General Warping Questions

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General Warping Questions

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This chapter covers some general warping questions related to the Mesh Editing Mode in the Pandoras Box Preview.
- What is the difference between a Mesh and a FFD?
- How many FFD and Mesh points should I choose?
- What to get a good Mesh?
For other topics regarding the Preview tab please see the introductory chapter. If you are interested in the Warper tool, please follow this link.

Mesh versus FFD

This paragraph describes the difference between a Mesh point (Vertex) and an FFD point, thus it is covering a fundamental function of the warping process.
The pictures below show a 2D plane with a green-colored 10x10 Mesh and orange-colored 3x3 FFD.

See here the differences between moving a FFD control point and moving a Mesh point. Please note that the FFD is only a helping tool to set up the Mesh, you will not see the FFD in the final object.


This shows the grid without any editing. The green lines represent the Mesh, the orange lines the FFD.


The top left FFD control point is moved further down. The whole Mesh is affected by this change: the horizontal lines are bend together on the top left side, the Meshes outline gets curved.
This effect can be of advantage or disadvantage. In the beginning of the warping process it can simplify and accelerate the workflow as it is not necessary to move each individual Mesh point. The further the warping process develops the more it is necessary to apply changes to particular pixels only. At this point the FFD is not sufficient any more as it affects large areas of the grid.


Now only one Mesh point is moved. Only the segment lines between the moved point and the four neighbor Mesh points are affected by this.
The more you are experienced with warping the better you will be able to answer the question how many Mesh points a grid should have. If too little points are chosen it won't be possible to apply the detail changes that are necessary. This is especially crucial when setting up Meshes for a softedge projection as the pixels must overlap each other exactly in the overlapping area.
If too many points are chosen, the warping process is lengthened unnecessarily as all points must be adjusted.

Setting up FFD Control Points and Mesh Segments

Before you start moving the control points you have to decide with how many FFD control points and Mesh segments you want to work. The FFD and Mesh count is set up in the Sub Mesh Inspector. The perfect amount depends on your screen surface and outline.
The more warping projects you have done, the faster you will be able to tell the best FFD and Mesh count. If you are not sure in the beginning, simply make a guess and start warping. You will see quite fast, that you have picked too many or too less FFDs. Too many FFD points are not that bad, it might take more time during the FFD-phase but you could save time during the Mesh-phase - at least if not way too many FFDs were picked. If you have picked too less FFD points you will notice that you will have to move Mesh points at a very early stage of warping. This will definitely be more time-consuming than starting all over with a new Mesh. In addition, it is easier to obtain a good quality Mesh (as described below) with FFDs instead of offsetting single Mesh points too far. If you like, you can save the current project and have a quick look whether a new Mesh with more FFDs does give you better results and eases your work. This can be found out in a few seconds but save minutes or even hours.


Example 1, a 4x3 FFD
A simply bend screen will go well with only 3 vertical FFD control points. Horizontally there needs to be done more warping, thus 4 FFDs work better. The more smooth the outline has to be, the more horizontal Mesh segments you should take. In this case there are 20 Mesh segments.


Example 2, a 6x3 FFD
Curved screens that are more complex will be easier to handle if you increase the amount of FFD control points. In this case there are 6 horizontal control points and still 3 vertical ones.
The curved outline of the Mesh is the result by only moving the FFD control points.


Example 3, a 5x5 FFD
Spherical screens are bend in all directions. They require to increase the amount of vertical FFD points as well. In the depicted example a Mesh with an 5x5 FFD is shown. If the projector looks straight on the equator, the Mesh deformation will be quite homogenous and the FFD should look similar to the example.

The amount of Mesh segments can be altered as long as working only with the FFD control points. As soon as a Mesh point is moved, the amount should not be changed any more.
In general, the amount of Mesh segments depends on how exact the warp needs to be. Firstly this is a question how complex or detailed the screen is, including the outline, as shown in the above example with the simply bend screen. If the screen is quite flat itself but has a very detailed outline, it could be a faster solution to create a mask instead of increasing the Mesh count.

Secondly, it is important whether you are projecting with single projectors only, or if several projectors overlap each other. Within the softedge area the pixels from both projectors must overlay each other perfectly. This requires a higher Mesh count. As a rule of thumb, at least 7 Mesh segments should lie within the overlap area.

By the way, it could be helpful to work with Mesh segments that have the same height as width. If your projector has an aspect ratio of 16:9, you could set up a Mesh count of 16 by 9 during the FFD-phase and increase it to 32 by 18 or even 48 by 27 before starting the Mesh-phase.

The Warping Workflow

A good quality Mesh refers to a Mesh where the Mesh lines are uniformly distributed on the screen. For example, if the screen is 2m wide, and there are 20 horizontal Mesh segments, each segment should be 10cm wide. If this is not achieved sufficiently, and you project text that moves across the screen, it would scroll unevenly. Wherever there are smaller distances between Mesh lines, the text would be smaller too. Wherever there are larger Meshes, the text is enlarged.
If your content does not contain critical movements or visible geometrical forms, you can warp a little more rough. So before you start warping, or before you spend too much time within the last phase, check the content and decide how perfect the result really needs to be.

For some people it is quite hard to perceive equal distances. To fasten and ease the warping workflow, try to mark certain points on the screen. If you mark for example every 40cm with tape, it will be much easier to arrange the Mesh equally. If you cannot tape on the screen, a rotatory laser can be helpful as well.
For the same reason it can be worth the time to create special test patterns. This is definitely recommended when projecting on complex geometries and the later on used content refers to the geometry.

Keeping this mind we can now start warping. The golden rule is always to warp as much as possible with the FFD, but not more than necessary. Or in other words: the FFD is for the coarse adjustment and the Mesh for fine-tuning.

Whilst moving the FFD points, match the Mesh outline (= content outline) as good as possible with your screen outline. At the same time keep an eye on the distance between the horizontal lines and between the vertical lines. As soon as you recognize that moving an FFD point helps within a small Mesh area but "destroys" an higher number of other Mesh areas it is better to finish with the FFD-phase. Decide for a final Mesh count and move on to moving Meshes. Here you will see that the better the FFD was adjusted, the less time needs to be spend for finishing.

When warping with overlapping Meshes, you are done with warping as soon as all Mesh lines overlay each other. This can be seen easily when both Meshes have a different colors as the resulting color will be the sum. The closer the audience sees the projection, the more perfect the overlay needs to be.

In general it is possible to do a rough warp and start programming with it in the timeline. Later on, when there is enough time or when it is sure the projector or screen will not move, you can go back in the Editing Mode and finalize it.

The next chapter explains the Planar and Perspective UV Mapping Mode.