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https://github.com/SatDump/SatDump
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138 lines
No EOL
5.3 KiB
Markdown
138 lines
No EOL
5.3 KiB
Markdown
# Image Product Expression
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The image product expression system is specifically used to generate (more or less complex) RGB composites from an ImageProduct.
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## Expression
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An expression is split into 2 parts, one of them optional. The first part is the actual pixel expression, where you can express the desired output value for each channel via a mathematical expression based on the raw channel values. Possible combinations are summarized in this table :
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| Expression | Output |
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|--------------------|-----------------|
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| ch1 | Grayscale |
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| ch1, ch2 | Grayscale Alpha |
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| ch1, ch2, ch3 | RGB |
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| ch1, ch2, ch3, ch4 | RGB Alpha |
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The only variables available by default are raw channel values, named `chCHNAME`, with `CHNAME` being the instrument channel name, as displayed in the explorer under "Channels" (eg, `Channel 3a` becomes `ch3a`).
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Those channel variables will be scaled from 0 to 1, with 0 being the minimum possible pixel value, and 1 the maximum. The expected output range is the same.
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## Additional definitions
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In many situations it will be desirable to utilize more than just raw instrument channels. To this effect, it is possible to define many other types of variables.
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Those definitions must be present in the expression because pixel values, and separated by `;` instead of a normal coma. For example :
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```
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cch1=(1, reflective_radiance, 0.000000, 46.203000);
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cch2=(1, reflective_radiance, 0.000000, 46.203000);
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ch2, ch2, ch1
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```
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### Calibrated Channel
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This allows getting calibrated channels values as a variable.
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```
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variable_name=(CHNAME, CALIBRATION_UNIT, MINCALVAL, MAXCALVAL);
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```
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- `variable_name` should be the desired variable name. Often `cchCHNAME`
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- `CALIBRATION_UNIT` should be calibration unit ID string (TODOREWORK document)
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- `MINCALVAL` value of the unit to be used as a minimum (scaled to 0)
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- `MAXCALVAL` value of the unit to be used as a maximum (scaled to 1)
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If `CALIBRATION_UNIT` is set to `equalized`, this will return the raw channel equalized (useful to stretch it on uncalibrated instruments for example).
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### Look-up table (1D/2D)
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This lets you loading and utilizing a Look-Up Table in the expression.
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```
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lut_name=lut(path/to/lut.png)
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```
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- `lut_name` desired LUT function name
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- `path/to/lut.png` path to a lut (can be any image format) relative to SatDump's resources folder
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The lut can then be used as follow in an expression :
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```
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lut_name(x_pos, y_pos, lut_channel)
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```
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- `x_pos` LUT X position, expects 0 to 1
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- `y_pos` LUT Y position, expects 0 to 1
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- `lut_channel` LUT Channel index to get. Can be 0 up to the number of channels in the LUT image minus 1
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Example to generate RGB with a LUT from a single channel :
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```
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lut=lut(path/to/lut.png);
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lut(ch1, 0, 0), lut(ch1, 0, 1), lut(ch1, 0, 2)
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```
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### Equirectangular Projected
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Loads an equirectangular image (such as a NASA Blue Marble) and (given the instrument has projection information) will return its pixel value so it can be used just like any other channel.
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```
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equ_name=equp(path/to/image.png)
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```
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- `equ_name` desired EquP function name
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- `path/to/image.png` Equirectangular image to load, relative to SatDump's resources folder
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```
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equ_name(equ_channel)
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```
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- `equ_channel` Equirectangular Channel index to get. Can be 0 up to the number of channels in the equirectangular image minus 1
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Example to generate a RGB "MCIR" composite :
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```
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bm=equp(maps/nasa_hd.jpg);
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ch4 * (ch4) + (1-ch4) * bm(0),
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ch4 * (ch4) + (1-ch4) * bm(1),
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ch4 * (ch4) + (1-ch4) * bm(2)
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```
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### Macros
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Since some equations can become relatively complex, this allows having a simple concept of macros to simplify them. **Do note that macros cannot be nested!**
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```
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macro_name = equation
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```
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## Channel alignement/scaling
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For several instruments, channels are not actually aligned with one or the other, or may be of different resolutions. SatDump will automatically deal with this.
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The output alignment and size will be dictated by the first channel found in the equation. Therefore `ch2, ch2, ch1` will lead to `ch2` being used as the output size/offset, while `ch1 * 0 + ch2, ch2, ch1` will generate the same output but using `ch1` as an output reference. This behaviour can be overwritten by explicitely specifying `ref_channel=1` if necessary. Do note said channel *must* be part of the expression still.
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## Wavelength/Frequency-based Channel selection
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A lot of composites are common between several instruments, and re-writing them for each can be cumbersome. Instead, it is possible to automatically select a channel by specifying a wavelength or frequency.
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Using `{10.4um}` will select the channel closest to 10.4um in the loaded instrument, and effectively will be getting replaced by `4` or whatever the actual channel number is. If you had `ch{10.4um}`, this would become `ch4`.
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This works the same with calibrated channels :
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```
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cch2=({830nm}, sun_angle_compensated_reflective_radiance, 0.000000, 97.050700);
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cch1=({630nm}, sun_angle_compensated_reflective_radiance, 0.000000, 90.000000);
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cch2, cch2, cch1
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``` |