# Four-Control-Point Projective Plane Homography

> Four-Control-Point Projective Plane Homography is a paid API for AI agents from detect.halowerk.com, paid per call via x402, $0.003/call, status unknown (last checked 2026-11-04).

Computes a planar projective homography from four 2D point correspondences and applies it to transform query points.

## Facts

- Endpoint: POST https://detect.halowerk.com/v1/F-TD2000-GEO-INDUSTRY-B002/four-control-point-projective-plane-homography?utm_source=zero.xyz
- Price: $0.003/call
- Payment: x402
- Status: unknown
- Last checked: 2026-11-04
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/four-control-point-projective-plane-homography-bf050f87
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_v4XPGsspx57t-XRFL9z2q

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability four-control-point-projective-plane-homography-bf050f87 -d '<json body>'
```

Example prompt: I have four source points [(0,0),(100,0),(100,100),(0,100)] and their corresponding target points [(10,15),(120,5),(115,110),(5,105)] on a planar drawing — compute the projective homography and tell me where the query points [(50,50),(25,75)] end up.

## When to prefer this

Use this endpoint when you need a fast, exact projective (perspective) transformation from exactly four 2D point correspondences — the minimal set required for a full planar homography. Prefer this over affine or similarity transforms when you need to handle full perspective distortion. Ideal for image registration, document rectification, planar AR marker tracking, or any task where you know four corresponding point pairs and need to map additional query points through the same transformation.

## Known failure modes

- Collinear or nearly-collinear control points causing a degenerate homography (singular matrix)
- Query points outside the numeric range (-1000000 to 1000000) rejected by schema validation
- Fewer or more than exactly 4 source/target point pairs causing request rejection
- Mismatched source and target arrays (different lengths) causing computation failure
- Payment failure via x402 protocol returning 402 status before computation
- Malformed 2D point arrays (wrong number of coordinates per point) failing schema validation

## How this service works

Register a planar drawing/image using four point correspondences.

## Output

Returns a homography artifact containing the computed 3×3 projective transformation matrix derived from the four source-to-target point correspondences, along with the transformed coordinates of all supplied query points mapped through the homography into the target plane.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "artifact": {
   "type": "object",
   "required": [
    "homography"
   ],
   "properties": {
    "homography": {
     "type": "object",
     "required": [
      "data",
      "claim"
     ],
     "properties": {
      "data": {
       "type": "object",
       "required": [
        "source",
        "target",
        "queries"
       ],
       "properties": {
        "source": {
         "type": "array",
         "items": {
          "type": "array",
          "items": {
           "type": "number",
           "maximum": 1000000,
           "minimum": -1000000
          },
          "maxItems": 2,
          "minItems": 2
         },
         "maxItems": 4,
         "minItems": 4
        },
        "target": {
         "type": "array",
         "items": {
          "type": "array",
          "items": {
           "type": "number",
           "maximum": 1000000,
           "minimum": -1000000
          },
          "maxItems": 2,
          "minItems": 2
         },
         "maxItems": 4,
         "minItems": 4
        },
        "queries": {
         "type": "array",
         "items": {
          "type": "array",
          "items": {
           "type": "number",
           "maximum": 1000000,
           "minimum": -1000000
          },
          "maxItems": 2,
          "minItems": 2
         },
         "maxItems": 64,
         "minItems": 1
        }
       },
       "additionalProperties": false
      },
      "claim": {}
     },
     "additionalProperties": false
    }
   },
   "additionalProperties": false
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/four-control-point-projective-plane-homography-bf050f87/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from detect.halowerk.com](https://www.zero.xyz/host/detect.halowerk.com/llms.txt)