Convert a tiff Float32 image to 8bit tif (or jpeg) image using GDAL and Python

Question

I need to convert a Float32 tiff image to 8bit format in order to display the image in a web page. In order to do that I need a jpg image but this conversion is not correctly done with GDAL because the image is Float32 type. So I want to convert to 8bit type with Python.

Can you help me?

I've tried something like this:

gdal.AllRegister()  
src = gdal.Open(str(input_file)) 
src_data = src.ReadAsArray()
cols = src.RasterXSize
rows = scr.RasterYSize
driver = gdal.GetDriverByName("GTiff")
dst = driver.Create(str(output_file), cols, rows, 1, gdal.GDT_Byte)
dst.GetRasterBand(1).WriteArray(src_data)
#dst = drv.CreateCopy(output_file, src, 0, saveOptions)

I try with Create() and CreateCopy() but I always get a black image.

Answer 1

You can do that directly in QGIS with Raster -> Conversion -> Translate (Convert Format). To test it, I used a Float64 (Sixty four bit floating point) raster (named test) loaded in the Map View of QGIS. To convert it to Byte (8 bit type), I named it first at the "output file space" as test_byte.tif and afterward, I click in the icon pencil of "Translate Window" to modify gdal_translate command to:

gdal_translate -ot Byte -of GTiff /home/zeito/proyectosR/test.tif /home/zeito/proyectosR/test_byte.tif

as in the following image:

After click in OK, it can be observed at the layer properties of test_byte.tif that this raster is "Eight bit unsigned integer":

Editing Note:

This code works (to change the data type) in my bash console (in Windows is necessary to use IDLE Python GUI):

from osgeo import gdal, osr

dataset = gdal.Open('tiznados_canoa.tif')

geotransform = dataset.GetGeoTransform()

prj=dataset.GetProjection()

values = dataset.ReadAsArray()

band = dataset.GetRasterBand(1)

#Get minimum and maximum values of raster
min = band.GetMinimum()
max = band.GetMaximum()

#if not exist minimum and maximum values
if min is None or max is None:
    (min,max) = band.ComputeRasterMinMax(1)

print "Min=%.3f, Max=%.3f" % (min,max) #print minimum and maximum values

# Set output file
output_file = "out.tif"

# Create new GTiff (Byte type)
driver = gdal.GetDriverByName("GTiff")
dst_ds = driver.Create(output_file, band.XSize, band.YSize, 1, gdal.GDT_Byte)

print "rows = ", band.YSize, "columns = ", band.XSize

print "Executing..."

for i in range(band.YSize):
    for j in range(band.XSize):

        values[i][j]

dst_ds.GetRasterBand(1).WriteArray( values )

# top left x, w-e pixel resolution, rotation, top left y, rotation, n-s pixel resolution
dst_ds.SetGeoTransform( [ geotransform[0], geotransform[1], 0, geotransform[3], 0, geotransform[5] ] )

# set projection of new raster
dst_ds.SetProjection( prj )

dataset = None

Answer 2

The most easy way to do this in a script is to use the subprocess module and gdal_translate

import subprocess

image_in = "path_input_image"
image_out = "path_output_image.tif"
subprocess.call(["gdal_translate.exe","-co", "TILED=YES", "-co", "COMPRESS=LZW" "-ot", "Byte", "-scale", image_in, image_out ])

f you are in Linux, you don't need the ".exe" after gdal_translate, and if gdal is not installed, you might need to provide the full path to the application

alternatively, this can be done with OTBApplication

#!/usr/bin/python

# Import the otb applications package
import otbApplication

image_in = "path_input_image"
image_out = "path_output_image.tif"

# The following line creates an instance of the Rescale application
Rescale = otbApplication.Registry.CreateApplication("Rescale")

# The following lines set all the application parameters:
Rescale.SetParameterString("in",image_in )

Rescale.SetParameterString("out",image_out )
Rescale.SetParameterOutputImagePixelType("out", ImagePixelType_uint8)

Rescale.SetParameterFloat("outmin", 0)

Rescale.SetParameterFloat("outmax", 255)

# The following line execute the application
Rescale.ExecuteAndWriteOutput()