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Store logarithmic sample values for better visualization #39

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Merged
brawer merged 1 commit into from
Jun 1, 2025
Merged

Store logarithmic sample values for better visualization #39

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23 changes: 19 additions & 4 deletions cmd/osmviews-builder/raster.go
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Original file line number Diff line number Diff line change
Expand Up @@ -174,7 +174,17 @@ func (w *RasterWriter) Write(r *Raster) error {
return w.WriteUniform(r.tile, color)
}

offset, size, err := w.compress(r.tile, r.pixels[:])
// For each pixel, we emit the natural logarithm of 1 plus its
// original value. While this logarithmizing does not affect
// the relative order of pixels, it produces a nicely
// near-linear distributino of absolute values. Tools such as
// QGIS can more easily visualize our GeoTIFF image when the pixel
// values have a somewhat linear distribution.
var logPixels [256 * 256]float32
for i := 0; i < 256*256; i++ {
logPixels[i] = float32(math.Log1p(float64(r.pixels[i])))
}
offset, size, err := w.compress(r.tile, logPixels[:])
if err != nil {
return err
}
Expand All @@ -199,12 +209,13 @@ func (w *RasterWriter) WriteUniform(tile TileKey, color uint32) error {
return nil
}
col := float32(color)
logCol := float32(math.Log1p(float64(color)))
if col > w.maxValue {
w.maxValue = col
}
var pixels [256 * 256]float32
for i := 0; i < len(pixels); i++ {
pixels[i] = col
pixels[i] = logCol
}
offset, size, err := w.compress(tile, pixels[:])
if err != nil {
Expand Down Expand Up @@ -490,10 +501,14 @@ func (w *RasterWriter) writeIFD(zoom uint8, f *os.File) error {
extraBuf.Write(s)

case sMinSampleValue:
typ, count, value = floatFormat, 1, math.Float32bits(0)
typ, count = floatFormat, 1
logMinSampleValue := math.Log1p(0.0)
value = math.Float32bits(float32(logMinSampleValue))

case sMaxSampleValue:
typ, count, value = floatFormat, 1, math.Float32bits(w.maxValue)
typ, count = floatFormat, 1
logMaxSampleValue := math.Log1p(float64(w.maxValue))
value = math.Float32bits(float32(logMaxSampleValue))

case geoKeyDirectory:
typ, count, value = shortFormat, uint32(len(geoKeys)), uint32(extraPos)+uint32(extraBuf.Len())
Expand Down