Cross (Variation 1): Geometry, Mathematical Formulas & Visual Guide

The photographic specimen captures an authoritative two-dimensional geometric cross, mathematically classified as an equilateral orthogonal Greek cross or concave dodecagon, rendered as an architectural slate bento tile within a square 1024 by 1024 pixel canvas. The figure consists of four congruent rectangular arms extending orthogonally from a central square core at 90-degree intervals. The outer boundary is defined by twelve straight edges of equal length meeting at twelve right angles: eight salient convex exterior corners (90 degrees) and four re-entrant reflex interior corners (270 degrees). The specimen exhibits perfect four-fold rotational and reflectional symmetry across its horizontal, vertical, and diagonal midlines.

Conforming to the Slot 1 Obsidian Black Matte studio specification, the planar face of the cross is filled with an architectural slate gradient. The tonal scale begins with deep charcoal slate (#2D3748) across the top vertical arm, transitioning smoothly through mid-tone graphite (#1E293B) across the horizontal crossbar and settling into dark obsidian (#0F172A) along the bottom arm. Grazing the upper horizontal edges of the arms, an ultra-fine 1.5-pixel specular rim highlight simulates an overhead directional keylight, lending the polygonal tile the tactile feel of precision-cut matte slate or honed carbon composite.

The cross is elevated above the white studio floor by a multi-tiered elevation drop shadow. Three Gaussian blur convolution passes (4px, 12px, and 28px blur radii) cast soft, natural shadows beneath the horizontal arms and lower vertical extremity, accentuating dimensional depth while maintaining crisp corner definition. The entire bounding box spans exactly 778 by 778 pixels, centered strictly at coordinates (cx = 512, cy = 512). An unblemished 83-pixel clear buffer surrounds the outer drop shadow envelope, guaranteeing visual poise and zero boundary clipping across high-resolution displays.

In Euclidean plane geometry, an equilateral orthogonal cross (commonly termed a Greek cross or plus symbol) is classified as a non-convex (concave) equilateral dodecagon (a 12-sided polygon). It is formed by the orthogonal intersection of two congruent rectangles or, equivalently, by adjoining four congruent squares to the four edges of a central core square.

Geometry Taxonomy Summary:
- Canonical Name: Greek Cross (Equilateral Orthogonal Cross, Plus Sign)
- Geometric Classification: Non-Convex Equilateral Polygon (Concave Dodecagon)
- Number of Vertices (V): 12 (8 convex 90-degree corners, 4 reflex 270-degree re-entrant corners)
- Number of Edges (E): 12 straight segments of equal length s
- Dihedral Symmetry Group: D4 (Symmetry group of the square; order 8)
- Rotational Symmetry: Order 4 (90, 180, 270, 360 degrees invariance)
- Reflectional Symmetry Axes: 4 axes (2 orthogonal midlines, 2 diagonal bisectors)
- Interior Angle Sum: (12 - 2) * 180 degrees = 1800 degrees (10*pi radians)
- Topological Invariant: Simply connected planar polygon, Euler characteristic chi = 1

Discrete Counts and Interior Angles

The Greek cross possesses twelve vertices and twelve edges:

• Convex Exterior Vertices (8): Each of the four arms terminates in two 90-degree exterior corners:

8 \* 90 degrees = 720 degrees.

• Concave Interior Vertices (4): The four re-entrant reflex junctions where adjacent arms meet have interior angles of 270 degrees (3*pi/2 radians):

4 * 270 degrees = 1080 degrees.

• Total Interior Angle Sum:

720 degrees + 1080 degrees = 1800 degrees = (12 - 2) \* 180 degrees.

This exact sum verifies the polygon's adherence to the general Euclidean polygon interior angle theorem.

Closed-Form Algebraic Derivations

Let s denote the side length of each of the twelve equal edges. The overall span of the cross along both horizontal and vertical axes is:

Span (W = H) = 3 * s

1. 1Area Formula (A):

The cross can be decomposed into five congruent non-overlapping squares of side length s (one central square and four arm squares):

A_central = s^2
A_arms = 4 * s^2
A_total = 5 * s^2

In terms of the total bounding span W:

s = W / 3
A_total = 5 * (W / 3)^2 = (5 / 9) * W^2

The cross encloses exactly 5/9 (approximately 55.56 percent) of the area of its square bounding box.

1. 2Perimeter Formula (P):

Because all twelve boundary edges have equal length s:

P = 12 * s = 12 * (W / 3) = 4 * W

Remarkably, the perimeter of a Greek cross is exactly equal to the perimeter of its square bounding box (4 \* W), despite enclosing only 55.56 percent of its area.

1. 3Inradius (r_in) and Circumradius (R):

The maximum circle that can be inscribed in the central square core has radius:

r_in = s / 2 = W / 6

The circumradius R from the center (0, 0) to the eight outermost corner vertices (+-3s/2, +-s/2) and (+-s/2, +-3s/2) is:

R = sqrt((3s/2)^2 + (s/2)^2) = sqrt((9s^2/4) + (s^2/4)) = (s / 2) * sqrt(10) = (W / 6) * sqrt(10)

1. 4Cartesian Coordinate Representation:

Centered at the origin (0, 0) with arm half-width s/2 and total half-span 3s/2, the cross is the union of two orthogonal rectangles:

Region: { (x, y) in R^2 :
  (|x| <= 3s/2 and |y| <= s/2) or (|x| <= s/2 and |y| <= 3s/2) }

1. 5SVG Coordinate Definition:

Within a 1024 by 1024 pixel canvas centered at (cx = 512, cy = 512) with span W = 778 pixels (s = 778 / 3 = 259.33 pixels):

<polygon points="
  426,123 598,123 598,426 901,426 901,598 598,598
  598,901 426,901 426,598 123,598 123,426 426,426" />

1. Architectural History & Byzantine Greek Cross Plans

In the history of monumental architecture, the equilateral Greek cross (crux immissa quadrata) is the defining geometric blueprint of Byzantine, Eastern Orthodox, and High Renaissance ecclesiastical architecture:

1. 1Cross-in-Square (Catholicon) Structural Engineering: Developed in the middle Byzantine period (9th to 12th centuries), the cross-in-square architectural plan features four barrel-vaulted cross arms that act as monumental structural buttresses for a central hemispherical dome elevated on pendentives. Gravitational dead loads and lateral thrust from the dome are channeled down through the four internal pier columns and dissipated outward into the four barrel vaults.
2. 2Renaissance Centralized Plans: During the Italian High Renaissance, architects such as Donato Bramante, Leonardo da Vinci, and Michelangelo Buonarroti favored the Greek cross plan over the elongated Latin cross for the rebuilding of St. Peter's Basilica in Rome. The four-fold D4 symmetry of the Greek cross was celebrated as the ultimate architectural embodiment of cosmic harmony, mathematical proportion, and geometric perfection.

2. Regular Tessellation & Pentomino Geometry

The Greek cross is one of the few non-convex polygons capable of forming regular edge-to-edge monohedral tessellations of the Euclidean plane:

• The Cross Pentomino (X-Pentomino): In discrete mathematics and recreational geometry, the Greek cross is the X-pentomino, formed by five unit squares.
• Plane Tiling: Greek crosses can tile the Euclidean plane without gaps or overlaps by staggering adjacent crosses in an interlocking grid. Each cross shares its twelve edges with adjacent crosses, creating a dense, structurally cohesive tessellation with periodic lattice vectors.
• Fractal Generation (Vicsek Fractal): In fractal geometry, the Greek cross is the generator of the Vicsek fractal (or box fractal). Starting from a square, the square is partitioned into a 3 by 3 grid of 9 smaller squares, and the 4 corner squares are removed, leaving a Greek cross. Repeating this recursive decomposition infinitely produces a self-similar fractal with Hausdorff dimension:

D_H = log(5) / log(3) approximately equal to 1.46497

3. Studio Material Shader Architecture: Obsidian Black Matte (Slot 1)

The visual rendering of cross-v1 relies on the Slot 1 Obsidian Black Matte shader architecture:

Shader Property Matrix (Slot 1: Obsidian Matte):
- Top Arm Gradient Stop: #2D3748 (slate charcoal, luminance 28%)
- Central Core Stop: #1E293B (graphite midnight, luminance 17%)
- Bottom Arm Stop: #0F172A (obsidian foundation, luminance 9%)
- Hairline Rim Highlight: 1.5px stroke at rgba(255, 255, 255, 0.18)
- Primary Contact Drop Shadow: 4px blur, dy = 2px, rgba(0, 0, 0, 0.35)
- Ambient Mid-Field Occlusion: 12px blur, dy = 8px, rgba(0, 0, 0, 0.22)
- Far-Field Elevation Diffusion: 28px blur, dy = 16px, rgba(0, 0, 0, 0.12)

The shader executes a smooth linear color ramp across the cross along the vertical axis. To define the twelve straight edges without rounding the sharp 90-degree corners, the shader uses a polygon clipping mask. Along the upper horizontal surfaces of the top and side arms, an inner stroke filter isolates upward-facing normals, creating a 1.5-pixel specular rim that catches the overhead keylight.

4. Cognitive Psychology & Gestalt Perception

Human neurological processing of the cross geometry is characterized by high salience and rapid perceptual binding:

• Orthogonal Symmetry and Foveal Fixation: The human visual system possesses specialized orientation-sensitive simple cells in primary visual cortex area V1 tuned to vertical and horizontal line segments. The presence of four orthogonal arms activates both vertical and horizontal cell columns simultaneously, anchoring foveal fixation at the exact geometric center in under 25 milliseconds.
• Law of Good Continuation: The intersecting rectangular beams stimulate the Gestalt Law of Good Continuation, allowing the brain to perceive two continuous perpendicular bars crossing each other rather than eight separate angular fragments.
• Universal Semiotics: In semiotics and graphic design, the cross is a universal archetypal glyph. Depending on context, it represents mathematical addition (+), positive charge, medical first aid (the International Red Cross), navigation reticles, and spiritual iconography.

In rapid PicRush competitive gameplay, distinguishing the Greek cross from related orthogonal and multi-armed figures is critical:

Key Diagnostic Distinctions

• Cross vs. Saltire (X-Cross): A Greek cross is oriented strictly along horizontal and vertical axes (arms at 0, 90, 180, and 270 degrees). A saltire (Saint Andrew's cross) is rotated 45 degrees, forming an "X" shape.
• Cross vs. Octagon: An octagon is a convex eight-sided polygon with zero concave corners or arms. A Greek cross is a concave twelve-sided polygon with four distinct extending arms and four reflex corners.
• Cross vs. Latin Cross: A Latin cross has an elongated vertical lower arm, giving it bilateral D1 symmetry. A Greek cross has four arms of equal length, giving it four-fold D4 dihedral symmetry.

To identify the cross with lightning speed in the PicRush Arena:

1. 1Verify Four Orthogonal Arms: Look for four equal rectangular arms extending up, down, left, and right at 90-degree angles.
2. 2Count the Twelve Right Angles: Note the distinct Greek cross profile with eight exterior corners and four interior reflex corners.
3. 3Eliminate the Diagonal Saltire: If the arms point horizontally and vertically like a plus sign (+), it is a cross. If the arms are tilted at 45 degrees like an "X", it is a saltire.
4. 4Identify the Slate Tile Shading: The dark charcoal-to-obsidian gradient indicates Slot 1 Obsidian Black Matte, but the underlying geometric entity remains strictly cross.
5. 5Submit Recognized Keywords Promptly: Type cross into the answer field, or enter accepted alternatives such as plus, plus sign, or greek cross to secure top leaderboard points.