drawPolygonRGBA

DrawPolygonRGBA is a routine found in several low‑level graphics libraries that renders a filled convex or non‑convex polygon directly onto an image buffer. The function accepts an array of vertex coordinates, the number of vertices, and buffer parameters such as width, height, and pixel stride. It performs rasterization by converting the polygon into a set of scan lines and filling each line with the supplied pixel data. The color argument is typically given as four unsigned bytes (red, green, blue, alpha), making it convenient for blending against existing image content.

The signature of the routine varies slightly between implementations. In the most common form it is declared as

drawPolygonRGBA(unsigned char image, int imageWidth, int imageHeight,

const short xs, const short ys, int n, unsigned char r,

unsigned char g, unsigned char b, unsigned char a);

Here xs and ys hold the x and y coordinates of each vertex in image space, and n specifies how many vertices are passed. The image buffer is assumed to be laid out in row-major order with 4 bytes per pixel (RGBA).

This function is typically used in applications that require quick, software‑based polygon filling such as GPS map rendering, raster graphic editors, or simple GUI frameworks. It bypasses the GPU and allows rendering on machines lacking hardware acceleration or on embedded devices. Because it performs its own scan‑line algorithm it works well for arbitrarily shaped polygons but can suffer from performance issues when processing thousands of vertices or very large images.

Typical usage patterns involve creating a blank image buffer, calling drawPolygonRGBA with the desired vertices and color, and then writing the buffer to disk or displaying it in a window. Some libraries provide wrapper functions that take vector or array types for convenience, but at the core the operation remains the same: convert polygon edges to horizontal segments and fill them with the given RGBA color.