Difference between revisions of "Talk:Link to BLine"

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In an [http://dooglus.rincevent.net/synfig/logs/2008/%23synfig-2008-03-03.log IRC discussion] it was suggested that it should to be possible to:
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== Circular references problem ==
  
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1) Provide an easy way for the user to attach a vertex of a bline to another bline but without the need to create a new vertex over that bline. That allows the simplification of the animation of complex compositions due to the use of less vertices.
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As of http://dooglus.rincevent.net/synfig/logs/2008/%23synfig-2008-03-06.log the circular references problem appeared.
  
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See Yoyobuae's example:
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In my (Zelgadis) opinion we have 3 problems:
  
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[[Image:sif_icon.png|16px]][http://members.lycos.co.uk/yoyobuae/bline_attached_2sections.sifz bline attached]
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# Detecting loops
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# Creating loops (handling detected loop while creating)
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# Dealing with loops (handling modifications of linked-to-bline vertices engaged inloops)
  
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In that example there are some vertices converted to "BLine Vertex" and some tangents converted to "BLine Tangent". Manipulating the exported parameter that represents the position of the vertex along the curve it is possible to easily modify the shape of the composition allowing a better morphing of the shapes created.
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=== Detecting loops ===
  
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Currently it is done through various export/connect manipulations.
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00:22 < dooglus> it turns out that every node in the network has a 'time last modified' field, so checking that will allow us to detect loops
  
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'''Suggestion:''' It's desired that this attachment be achieved visually, by using the mouse and context menus.  The current method requires the use of exported values and parameter linking, which is less intuitive and also is slower.
 
  
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'''Possible realization way:'''
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=== Creating loops ===
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# Select the duck you want to link to bline. It could be
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While creating loop we have a Circular references problem.
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## bline vertex duck
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## bline tangent duck
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## or real duck
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# Right click on the bline (not the bline vertex!)
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# Link
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## Linking creating appropriate convert sequences.
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## if selected vertex duck, it placed on bline at position where clicked
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## if selected tangent duck, it links with bline tangent at click position
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### Which one of two? Maybe both? --[[User:Zelgadis|Zelgadis]] 12:27, 3 March 2008 (EST)
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### We need to scale tangents as in example above
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## if selected real duck, it links with bline width at click position
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### currently width value at specific point of bline couldn't be easily retrieved via export/convert combinations --[[User:Zelgadis|Zelgadis]] 12:27, 3 March 2008 (EST)
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# If was more than one duck selected for linking (i.e. bline vertex duck and bline tangent duck) then they Amount values (in "Bline Vertex"/"Bline Tangent" converts) must be linked  to ensure that they retrieved from the same position on a bline (if changed).
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''' Another possible realization way:'''
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In example http://zelgadis.profusehost.net/files/synfig/circular_references/bline_link_loop_crash.sifz we have 3 blines A,B,C.
  
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The main idea is to change the way that vertex deletion is handled. If a vertex is deleted and there are other vertices linked to that vertex, then those vertices should become linked to a specific position on the bline (where the deleted vertex was)
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http://zelgadis.profusehost.net/files/synfig/circular_references/cr01.jpg
  
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Example: We have bline A and bline B. Bline A have vertices A1,A2,A4. Bline B have vertices B1 and B2. To link vertex B1 to bline A between vertices A2 and A4 user must:
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We want to link C2 to Bline A.
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# create new vertex A3 between A2 and A4 on bline A. (Insert item (smart))
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# do linking of B1 to A3 as he likes (user can link tangents or not and can define which tangent linked to which)
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# remove vertex A3 (Delete item (smart))
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Result: B1 vertex becomes linked to bline A according to way it was linked to A3.
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2) Derived from that request it appears the need to be able to manipulate the ducks that have been converted to a composition. That is: inverse manipulation of valuenodes.
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But if we do it, C2 will change it's position (it must lay on the Bline A). As result, the shape of Bline C will change and B2 (which linked to Bline C) will move. It will change the shape of Bline B -> A2 will move -> shape of Bline A changed -> C2 not on Bline A. Сontradiction.  
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For the moment there are some convert types that allow inverse manipulation and only for certain parameters. For example tangents allow inverse manipulation (you modify the x/y position and it calculates the r,theta values. It is know that inverse manipulation leads on some inconsistencies. If the the function that convert a type into other is not biunivocal then it can produce undesired results form the point of view of the final user. For example, the Scale convert type would not have any inconsistency due to it has a easy computable reverse function (the reciprocal)
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Considering that and taking account that inverse calculation of ValueNodes based on ducks position could need some extra computation, it would be good if some convert types can have inverse manipulation.
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We could move C2 to place it on Bline A again but then it'll end up not laying on Bline A again.
  
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'''The proposal is''' that BLine Vertex converted types allow inverse manipulation. So, manipulating a vertex duck you move the attached vertex along the bline and also match the tangent of the attached vertex to the bline's tangent at that position.
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So, if we detected loop, we must do linking in special way.
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# As the user invokes "Link to Bline" command for C2, we detected loop C2->B2->A2->C2. <br> http://zelgadis.profusehost.net/files/synfig/circular_references/cr02.jpg
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# We must solve  system of equations to determine position of C2 when: a) it belongs to Bline A; b) the shape of Bline A determined by position of C2;
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## How such system of equations will look like? See [[Linking to Blines - Equations]].
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# We placing C2 to determined position. So it definitely on Bline A. Blines A, B, C are updated according to new position of C2.
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# We linking C2 to Bline A. Position of C2 not changed while linking, so all blines not recalculated and Bline A keeps its shape and recursion not taking place.
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'''Another possible solution (ah-hoc):'''
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# As the user invokes "Link to Bline" command for C2, we detected loop C2->B2->A2->C2. <br> http://zelgadis.profusehost.net/files/synfig/circular_references/cr02.jpg
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# We adding a new vertex A3 on Bline A where C2 must be connected. Then we moving (moving, not linking) C2 to the position Bline A where user had clicked. The blines C,B,A will change so C2 probably won't be on A.
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# Just link C2 -> A3
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=== Dealing with loops ===
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To be written... if needed to.

Latest revision as of 16:50, 22 October 2008

Circular references problem

As of http://dooglus.rincevent.net/synfig/logs/2008/%23synfig-2008-03-06.log the circular references problem appeared.

In my (Zelgadis) opinion we have 3 problems:

  1. Detecting loops
  2. Creating loops (handling detected loop while creating)
  3. Dealing with loops (handling modifications of linked-to-bline vertices engaged inloops)

Detecting loops

00:22 < dooglus> it turns out that every node in the network has a 'time last modified' field, so checking that will allow us to detect loops


Creating loops

While creating loop we have a Circular references problem.

In example http://zelgadis.profusehost.net/files/synfig/circular_references/bline_link_loop_crash.sifz we have 3 blines A,B,C.

cr01.jpg

We want to link C2 to Bline A.

But if we do it, C2 will change it's position (it must lay on the Bline A). As result, the shape of Bline C will change and B2 (which linked to Bline C) will move. It will change the shape of Bline B -> A2 will move -> shape of Bline A changed -> C2 not on Bline A. Сontradiction.

We could move C2 to place it on Bline A again but then it'll end up not laying on Bline A again.

So, if we detected loop, we must do linking in special way.

  1. As the user invokes "Link to Bline" command for C2, we detected loop C2->B2->A2->C2.
    cr02.jpg
  2. We must solve system of equations to determine position of C2 when: a) it belongs to Bline A; b) the shape of Bline A determined by position of C2;
    1. How such system of equations will look like? See Linking to Blines - Equations.
  3. We placing C2 to determined position. So it definitely on Bline A. Blines A, B, C are updated according to new position of C2.
  4. We linking C2 to Bline A. Position of C2 not changed while linking, so all blines not recalculated and Bline A keeps its shape and recursion not taking place.

Another possible solution (ah-hoc):

  1. As the user invokes "Link to Bline" command for C2, we detected loop C2->B2->A2->C2.
    cr02.jpg
  2. We adding a new vertex A3 on Bline A where C2 must be connected. Then we moving (moving, not linking) C2 to the position Bline A where user had clicked. The blines C,B,A will change so C2 probably won't be on A.
  3. Just link C2 -> A3

Dealing with loops

To be written... if needed to.

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