View Source

h1. OAC Representation
We use the [Open Annotation Collaboration (OAC)] (OAC) ontology to represent image annotations.
- the target is an [oac:ConstrainedTarget|] that uses:
-- [cnt:ContentAsText|] (Inline Constraint) to embed the SVG info in RDF
-- [oac:SvgConstraint|] to represent the annotated region
-- The OAC spec says "should be a single SVG element, not an entire SVG canvas" so it seems an implicit <svg> tag is assumed around the SVG data:
{code:xml}<svg version="1.1" xmlns="">...</svg>{code}
-- We use an [Image Fragment|] (#xywh) to associate a [#View Configuration] with the annotation.
-- The SvgConstraint is given in absolute image coordinates, even for deep-zoom images, no matter what the View Configuration is
- the body is an oac:Body and includes the same properties as for [Data Annotation|Open Annotation Collaboration (OAC)#RS Data Annotation in OAC]
-- if a reply is made, it is attached to the same oac:ConstrainedTarget and has a link rso:reply_to pointing to the original annotation

h2. Example
We start with an example of the basic case, then treat some special requirements.
@base <>.
@prefix oac: <>. # Open Annotation Collaboration
@prefix dcterms: <>. # Dublin Core Terms
@prefix rso: <>. # ResearchSpace Ontology
@prefix cnt: <>. # Content in RDF
@prefix xsd: <>. # XML Schema Datatypes

<DT219363.tif/annot/1> a oac:Annotation;
oac:hasBody <DT219363.tif/annot/1/body>;
oac:hasTarget <DT219363.tif/annot/1/target>.
<DT219363.tif/annot/1/target> a oac:ConstrainedTarget;
oac:constrainedBy <DT219363.tif/annot/1/constraint>;
oac:constrains <DT219363.tif#xywh=1000,900,250,250>.
<DT219363.tif#xywh=1000,900,250,250> dcterms:isPartOf <DT219363.tif>.
<DT219363.tif/annot/1/constraint> a oac:SvgConstraint, cnt:ContentAsText;
dc:format "SVG";
cnt:chars """<polyline points="1125,1025 1150,1025 1135,1050 1125,1025"/>""".
<DT219363.tif/annot/1/body> a oac:Body;
rso:P2_annotation_category rst-annotation-category:observation;
rso:P3_has_title "The nose is large and wrinkled";
dcterms:creator rs-users:VladimirAlexiev;
dcterms:created "2002-03-15"^^xsd:date...

h1. Special Requirements

h2. View Configuration
Image annotations can be checked/unchecked (made visible/invisible). But we also need a concept of "currently selected annotation" so that:
- When the user Replies, the reply is attached to the current annotation
- When the user selects an annotation, the image view is set so that the annotation is fully visible

There are two alternatives to accomplish the latter:
# *Bounding Box*: set the view to the bounding box of the annotated region, thus maximizing it
# (current decision) *Saved View*: set the view to what it was when the annotation was created or last edited.
This is better since:
#- The user may want to see more than the annotated region (eg more annotations or something outside of the region)
#- If the same view was shared by several annotations then the view won't "jump"

- (current decision) Use [Image Fragment|] (#xywh) to select the view area.
-- This should work even for deep-zoom images, since we can compute the required zoom level from the fragment area
-- If we need to save the zoom level, we should represent it as a real number to accommodate smooth zoom in the future, although IIP represents the zoom level as log2 (eg 5 means 2^5=32)
-- Depending on [answer from OAC mlist|!topic/oac-discuss/VEJrBFJAvks] we may decide to store the view configuration in a different way
- Could use [viewBox|] attribute [that is available|] on svg, symbol, image
-- Could specify the <svg> canvas area to include the complete image, then view part of it:
{code:xml}width="2000" height="2000" viewBox="1000 900 250 250"{code}
- could use
transform="translate(1000,900) scale(5)"
- Could use [preserveAspectRatio|] attribute that is available on svg, symbol, image, marker, view.
-- The screens of two users may be different, so the available image area may be different (including different aspect ratios).
To preserve center of image area, aspect ratio, zoom level; while allowing the image fringes to be cropped, maybe this can help:
{code:xml}preserveAspectRatio="XMidYMid slice"{code}
- [SVG Linking|] describes a number of options to link to a particular area of an SVG canvas

h2. Markers
A marker (point, pin) is a point carrying a particular symbol (shape).
OAC does not support markers, since an image annotation target is represented as an [oac:SvgConstraint|], which consists of an [SVG shape|]: path, rect, circle, ellipse, line, polyline, polygon, g(roup) (eg used to create a region with a hole).

A ResearchSpace requirement is to allow *markers* as annotation targets
- A marker is characterized by:
-- Point (coordinates)
-- Symbol (marker type or shape)
--- Can be any geometry or image
--- Should not scale with image zoom (should remain constant in size)
--- RS3.3 will support a small circle and a pin (as on Google Maps)
- It could also be useful to support *multi-markers* consisting of several points, where the symbol is fixed

SVG does not have the concept of a standalone point/marker, but there are several alternatives:

h3. 1. Use-Symbol
Represent the point as [use|] and the shape as a [symbol|]
- "use" carries x,y,width,height
- "symbol" carries the shape geometry or image
- pro: supports constant size through explicit width,height
- cons: doesn't support multi-markers
- cons: requires <svg><defs> while it's unclear whether OAC supports <svg>
<defs> <symbol id="Triangle"> <polyline points="0,0 10,5 0,10 0,0" /> </symbol> </defs>
<use x="1125" y="1025" width="10" height="10" xlink:href="#Triangle" />
- if the symbol contains a single graphics element, then <symbol> can be skipped:
<defs> <polyline id="Triangle" points="0,0 10,5 0,10 0,0" /> </defs>
<use x="1125" y="1025" width="10" height="10" xlink:href="#Triangle" />

h3. 2. Polyline-Marker
Represent the point as [polyline|] and the shape as [marker|]
- Markers are styling elements (symbols) "attached to one or more vertices of ‘path’, ‘line’, ‘polyline’ and ‘polygon’ elements"
-- the marker carries markerWidth,markerHeight
-- use markerUnits="userSpaceOnUse" so the marker size is absolute (the default markerUnits="strokeWidth" scales the marker according to the path's stroke)
-- use the default orient="0" so the marker has absolute orientation (orient="auto" rotates the marker along the path's tangent)
- A single point can be represented as a singular (degenerate) polyline; a multi-marker as a polyline with no stroke
- pro: supports multi-markers
- cons: requires <svg><defs> while it's unclear whether OAC supports <svg>
<defs> <marker id="Triangle"> <polyline points="0,0 10,5 0,10 0,0" /> </marker> </defs>
<polyline points="1125,1025" marker="url(#Triangle)" />

h3. 3. Class "marker"
Use a normal graphics element (shape or image), signify it as a marker using [class|]
- cons: to achieve constant size, requires a [transform|] or manual size recalculation
- cons: does not support multi-markers
- pro: does not require <svg><defs>
{code:xml}<polyline points="0,0 10,5 0,10 0,0" transform="translate(1125,1025)" class="marker" />{code}

This is the current decision, but it's unclear whether it is optimal

h2. Image URLs
Since Image URLs are used to bind the annotation, they need to be stable. The following should be taken into account (see [RS URLs and URIs]):
- the possibility of per-project domains
- server migration from development to testing to production: research and finally publication
- need to support deep zoom images
- possible changes in image format or server technology
- for a deep zoom image, the IIP URL specifies the zoom level and tile origin
- eg an image served by IIP and converted from TIF to JPG has URL like ([Rembrandt data#Image URLs]):

TODO TODO: not yet resolved

|T|M|F| |1331577959401||Vlado: ask in OAC mailing list if an implicit <svg> tag is assumed around the SVG data|
|F|M|F| | | |Stanislav: test if SVG singular polyline with marker works|
|F|M|F| | | |Vlado, Jana: figure out [#Image URLs] in the case of IIP|
|F|M|F| | | |Vlado, Stanislav: work out and document a [#View Configuration] example|