Here's an example of how you can do that using the png crate for encoding/decoding PNGs and the Cairo graphics library (via the cairo-rs crate) for drawing:
extern crate png;
extern crate cairo;
use std::io::BufWriter;
use std::fs::File;
// for `encoder.set()`
use png::HasParameters;
fn main() {
// open the image file
let file = File::open("/path/to/some/file.png")
.expect("Failed to open the file");
// parse the metadata
let decoder = png::Decoder::new(file);
let (info, mut reader) = decoder.read_info()
.expect("Invalid PNG");
// extract the raw image
let mut data = vec![0; info.buffer_size()];
reader.next_frame(&mut data)
.expect("Invalid PNG");
// create a Cairo image surface
let mut surface = cairo::ImageSurface::create_for_data(
data.into_boxed_slice(),
|_| {}, // just drop it
cairo::Format::ARgb32,
info.width as i32,
info.height as i32,
info.line_size as i32
);
// initialize a Cairo drawing context
let context = cairo::Context::new(&surface);
// draw some text
context.select_font_face(
"serif",
cairo::FontSlant::Normal,
cairo::FontWeight::Normal
);
context.set_font_size(20.0);
context.set_source_rgb(1.0, 0.0, 0.0);
context.move_to(50.0, 50.0);
context.show_text("Hello");
drop(context);
let data = surface.get_data().unwrap();
// save the image back
let file = File::create("/path/to/output/file.png")
.expect("Failed to create the output file");
let buf_writer = BufWriter::new(file);
let mut encoder = png::Encoder::new(buf_writer, info.width, info.height);
encoder.set(png::ColorType::RGBA).set(png::BitDepth::Eight);
let mut writer = encoder.write_header().unwrap();
writer.write_image_data(&data)
.expect("Failed to write the image data");
}
