Questions regarding DEPARE visualizaton

[thomas.e]

Hi forum,

I apologize if this is the incorrect place to post this questions.

I'm working on visualizing some ECN data, where I have been validating my efforts so far using OpenCPN.

However, I have a couple of questions regarding the data visualization procedure, since the data format is a bit odd.


When extracting the DEPARE layer, it contains various features. A given feature seems to at least contain one polygon describing the depth areas. However, sometimes there seems to be a list of multiple polygons, where the first listed one is the bounding one, and the subsequent ones are within it.

1) For a given feature, are all the polygons within bounding polygon supposed to be "blank"/empty, i.e. not related to the overall depth value at hand?

2) Is there ever any overlap between the bounding polygons?

3) What is the plotting order in OpenCPN? I'm thinking something like: Depth areas -> tracks -> land -> top marks

Please excuse me if some of the terminology is lacking, I'm fairly new at this.
I have also attempted to consult the Github repository for clues.

Thank you for reading this.

Best regards,
Thomas

[GordMay]

Greetings and welcome aboard the CF, Thomas.

[bdbcat]

Thomas...


1. You are effectively correct. Polygons are rendered using OpenGL "Winding rules". Clockwise regions are filled, anti-clockwise are empty.


2. Polygons can overlap, but not generally if they are intended to be filled. DEPARE polys do not overlap.


3. There is no defined plotting order in S57. The S52 PLIB defines a "display priority" parameter that determines the final rendering order. Generally, LNDARE and DEPARE are drawn first, so that POINT and LINE objects render on top,


Good Luck
Dave

[thomas.e]

Originally Posted by bdbcat

Thomas...


1. You are effectively correct. Polygons are rendered using OpenGL "Winding rules". Clockwise regions are filled, anti-clockwise are empty.


2. Polygons can overlap, but not generally if they are intended to be filled. DEPARE polys do not overlap.


3. There is no defined plotting order in S57. The S52 PLIB defines a "display priority" parameter that determines the final rendering order. Generally, LNDARE and DEPARE are drawn first, so that POINT and LINE objects render on top,


Good Luck
Dave

Hi Dave, thank you very much for your response.

I have some follow-up questions regarding 1), since I'm unsure about this CW and CCW notation. I'm using GDAL/OGR (https://gdal.org/) to obtain a given feature for the DEPARE layer. Where the code may return something like the following:

feat is some feature within DEPARE

feat.GetGeomFieldCount() #Returns 1
feat.GetGeometryRef().GetGeometryCount() # Returns 9

From the above I assume there are 9 geometries within the feature, where the following

feat.GetGeometryRef().GetGeometryRef(0).GetPoints()

represents the boundary polygon and the subsequent (1-8) geometries represent polygons within the bounding polygon.

My assumption is that the first polygon is always the bounding one, is this always the case?

The current approach is to colour the bounding polygon and remove colour specified by the geometries following the bounding ones.


I hope my questions make sense. Thank you in advance.

Have a nice weekend,
Thomas

[thomas.e]

Originally Posted by bdbcat

Thomas...


1. You are effectively correct. Polygons are rendered using OpenGL "Winding rules". Clockwise regions are filled, anti-clockwise are empty.


2. Polygons can overlap, but not generally if they are intended to be filled. DEPARE polys do not overlap.


3. There is no defined plotting order in S57. The S52 PLIB defines a "display priority" parameter that determines the final rendering order. Generally, LNDARE and DEPARE are drawn first, so that POINT and LINE objects render on top,


Good Luck
Dave

Hi Dave,

Thank you very much for your response.

I have some follow-up questions regarding 1), since I'm unsure what you mean exactly by the CW and CCW notation.

My implementation interfaces with the maps using GDAL/OGR (Similar to OpenCPN). For a given feature in layer DEPARE, a situation may be as follows:

feat.GetGeomFieldCount() # Returns 1
feat.GetGeometryRef().GetGeometryCount() # Returns 9

For a given feature, a geometry with 9 geometries exist, where

feat.GetGeometryRef().GetGeometryRef(0).GetPoints()

contains the polygon information for the bounding polygon, and the subsequent indices (1-8) contains information about the polygons within the bounding polygon. My approach is then to colour the entire bounding polygon, and then remove colour as stated by the remaining polygons.

It is merely an assumption on my end, that the first geometry is a the bounding polygon. Can you confirm this?

I hope my question makes sense. Thank you very much in advance.

Have a nice weekend,
Thomas

[thomas.e]

I'm sorry about the double post. I am having some issues with the forum in my browser.