr 28, 2018

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antonfirsov added 2 commits April 28, 2018 02:33

use "fixed" reference output + reduce tolerance

368903a

OK, I give it up,let's increase the tolerance again.

92d50ba

JimBobSquarePants added 4 commits April 28, 2018 18:36

Merge branch 'master' into js/projective-transforms

bca48a6

Merge branch 'master' into js/projective-transforms

0d08cb7

Merge branch 'master' into js/projective-transforms

003debc

Enable comparison tests.

ec04a6f

Merge branch 'master' into js/projective-transforms

4f9edfa

antonfirsov approved these changes May 2, 2018

View reviewed changes

antonfirsov added 2 commits May 2, 2018 23:54

increase tolerance for Transform_WithTaperMatrix

df19dd0

floating points from hell

775aa90

JimBobSquarePants merged commit 3cc7caa into master May 4, 2018

JimBobSquarePants deleted the js/projective-transforms branch May 4, 2018 05:54

dependabot bot mentioned this pull request Oct 19, 2025

Bump the nuget group with 1 update ewdlop/bepuphysics2Fork#1

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JimBobSquarePants commented Apr 26, 2018

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