Coordinate Systems

Gondwana currently provides seven coordinate systems supporting different tile map geometry.

A coordinate system defines how a SceneLayer interprets its grid:

• where each grid coordinate is placed in world pixel space
• what shape a tile occupies
• how world pixels map back to tiles
• which tiles count as neighbors
• how tile bounds, overhang, and wrapping are calculated

This is related to, but different from, Coordinate Spaces. Coordinate spaces describe where a value lives (Grid -> World -> Screen). A coordinate system supplies the layer-specific rules for the Grid -> World part of that pipeline.

In the diagram, blue is tile (0,0), green is (1,0), and orange is (0,1).

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Selecting a coordinate system

The usual approach is to select the coordinate system when adding the layer to a scene:

using System.Drawing;
using Gondwana.Drawing.Coordinates;
using Gondwana.Scenes;

var scene = new Scene();

SceneLayer groundLayer = scene.AddLayer(
    columnCount: 100,
    rowCount: 100,
    width: 64,
    height: 32,
    coordinateSystem: CoordinateSystemTypes.IsometricRhombic);

Orthogonal is the default when coordinateSystem is omitted.

An existing layer can also be changed:

groundLayer.CoordinateSystemType = CoordinateSystemTypes.ObliqueLeft;

Changing the property replaces the layer's internal coordinate strategy. It does not convert the map, resize the tiles, or reshape the artwork for the new geometry. In normal use, choose the system when the layer is created and make the tile dimensions and artwork match it.

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ISceneLayerCoordinates

ISceneLayerCoordinates is the internal strategy interface behind every coordinate system. SceneLayer.CoordinateSystemType selects one of its internal implementations.

The interface is deliberately internal: game code normally works through SceneLayer, not through a coordinate implementation directly. The principal public entry points are:

PointF worldPx = groundLayer.GridToWorldPx(new PointF(4, 7));
PointF grid = groundLayer.WorldPxToGrid(worldPx);

SceneLayerTile? tile = groundLayer[4, 7];
SceneLayerTile? neighbor = tile is null
    ? null
    : groundLayer.GetAdjacentTile(tile, CardinalDirections.E);

PointF wrapped = groundLayer.WrapGrid(new PointF(-1, 7));

Internally, the interface provides eight operations.

Member	Responsibility
GetAnchorPixelAtSceneLayerCoordinates	Converts a grid position to the projection-defined world-pixel anchor used to place the tile. For a rectangle this is its top-left corner; for an isometric diamond it is its top vertex.
GetSceneLayerCoordinatesAtPixel	Converts a world-pixel position back to grid coordinates. Orthogonal, isometric, and oblique systems can return fractional coordinates. The hex systems resolve the containing or nearest hex and return integer-valued coordinates in a PointF.
GetSceneLayerTilesInPixelRange	Finds tiles whose rendered bounds intersect a world-pixel rectangle. Rendering and dirty-region logic use this to gather candidate tiles efficiently.
GetPixelRangeForTile	Returns one tile's world-pixel bounding rectangle.
GetPixelRangeForTileList	Returns the union of the world-pixel bounds for several tiles.
GetAdjacentSceneLayerTile	Applies the topology's neighbor rules. Square-derived systems can expose eight directions; hex systems expose six.
GetPolygonPts	Returns the tile footprint as world-pixel polygon vertices for hit testing, grid outlines, bounds, and debug drawing.
FindEquivalentSceneLayerCoordinates	Wraps grid coordinates into valid layer bounds using modulo arithmetic.

Several methods accept includeOverhang. When false, the result describes the base tile footprint. When true, the result is expanded to account for artwork that extends beyond that footprint.

The interface is concerned only with grid space and world pixel space. Camera position, viewport placement, parallax, and zoom are handled later when world pixels become screen pixels.

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Supported coordinate systems

1. Orthogonal

CoordinateSystemTypes.Orthogonal

Orthogonal is the conventional rectangular tile grid. Columns run left to right, rows run top to bottom, and every tile occupies an axis-aligned rectangle.

For a tile size of W x H:

worldX = column * W
worldY = row    * H

The layer origin is also applied to the final world position.

Orthogonal supports the eight compass neighbors: N, NE, E, SE, S, SW, W, and NW.

Use it for traditional top-down maps, platformers, board layouts, and any map whose logical and visual axes should remain horizontal and vertical.

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2. Isometric rhombic

CoordinateSystemTypes.IsometricRhombic

Isometric rhombic is Gondwana's classic tightly packed diamond projection. Moving along one grid axis travels down and to the right; moving along the other travels down and to the left.

Ignoring the layer origin, its anchor transform is:

worldX = (column - row) * W / 2
worldY = (column + row) * H / 2

The anchor is the diamond's top vertex. A rectangular grid therefore becomes a rhombus-shaped arrangement in world space.

The underlying grid still has eight addressable compass directions. The projection changes where those neighbors appear visually; it does not change the layer's rectangular (column, row) storage.

Use it for the familiar 2.5D isometric-map appearance in which adjacent diamond tiles share edges.

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3. Isometric axial

CoordinateSystemTypes.IsometricAxial

Isometric axial uses a horizontal column axis and a diagonal row axis. Each tile is drawn as a diamond, and the two axes form an affine basis that packs those diamonds edge-to-edge.

Ignoring the layer origin, its anchor transform is:

worldX = column * W + row * W / 2
worldY = row * H / 2

One column advances horizontally by (W, 0). One row advances diagonally by (W/2, H/2). The area of that basis is exactly the area of one W x H diamond, so the tiles neither overlap nor leave holes.

Including the layer origin, the forward and inverse transforms are:

worldX = -OriginX + column * W + row * W / 2
worldY = -OriginY + row * H / 2

row    = (worldY + OriginY) / (H / 2)
column = (worldX + OriginX - row * W / 2) / W

The inverse is continuous: a world-pixel position between tile anchors can produce fractional grid coordinates.

The important difference from IsometricRhombic is the choice of basis. Rhombic uses two symmetric diagonal axes; Axial uses one horizontal axis and one diagonal axis. A finite Axial layer therefore forms a right-sheared parallelogram rather than a symmetric rhombus.

Adjacency follows the same eight-direction rectangular index space as Orthogonal.

Use it when a tightly packed diamond map should use a continuous axial basis rather than the classic symmetric rhombic projection.

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4. Hex axial, flat top

CoordinateSystemTypes.HexAxialFlatTop

This system draws flat-topped hexagons using an even-column offset layout (even-q). Columns overlap horizontally and alternate columns are vertically staggered.

• columns advance by 0.75 * TileWidth
• odd columns are shifted down by TileHeight / 2
• rows within a column advance by TileHeight

A hex has six direct neighbors. For flat-top hexes they are E, W, NE, NW, SE, and SW. N and S are not direct neighbors and return null.

Pixel-to-grid conversion first estimates the nearby cell, then uses the hex polygons and nearest centers to resolve the containing tile. This avoids treating transparent corners of a hex's rectangular image bounds as part of the wrong tile.

Use flat-top hexes when the map's strongest visual axis should run horizontally through the flat sides.

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5. Hex axial, pointed top

CoordinateSystemTypes.HexAxialPointedTop

This system draws pointed-top hexagons using an even-row offset layout (even-r). Rows overlap vertically and alternate rows are horizontally staggered.

• rows advance by 0.75 * TileHeight
• odd rows are shifted right by TileWidth / 2
• columns within a row advance by TileWidth

Its six direct neighbors are N, S, NE, NW, SE, and SW. E and W are not direct neighbors and return null.

Like the flat-top implementation, reverse conversion uses polygon hit testing and nearest-center selection so the result follows the actual hex footprint.

Use pointed-top hexes when the map's strongest visual axis should run vertically through the pointed ends.

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6. Oblique right

CoordinateSystemTypes.ObliqueRight

Oblique right is the renamed form of the original Oblique coordinate system. It uses a right-receding, sheared square lattice whose tile footprints are parallelograms.

Let skewX = W / 2 and faceWidth = W - skewX. Its complete transform is:

worldX = -OriginX + column * faceWidth + row * skewX
worldY = -OriginY + row * H

row    = (worldY + OriginY) / H
column = (worldX + OriginX - row * skewX) / faceWidth

A column advances horizontally by (faceWidth, 0); a row advances down and right by (skewX, H).

Tile images still occupy W x H rectangular bitmap bounds. Artwork normally uses transparency in the unused corners around the parallelogram. Adjacency uses the same eight-direction rectangular index space as Orthogonal.

Use it for cabinet, tactics, pseudo-3D, or stylized maps whose depth axis should recede to the right.

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7. Oblique left

CoordinateSystemTypes.ObliqueLeft

Oblique left mirrors the oblique geometry so rows recede to the left. It is a separate coordinate implementation, including its own forward transform, inverse transform, range query, bounds, polygon, adjacency, and wrapping operations.

Using the same skewX and faceWidth definitions:

worldX = -OriginX + column * faceWidth - row * skewX
worldY = -OriginY + row * H

row    = (worldY + OriginY) / H
column = (worldX + OriginX + row * skewX) / faceWidth

A column still advances by (faceWidth, 0), while a row advances down and left by (-skewX, H). Its tile polygon is the horizontal mirror of the right-receding parallelogram.

Use it when the same pseudo-3D treatment should recede toward the left. Artwork must match the left-sheared footprint; right-receding artwork is not mirrored automatically.

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Quick comparison

Type	Tile footprint	Grid layout in world space	Direct neighbors
Orthogonal	Rectangle	Axis-aligned rectangular lattice	8
IsometricRhombic	Diamond	Classic diagonal rhombic lattice	8
IsometricAxial	Diamond	Horizontal/diagonal axial lattice	8
HexAxialFlatTop	Flat-top hexagon	Staggered columns (even-q)	6
HexAxialPointedTop	Pointed-top hexagon	Staggered rows (even-r)	6
ObliqueRight	Parallelogram	Right-sheared rectangular lattice	8
ObliqueLeft	Parallelogram	Left-sheared rectangular lattice	8

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Key insight

A SceneLayer always stores tiles by (column, row). The selected coordinate system decides what those indices mean geometrically.

Changing the system changes more than appearance. It changes projection, reverse lookup, tile footprints, visible-range queries, and neighbor topology together. That is why these rules live behind one shared interface.

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Mental model

SceneLayer grid
      |
      v
ISceneLayerCoordinates
      |
      +-- placement and reverse lookup
      +-- tile polygon and pixel bounds
      +-- adjacency and wrapping
      |
      v
World pixel geometry

The camera and view system then transform that world geometry into screen space.

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Where to read next

• Coordinate Spaces
• Gondwana/Drawing/Coordinates/ISceneLayerCoordinates.cs
• Gondwana/Drawing/Coordinates/CoordinateSystemTypes.cs
• coordinate implementations under Gondwana/Drawing/Coordinates/
• Gondwana/Scenes/SceneLayer.cs
• Gondwana/Scenes/Scene.cs