[GIS] Scale 16 bit to 8 bit within range using GDAL

gdalgdal-translateimagery

I am looking to scale a 16bit unsigned tiff image to an 8bit image using GDAL (OSGeo4W). However I want to constrain the pixel values so that they stay within the range of the 8bit imagery. What I mean is, I want to ensure the visual proportion in the 16bit image is represented in the 8bit image (0-255). Using gdal_translate to convert from 16bit to 8bit cuts the pixel values and does not produce a RGB image. I am unsure of what function and setting to call from within gdal_translate, or if there is a better option.

GDALINFO

C:\>gdalinfo C:\Projects\Stormwater\ForPCI\images\1537TORO_6cm_04761_RGBI.tif
Driver: GTiff/GeoTIFF
Files: C:\Projects\Stormwater\ForPCI\images\1537TORO_6cm_04761_RGBI.tif
Size is 17310, 11310
Coordinate System is `'
Metadata:
  TIFFTAG_DATETIME=2015:05:03 12:27:13
  TIFFTAG_IMAGEDESCRIPTION=UltraCam-Lvl03
--------------
CAM_ID: UC-SXp-1-50215465 [3]
IMG_NO: 4761
RECORD_GUID: 8fc3d7a9-d2e5-40e5-babf-939f1f803dcf
IMG_GUID: C850D0A4-39CA-4BC8-ACD7-81A363D78C6E
FILE_GUID: 86C4F8E4-A2FD-43FB-B3B1-4C4ED8DF0E42
LICENSE_ID: 820342819
SOFTWARE: UltraCam Aerial Radiometry Core 12.6.1408.2501
PIXEL_SIZE_WIDTH: 6 [micron]
PIXEL_SIZE_HEIGHT: 6 [micron]
APERTURE: F_8
EXPOSURE_TIME: 0.002000000000 [s]
HIGH_ISO_MODE_CAPTURE: off
HIGH_ISO_MODE_PROCESSING: off

----- Inner Orientation -----
PRINCIPAL_DISTANCE: 100.500000000000 [mm]
PRINCIPAL_POINT_X: -0.000000000000 [mm]
PRINCIPAL_POINT_Y: 0.180000000000 [mm]
SENSOR_AREA_WIDTH: 103.860000000000 [mm]
SENSOR_AREA_HEIGHT: 67.860000000000 [mm]
-----------------------------

----- Exposure Annotation Data -----
MID_EXPOSURE_CORRECTION: 0.000762000000 [s]
FMS_SENSOR_CODE: UCXp
FMS_CAMERA_PORT: 1
FMS_EXPOSURE_NUMBER: 4761
FMS_PROJECT: 1537TORO
FMS_AREA: 1537TORO_6cm_6030
FMS_LINE_NUMBER: 47
FMS_SEGMENT_NUMBER: 1
FMS_WAY_POINT_NUMBER: 0
GPS_DATE: 150412
GPS_TIME: 162517
GPS_LATITUDE: N43.745319 [degree]
GPS_LONGITUDE: W079.358953 [degree]
GPS_ALTITUDE: 1159 [m]
GPS_POSITION_SOLUTION: GPS
GPS_TRACK_OVER_GROUND: 73 [degree]
GPS_ABOVE_GROUND_LEVEL: 1008.7 [m]
GPS_GROUND_SPEED: 80.4 [mps]
------------------------------------

----- Level-3 Parameters -----
IMG_TYPE: High resolution multi channel RGBI
ROTATION: 0 [degree]
------------------------------------
TIFFTAG_RESOLUTIONUNIT=1 (unitless)
TIFFTAG_SOFTWARE=UltraCam Aerial Radiometry Core 12.6.1408.2501
Image Structure Metadata:
INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left  (    0.0,    0.0)
Lower Left  (    0.0,11310.0)
Upper Right (17310.0,    0.0)
Lower Right (17310.0,11310.0)
Center      ( 8655.0, 5655.0)
Band 1 Block=17310x1 Type=UInt16, ColorInterp=Red
Band 2 Block=17310x1 Type=UInt16, ColorInterp=Green
Band 3 Block=17310x1 Type=UInt16, ColorInterp=Blue
Band 4 Block=17310x1 Type=UInt16, ColorInterp=Undefined

Best Answer

If you don' want the values above 255 to be cut, you need to scale them down. For that purpose gdal_translate provides the option -scale:

From the Manual:

-scale [src_min src_max [dst_min dst_max]]: Rescale the input pixels values from the range src_min to src_max to the range dst_min to dst_max. If omitted the output range is 0 to 255. If omitted the input range is automatically computed from the source data.

So what you can use is the following:

gdal_translate -of GTiff -ot Byte -scale 0 65535 0 255 src_dataset dst_dataset

Why 255 and 65535 ?

You have 2^nbits values. As the min value is usually 0, the max value is 2^nbits-1.

8 bit --> 2^8 = 256 values --> 0-255
16 bit --> 2^16 = 65536 vaues --> 0-65535

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[GIS] Rasterize shapefile to a new gtiff file with gdal_rasterize utility,however,I cannot open the tif file

Three suggestions:

Try outputting to another format. Maybe Jpeg or PNG. These will likely work better with Windows Photo Viewer.
Download IrfanView. This will display your tiff quite well.
If neither work - could you run 'gdalinfo -stats Countries02.tif' and post the output.

Edit:

The output is float64. This is likely the issue. Try 'gdal_translate -ot Byte -scale Countries02.tif Countires02_scaled.tif'

I'm not sure why rasterize is creating 64-bit output, but not much reads 64-bit. Also, -scale should not be necessary, but is precautionary.

[GIS] Georeferencing png using gdal

Here's a world file for you (as per Gene's suggestion). Cut and paste the following into a text editor and name the file the same as your full-sized map but give it the extension '.pgw' instead on '.png':

1.0
0.0
0.0
-1.0
0.0
3231.0

I've made the following assumptions:

Your map has a resolution (pixel size) of 1m square (i.e. each pixel is a reasonable stride length for a fantasy game humanoid character). If I am wrong, edit lines 1 and 4 accordingly.
I have assumed that your map is not rotated.
I have assumed that your LL corner is at 0,0 in your world. If your pixel size is bigger than 1m square then you will need to multiply your Y coordinate (3231.0) accordingly.
Technically the UL coordinate (last two lines) is the center of your pixel. I have been lazy and set it to the corner. This in theory means that your coordinates would be consistently half a meter out but it's a fantasy world so nobody will know; it is consistent so won't screw up any distant calculations; and it saves a negative value as one of your origin coordinates. Adjust if you are sufficiently worried :)

I would then re-tile your map from the georeferenced full-sized one (this saves you making a world file for each tile)

Now, if you load your fantasy map into a GIS or similar system, you will probably be asked for the Spatial Reference System (SRS). The World file doesn't encapsulate that. It just locates and orients the image file in space relative to a nominal 0,0 origin. Since this is a fantasy map, all you need to do is pretend that it is in the real world somewhere and pick any coordinate system that gives you a flat projection with units in meters (or feet if you prefer but you will may have to adjust the values for resolution in the world file). A couple of suggestions for SRS could be:

EPSG: 3857 (aka 900913)
EPSG: 27700 (UK National Grid - the advantage of a national grid system UK or otherwise, is that you mostly stay clear of the poles and the international dateline and can hopefully avoid any weirdness which may result from such a location)

Your choice might not matter at all... on the other hand, you choice of SRS might be influenced by what you intend to do with your KML/CSV file (like you might be making a game with a post-apocalyptic theme but set in Quebec ...with zombies...). If you are using Google Earth or similar as a 'platform' for your game/fantasy world, so long as all your other data are in the same coordinate system, all should be well.

However, I do wonder if you may be better sticking with your PNG file as a text format like KML/CSV will be big. A PNG may be better for an internet-based game (reducing bandwidth). If you have a stand-alone scenario, most programming languages understand image formats well (especially if you call the GDAL libraries) and so I'm not convinced that a text-based format is even necessary here - but that's your choice of course.

Best Answer

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[GIS] Rasterize shapefile to a new gtiff file with gdal_rasterize utility,however,I cannot open the tif file

[GIS] Georeferencing png using gdal

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