Checking GEOSIntersection Results in C Programming

In C, how to check the result of GEOSIntersection

In C, the GEOS library provides functions for performing geometric operations, such as intersection, on geometries. To check the result of GEOSIntersection, you need to follow these steps:

1. Initialize the GEOS Environment: Before using any GEOS functions, you need to initialize the GEOS environment.

2. Create or Read Geometries: You need to have two geometries to perform the intersection operation.

3. Perform the Intersection: Use the GEOSIntersection function to compute the intersection of the two geometries.

4. Check the Result: After performing the intersection, you need to check if the result is valid and handle any errors.

Here is an example code snippet demonstrating these steps:

#include <stdio.h>
#include <geos_c.h>

// Error handling functions
void notice(const char *fmt, ...) {
    va_list ap;
    va_start(ap, fmt);
    vfprintf(stdout, fmt, ap);
    va_end(ap);
}

void log_and_exit(const char *fmt, ...) {
    va_list ap;
    va_start(ap, fmt);
    vfprintf(stderr, fmt, ap);
    va_end(ap);
    exit(1);
}

int main() {
    // Initialize GEOS
    initGEOS(notice, log_and_exit);

    // Create a GEOS context handle
    GEOSContextHandle_t context = GEOS_init_r();

    // Create geometries (for example, from WKT)
    const char *wkt1 = "POLYGON((0 0, 0 10, 10 10, 10 0, 0 0))";
    const char *wkt2 = "POLYGON((5 5, 5 15, 15 15, 15 5, 5 5))";

    GEOSWKTReader *reader = GEOSWKTReader_create_r(context);
    GEOSGeometry *geom1 = GEOSWKTReader_read_r(context, reader, wkt1);
    GEOSGeometry *geom2 = GEOSWKTReader_read_r(context, reader, wkt2);
    GEOSWKTReader_destroy_r(context, reader);

    if (!geom1 || !geom2) {
        log_and_exit("Error reading geometries\n");
    }

    // Perform the intersection
    GEOSGeometry *intersection = GEOSIntersection_r(context, geom1, geom2);

    if (!intersection) {
        log_and_exit("Error computing intersection\n");
    }

    // Check if the result is valid
    if (GEOSisEmpty_r(context, intersection)) {
        printf("The intersection is empty.\n");
    } else {
        // Convert the result to WKT and print it
        GEOSWKTWriter *writer = GEOSWKTWriter_create_r(context);
        char *resultWKT = GEOSWKTWriter_write_r(context, writer, intersection);
        printf("Intersection result: %s\n", resultWKT);
        GEOSWKTWriter_destroy_r(context, writer);
        GEOSFree_r(context, resultWKT);
    }

    // Clean up
    GEOSGeom_destroy_r(context, geom1);
    GEOSGeom_destroy_r(context, geom2);
    GEOSGeom_destroy_r(context, intersection);
    GEOS_finish_r(context);

    return 0;
}

Explanation:

1. Initialization:

   • initGEOS(notice, log_and_exit); initializes the GEOS library with custom error handling functions.
   • GEOSContextHandle_t context = GEOS_init_r(); creates a GEOS context handle for thread-safe operations.

2. Geometry Creation:

   • GEOSWKTReader is used to read geometries from WKT (Well-Known Text) strings.
   • GEOSWKTReader_read_r reads the WKT strings and creates GEOSGeometry objects.

3. Intersection:

   • GEOSIntersection_r computes the intersection of the two geometries.

4. Result Checking:

   • GEOSisEmpty_r checks if the resulting geometry is empty.
   • If the result is not empty, it is converted to WKT and printed.

5. Cleanup:

   • All created geometries and the context are properly destroyed to avoid memory leaks.

This example demonstrates how to use the GEOS library in C to perform an intersection operation and check the result. Make sure to link against the GEOS library when compiling your program.