Fixed mapping for negative values when saving as PNG

src/dsp.rs

@@ -6,7 +6,7 @@ pub type Signal = Vec<f32>;
 
 /// Get biggest sample in signal.
 pub fn get_max(vector: &Signal) -> &f32 {
-    let mut max: &f32 = &0_f32;
+    let mut max: &f32 = &vector[0];
     for sample in vector.iter() {
         if sample > max {
             max = sample;
@@ -16,6 +16,18 @@ pub fn get_max(vector: &Signal) -> &f32 {
     max
 }
 
+/// Get smallest sample in signal.
+pub fn get_min(vector: &Signal) -> &f32 {
+    let mut min: &f32 = &vector[0];
+    for sample in vector.iter() {
+        if sample < min {
+            min = sample;
+        }
+    }
+
+    min
+}
+
 /// Resample signal to given rate, using the default filter.
 ///
 /// The filter has a transition band equal to the 20% of the spectrum width of

src/noaa_apt.rs

@@ -147,10 +147,12 @@ pub fn decode(input_filename: &str, output_filename: &str) -> err::Result<()>{
 
     let aligned = dsp::resample_to(&aligned, WORK_RATE, FINAL_RATE);
     let max = dsp::get_max(&aligned);
+    let min = dsp::get_min(&aligned);
+    let range = max - min;
 
-    debug!("Mapping samples from 0-{} to 0-255", max);
+    debug!("Mapping samples from {}-{} to 0-255", min, max);
 
-    let aligned: Vec<u8> = aligned.iter().map(|x| (x/max*255.) as u8).collect();
+    let aligned: Vec<u8> = aligned.iter().map(|x| ((x-min)/range*255.) as u8).collect();
 
     info!("Writing PNG to '{}'", output_filename);