nitka/geometry.js

function clip(quads) {
    if (config.show_texture) {
        return quads;
    }

    const tl = screen_to_canvas({'x': 0, 'y': 0});
    const br = screen_to_canvas({'x': 0, 'y': canvas.height});

    const x1 = tl.x;
    const y1 = tl.y;

    const x2 = br.x;
    const y2 = br.y;

    const result = {
        'count': 0,
    };

    if (quads.count === 0) {
        return result;
    }

    let i0 = -1;
    let i1 = -1;

    for (let i = 0; i < quads.start.length - 1; ++i) {
        const index = quads.start[i];
        const next = quads.start[i + 1];

        const left = quads.pos[index];
        const top = quads.pos[index + 1];
        const bottom = top + quads.size[index + 1];
        const right = quads.pos[next - 2] + quads.size[next - 2];

        if (bottom < y1) {
            if (i < quads.start.length / 2 - 2) {
                const index_ahead = quads.start[i * 2];
                const top_ahead = quads.pos[index_ahead + 1];
                const bottom_ahead = top_ahead + quads.size[index_ahead + 1];

                if (bottom_ahead < y1) {
                    i *= 2;
                    continue;
                }
            }
        }

        if (bottom < y1 || right < x1) {
            continue;
        }

        if (top > y2) {
            i1 = quads.start[i + 1];
            break;
        }

        if (i0 === -1) {
            i0 = index;
        }
    }

    result.pos = quads.pos.subarray(i0, i1);
    result.size = quads.size.subarray(i0, i1);
    result.color = quads.color.subarray(i0 * 2, i1 * 2);
    result.uv = quads.uv.subarray(i0, i1);
    result.count = (i1 - i0) / 2;
    result.uploaded = false;

    return result;
}