Extract absolute altitude (abs_alt) from DJI SRT files

Extract-AbsAlt.ps1

# Extract-AbsAlt.ps1
# Verion 1.02
# Extract absolute altitude (abs_alt) from DJI SRT files
# Usage: .\Extract-AbsAlt.ps1 -InputFile "input.srt" -OutputFile "output.srt" -DownsampleMs 1000 -RemoveFont -RemoveTimestamp
# Required Parameters:
# -InputFile: Path to the input SRT file
# -OutputFile: Path to the output SRT file
# Optional Parameters:
# -DownsampleMs: Interval in milliseconds to downsample the data (0 = no downsampling)
# -RemoveFont: Remove <font>...</font> tags from the output
# -RemoveFrameCount: Remove frame count information from the output lines
# -RemoveTimestamp: Remove timestamp information from the output lines  
param (
    [Parameter(Mandatory = $true)]
    [string]$InputFile,

    [Parameter(Mandatory = $true)]
    [string]$OutputFile,

    # Downsample interval in milliseconds (e.g. 1000 = 1 second)
    [int]$DownsampleMs = 0,

    # Remove <font>...</font> tags
    [switch]$RemoveFont,

    # Frame count removal
    [Parameter()][switch]$RemoveFrameCount,

    # Timestamp removal
    [Parameter()][switch]$RemoveTimestamp
)

function Parse-TimeMs ($timestamp) {
    # Converts "HH:MM:SS,mmm" to milliseconds
    if ($timestamp -match '(\d+):(\d+):(\d+),(\d+)') {
        return (
            ([int]$matches[1] * 3600000) +
            ([int]$matches[2] * 60000) +
            ([int]$matches[3] * 1000) +
            [int]$matches[4]
        )
    }
    return 0
}

$lines = Get-Content $InputFile
$output = @()

$lastWrittenTime = -1
$block = @()

Write-Host ('Reading ' + $InputFile )

$CurentLine = 0

foreach ($line in $lines + "") {
    $CurentLine++
    if ($line.Trim() -eq "") {
        # End of SRT block

        $percent = ($CurentLine * 100 / $lines.Length)
        Write-Progress -Activity "Processing $InputFile"  -Status "$percent% complete" -PercentComplete $percent

        if ($block.Count -gt 0) {

            $indexLine = $block | Where-Object { $_ -match '^\d+$' }
            $timeLine  = $block | Where-Object { $_ -match '-->' }

            if ($indexLine -and $timeLine) {
                $startTime = ($timeLine -split '-->')[0].Trim()
                $startMs   = Parse-TimeMs $startTime

                if ($DownsampleMs -le 0 -or
                    $lastWrittenTime -lt 0 -or
                    ($startMs - $lastWrittenTime) -ge $DownsampleMs) {

                    if ($RemoveFont) {
                        $block = $block -replace '<\/?font[^>]*>', ''
                    }

                    if ($RemoveFrameCount) {
                        $newBlock = @()
                        foreach($b in $block) {
                            $in = $b.IndexOf('FrameCnt: ')
                            if ($in -ge 0) {
                                if ($in -gt 0) {
                                    $b = $b.Substring(0, $in).TrimEnd()
                                } else {
                                    continue
                                }
                            }
                            $newBlock += $b
                        }
                        $block = $newBlock
                    }

                    if ($RemoveTimestamp) {
                        $newBlock = @()
                        foreach($b in $block) {
                            if ($b -match '\d{4}-\d{2}-\d{2}\s\d{2}:\d{2}:\d{2}\.\d{3}') {
                                continue
                            }
                            $newBlock += $b
                        }
                        $block = $newBlock
                    }

                    foreach($b in $block) {
                        # To include other fields, modify here
                        $newdata = ""
                        if ($b -match 'rel_alt:\s*([0-9]+(?:\.[0-9]+)?)') {
                            $newdata = "rel_alt: $($matches[1])"
                        }
                        # remove the `n to keep on the same line
                        if ($b -match 'abs_alt:\s*([0-9]+(?:\.[0-9]+)?)') {
                            $newdata = "`nabs_alt: $($matches[1])"
                        }
                        # remove the `n to keep on the same line
                        if ($b -match 'latitude:\s*([0-9]+(?:\.[0-9]+)?)') {
                            $newdata += "`nlatitude: $($matches[1])"
                        }
                        if ($b -match 'longitude:\s*([0-9]+(?:\.[0-9]+)?)') {
                            $newdata += "`nlongitude: $($matches[1])"
                        }
                        if ($newdata -ne "") {
                            $b = $newdata
                        }
                        $output += $b
                    }
                    $output += ""

                    $lastWrittenTime = $startMs
                }

            }

        }

        $block = @()
    }
    else {
        $block += $line
    }
}

Write-Host ('Writing ' + $OutputFile)
Out-File -FilePath $OutputFile -InputObject $output