Skip to content

grib2: consolidate scaled value decoding #1605

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
lesserwhirls merged 1 commit into from
Sep 23, 2026
Merged

grib2: consolidate scaled value decoding #1605

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
2 changes: 1 addition & 1 deletion cdm-test/src/test/java/ucar/nc2/iosp/grib/TestGribSpheroids.java
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -107,7 +107,7 @@ public void code7_oblate_specified_m() throws IOException {
try (NetcdfFile ncfile = NetcdfFiles.open(filename, null)) {
Variable v = ncfile.findVariable("LambertConformal_Projection");
Attribute axis = v.findAttribute("semi_major_axis");
Assert.assertEquals(6377397., axis.getNumericValue().doubleValue(), 0.1);
Assert.assertEquals(6377397.16, axis.getNumericValue().doubleValue(), 0.1);
}
}
}
14 changes: 11 additions & 3 deletions grib/src/main/java/ucar/nc2/grib/GribNumbers.java
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -268,7 +268,7 @@ public static long int8(RandomAccessFile raf) throws IOException {
}

/**
* A signed byte has a sign bit then 1 15-bit value.
* A signed byte has a sign bit and a 7-bit value.
* This is not twos complement (!)
*
* @param v convert byte to signed int
Expand All @@ -280,8 +280,16 @@ public static int convertSignedByte(byte v) {
return sign * value;
}

public static int convertSignedByte2(byte v) {
return (v >= 0) ? (int) v : -(128 + v);
Comment thread
lesserwhirls marked this conversation as resolved.
/**
* A scaled value is encoded with the formula: L * 10^F = V, where L is the original value, F is the scale factor and
* V is the scaled value. Therefore, to decode: L = V / 10^F.
* See: https://codes.ecmwf.int/grib/format/grib2/regulations/
*/
public static double decodeScaledValue(int scaledValue, int scaleFactor) {
// Using p = 10^|F| ensures p is exactly representable. The result is then `L = V / p` or `L = V * p` based on the
// sign of F. This reduces float point rounding error.
double p = Math.pow(10, Math.abs(scaleFactor));
return scaleFactor <= 0 ? (scaledValue * p) : (scaledValue / p);
}
Comment thread
lesserwhirls marked this conversation as resolved.

// count number of bits on in bitmap
Expand Down
15 changes: 7 additions & 8 deletions grib/src/main/java/ucar/nc2/grib/grib2/Grib2Gds.java
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,8 @@
import java.util.Formatter;
import java.util.UUID;

import static ucar.nc2.grib.GribNumbers.decodeScaledValue;

/**
* Template-specific fields for Grib2SectionGridDefinition
* LOOK hashCode not right, cant use approximate float compare
Expand Down Expand Up @@ -100,7 +102,7 @@ public static Grib2Gds factory(int template, byte[] data) {
public int numberOfDataPoints;

public int center;
public float earthRadius, majorAxis, minorAxis; // in meters
public double earthRadius, majorAxis, minorAxis; // in meters
Comment thread
lesserwhirls marked this conversation as resolved.
protected int scanMode;
public int earthShape;

Expand Down Expand Up @@ -296,13 +298,10 @@ int getOctet4(int start) {
return GribNumbers.int4(getOctet(start), getOctet(start + 1), getOctet(start + 2), getOctet(start + 3));
}

private float getScaledValue(int start) {
int scaleFactor = getOctetSigned(start);
int scaleValue = getOctet4(start + 1);
if (scaleFactor != 0)
return (float) (scaleValue / Math.pow(10, scaleFactor));
else
return (float) scaleValue;
private double getScaledValue(int index) {
int scaleFactor = getOctetSigned(index);
int scaledValue = getOctet4(index + 1);
return decodeScaledValue(scaledValue, scaleFactor);
Comment thread
lesserwhirls marked this conversation as resolved.
}

/*
Expand Down
44 changes: 11 additions & 33 deletions grib/src/main/java/ucar/nc2/grib/grib2/Grib2Pds.java
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@
import java.util.StringJoiner;
import java.util.zip.CRC32;

import static ucar.nc2.grib.GribNumbers.decodeScaledValue;
import static ucar.nc2.grib.grib2.Grib2Utils.intervalToRangeDescriptor;

/**
Expand Down Expand Up @@ -318,11 +319,10 @@ public final int getRawLength() {
return input.length;
}


protected double getScaledValue(int start) {
int scale = getOctetSigned(start++);
int value = GribNumbers.int4(getOctet(start++), getOctet(start++), getOctet(start++), getOctet(start++));
return applyScaleFactor(scale, value);
public final double getScaledValue(int index) {
int scaleFactor = getOctetSigned(index);
int scaledValue = getInt4StartingAtOctet(index + 1);
return decodeScaledValue(scaledValue, scaleFactor);
}

public int getStatisticalProcessType() {
Expand Down Expand Up @@ -1019,18 +1019,13 @@ public int getProbabilityType() {
}

public double getProbabilityLowerLimit() {
int scale = getOctetSigned(38);
int value = GribNumbers.int4(getOctet(39), getOctet(40), getOctet(41), getOctet(42));
return applyScaleFactor(scale, value);
return getScaledValue(38);
}

public double getProbabilityUpperLimit() {
int scale = getOctetSigned(43);
int value = GribNumbers.int4(getOctet(44), getOctet(45), getOctet(46), getOctet(47));
return applyScaleFactor(scale, value);
return getScaledValue(43);
}


@Override
public int getProbabilityHashcode() {
if (probHash == 0) {
Expand Down Expand Up @@ -1113,7 +1108,7 @@ public String getProbabilityName() {
f.format("below_%s", Format.dfrac(getProbabilityUpperLimit(), scale2));
break;
default:
f.format("UknownProbType=%d", getProbabilityType());
f.format("UnknownProbType%d", getProbabilityType());
Comment thread
lesserwhirls marked this conversation as resolved.
}
result = StringUtil2.removeFromEnd(f.toString(), '0');
}
Expand Down Expand Up @@ -1479,9 +1474,7 @@ public SatelliteBand[] getSatelliteBands() {
sb.series = GribNumbers.int2(getOctet(pos), getOctet(pos + 1));
sb.number = GribNumbers.int2(getOctet(pos + 2), getOctet(pos + 3));
sb.instrumentType = getOctet(pos + 4);
int scaleFactor = getOctetSigned(pos + 5);
int svalue = GribNumbers.int4(getOctet(pos + 6), getOctet(pos + 7), getOctet(pos + 8), getOctet(pos + 9));
sb.value = applyScaleFactor(scaleFactor, svalue);
sb.value = getScaledValue(pos + 5);
pos += 10;
result[i] = sb;
}
Expand Down Expand Up @@ -1557,9 +1550,7 @@ public SatelliteBand[] getSatelliteBands() {
sb.series = GribNumbers.int2(getOctet(pos), getOctet(pos + 1));
sb.number = GribNumbers.int2(getOctet(pos + 2), getOctet(pos + 3));
sb.instrumentType = GribNumbers.int2(getOctet(pos + 4), getOctet(pos + 5));
int scaleFactor = getOctetSigned(pos + 6);
int svalue = GribNumbers.int4(getOctet(pos + 7), getOctet(pos + 8), getOctet(pos + 9), getOctet(pos + 10));
sb.value = applyScaleFactor(scaleFactor, svalue);
sb.value = getScaledValue(pos + 6);
pos += octetsPerBand;
result[i] = sb;
}
Expand Down Expand Up @@ -1634,9 +1625,7 @@ public SatelliteBand[] getSatelliteBands() {
sb.series = GribNumbers.int2(getOctet(pos), getOctet(pos + 1));
sb.number = GribNumbers.int2(getOctet(pos + 2), getOctet(pos + 3));
sb.instrumentType = GribNumbers.int2(getOctet(pos + 4), getOctet(pos + 5));
int scaleFactor = getOctetSigned(pos + 6);
int svalue = GribNumbers.int4(getOctet(pos + 7), getOctet(pos + 8), getOctet(pos + 9), getOctet(pos + 10));
sb.value = applyScaleFactor(scaleFactor, svalue);
sb.value = getScaledValue(pos + 6);
pos += octetsPerBand;
result[i] = sb;
}
Expand Down Expand Up @@ -2191,17 +2180,6 @@ protected CalendarDate calcTime(int startIndex) {
return CalendarDate.of(null, year, month, day, hour, minute, second);
}

/**
* Apply scale factor to value, return a double result.
*
* @param scale signed scale factor
* @param value apply to this value
* @return value ^ -scale
*/
double applyScaleFactor(int scale, int value) {
return ((scale == 0) || (scale == 255) || (value == 0)) ? value : value * Math.pow(10, -scale);
}

Comment thread
lesserwhirls marked this conversation as resolved.
TimeInterval[] readTimeIntervals(int n, int startIndex) {
TimeInterval[] result = new TimeInterval[n];
for (int i = 0; i < n; i++) {
Expand Down
88 changes: 78 additions & 10 deletions grib/src/test/java/ucar/nc2/grib/TestGribNumbers.java
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,25 +1,49 @@
package ucar.nc2.grib;

import static org.junit.Assert.*;
import static ucar.nc2.grib.GribNumbers.convertSignedByte;
import static ucar.nc2.grib.GribNumbers.convertSignedByte2;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.JUnit4;
import ucar.ma2.DataType;

import static com.google.common.truth.Truth.assertThat;
import static org.junit.Assert.assertNotEquals;
import static ucar.nc2.grib.GribNumbers.*;

@RunWith(JUnit4.class)
public class TestGribNumbers {

@Test
public void testConvertSignedByte() {
System.out.printf("byte == convertSignedByte == convertSignedByte2 == hex%n");
for (int i = 125; i < 256; i++) {
byte b = (byte) i;
System.out.printf("%d == %d == %d == %s%n", b, convertSignedByte(b), convertSignedByte2(b),
Long.toHexString((long) i));
assertEquals(convertSignedByte(b), convertSignedByte2(b));
}
assertThat(convertSignedByte((byte) 0x00)).isEqualTo(0);
assertThat(convertSignedByte((byte) 0x01)).isEqualTo(1);
assertThat(convertSignedByte((byte) 0x02)).isEqualTo(2);
assertThat(convertSignedByte((byte) 0x7d)).isEqualTo(125);
assertThat(convertSignedByte((byte) 0x7e)).isEqualTo(126);
assertThat(convertSignedByte((byte) 0x7f)).isEqualTo(127);

assertThat(convertSignedByte((byte) 0x80)).isEqualTo(-0);
assertThat(convertSignedByte((byte) 0x81)).isEqualTo(-1);
assertThat(convertSignedByte((byte) 0x82)).isEqualTo(-2);
assertThat(convertSignedByte((byte) 0xfd)).isEqualTo(-125);
assertThat(convertSignedByte((byte) 0xfe)).isEqualTo(-126);
assertThat(convertSignedByte((byte) 0xff)).isEqualTo(-127);
Comment thread
lesserwhirls marked this conversation as resolved.
}

@Test
public void testConvertSignedInt() {
assertThat(int4(0x00, 0x00, 0x00, 0x00)).isEqualTo(0);
assertThat(int4(0x00, 0x00, 0x00, 0x01)).isEqualTo(1);
assertThat(int4(0x00, 0x00, 0x00, 0x02)).isEqualTo(2);
assertThat(int4(0x7f, 0xff, 0xff, 0xfd)).isEqualTo(2147483645);
assertThat(int4(0x7f, 0xff, 0xff, 0xfe)).isEqualTo(2147483646);
assertThat(int4(0x7f, 0xff, 0xff, 0xff)).isEqualTo(2147483647);

assertThat(int4(0x80, 0x00, 0x00, 0x00)).isEqualTo(-0);
assertThat(int4(0x80, 0x00, 0x00, 0x01)).isEqualTo(-1);
assertThat(int4(0x80, 0x00, 0x00, 0x02)).isEqualTo(-2);
assertThat(int4(0xff, 0xff, 0xff, 0xfd)).isEqualTo(-2147483645);
assertThat(int4(0xff, 0xff, 0xff, 0xfe)).isEqualTo(-2147483646);
assertThat(int4(0xff, 0xff, 0xff, 0xff)).isEqualTo(UNDEFINED);
}

@Test
Expand All @@ -29,4 +53,48 @@ public void testConvertUnsigned() {
assertNotEquals(val, val2);
}

static void assertDecodes(int scaledValue, int scaleFactor, double expected) {
assertThat(decodeScaledValue(scaledValue, scaleFactor)).isWithin(Math.ulp(expected)).of(expected);
Comment thread
lesserwhirls marked this conversation as resolved.
}

@Test
public void testDecodeScaledValue() {

assertDecodes(9, 1, 0.9);
assertDecodes(9, 0, 9);
assertDecodes(9, -1, 90);

assertDecodes(9, 3, 9e-3);
assertDecodes(13, 3, 13e-3);
assertDecodes(18, 3, 18e-3);
assertDecodes(25, 3, 25e-3);
assertDecodes(26, 3, 26e-3);
assertDecodes(36, 3, 36e-3);
assertDecodes(3, 4, 3e-4);
assertDecodes(6, 4, 6e-4);
assertDecodes(9, 4, 9e-4);
assertDecodes(1, 5, 1e-5);
assertDecodes(2, 5, 2e-5);
assertDecodes(10, 5, 10e-5);
assertDecodes(1, -5, 1e5);
assertDecodes(8, -5, 8e5);
assertDecodes(25, -5, 25e5);
assertDecodes(1, -6, 1e6);
assertDecodes(8, -6, 8e6);
assertDecodes(25, -6, 25e6);
assertDecodes(1, -9, 1e9);
assertDecodes(33, -9, 33e9);
assertDecodes(66, -9, 66e9);

assertDecodes(1, -100, 1e100);
assertDecodes(1, 100, 1e-100);

assertDecodes(0x7fffffff, 0, 0x7fffffff);
assertDecodes(-1, 0, -1);

assertThat(decodeScaledValue(15, -127)).isEqualTo(1.5e+128);
assertThat(decodeScaledValue(UNDEFINED, 1)).isEqualTo(-999.9);
assertThat(decodeScaledValue(UNDEFINED, -127)).isEqualTo(-9.999e+130);
Comment thread
lesserwhirls marked this conversation as resolved.

}
}
Loading