Export GIS Images with Transparency in QGIS – How to

exportpngqgistransparency

I'm trying to export some images of Gulf of Mexico infrastructure that will later be used for non-GIS purposes as overlays onto sea current/weather maps (likely in photoshop).

I have no clue how to do this in either Arc or in QGIS. My student version of Arc just ran out but I still have access to Arc through school if there's an easy solution there ( I suppose Arc might have an "easy" solution).

I would rather a QGIS solution though as it is more sustainable for me in the long run. I have seen how to have a transparent background in the Print Composer background, but this doesn't help me when I add map data to the composer as the map data is imported into the composer with a white background.

Any tips? Maybe a way to change the background color of QGIS itself to transparent?

PS: To clarify, I'm looking to export with transparency, not have transparency in the program. Also, if it matters, the layers are vectors.

Best Answer

This is possible in QGIS using the print composer. You'll need to both:

Remove the background color from the map item. This is done by unchecking the "background" option in the map item's properties. (The default is a white background)
Set the page itself to a transparent style. This is done through the composition properties tab.

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[GIS] How to make transparent the background of topographic map in QGIS 1.8

Right-Click Layer Properties and select Transparency tab.
You would have a default Row in the transparency pixel list. Delete that Row(Use Icon 3)
If you just have to make the White part transparent, Create a new Row(Use Icon1). Enter 255,255,255 for Red,Green and white and Set 100 for percent transparent column.
Click Apply and you are done.

Some sample results.. enter image description here

enter image description here

Additionally, if you want to make any other color on the raster transparent, You can click the Icon2, This would ask you to select a pixel on the raster using mouse. This might make QGIS go minimized for some weird reason, but you can restore it from the taskbar. Then click the color on the raster which you want to make transparent. It would add a new row for that color and make that color transparent for the raster.

QGIS PNG – Saving Map Canvas as PNG with Transparent Background Programmatically with QGIS

You can set the page background to transparent by creating a transparent symbol and setting the page background to use that symbol:

transparent_fill = QgsFillSymbolV2.createSimple({ 'outline_style': 'no', 'style': 'no'})
c.setPageStyleSymbol( transparent_fill )

There's also a few other things wrong with your script:

First,

mapRenderer = iface.mapCanvas().mapRenderer()
c = QgsComposition(mapRenderer)

mapRenderer is deprecated and will be removed soon. The replacement is QgsMapSettings:

map_settings = iface.mapCanvas().mapSettings()
c = QgsComposition(map_settings)

Next,

# get all items and disable the backgrounds
itemList = c.items()
c.removeItem(itemList[2])
c.removeItem(itemList[2])
itemList[2].setBackgroundEnabled(False)
itemList[3].setBackgroundEnabled(False)
# add them to the composition
c.addItem(itemList[2])
c.addItem(itemList[3])

c.refreshItems()
c.refreshDataDefinedProperty(QgsComposerObject.AllProperties)

Most of this isn't required. Just set the map item to have no background after you've created it:

composerMap = QgsComposerMap(c, x ,y, w, h)
composerMap.setBackgroundEnabled(False)

Lastly:

# image = QImage(QSize(width, height), QImage.Format_ARGB32)
image = QImage(QSize(width, height), QImage.Format_ARGB32_Premultiplied)
image.setDotsPerMeterX(dpmm * 1000)
image.setDotsPerMeterY(dpmm * 1000)
#image.fill(0)

Should be:

image = QImage(QSize(width, height), QImage.Format_ARGB32)
image.setDotsPerMeterX(dpmm * 1000)
image.setDotsPerMeterY(dpmm * 1000)
image.fill( Qt.transparent )

Don't use a premultiplied image, and you MUST initialise the image with a color. In this case we'll initialise it by filling it with a transparent color.

Here's a working (and cleaned up) version:

from PyQt4.QtCore import *
from PyQt4.QtGui import *

# PARAMETERS
mainPath = '/path/to/folder/'
filename = 'filename'
imageType = "png"
imageWidth_mm = 400
imageHeight_mm = 160
dpi = 300

map_settings = iface.mapCanvas().mapSettings()
c = QgsComposition(map_settings)
c.setPaperSize(400, 160)
c.setPrintResolution(dpi)

#set page background to transparent
transparent_fill =QgsFillSymbolV2.createSimple({ 'outline_style': 'no', 'style': 'no'})
c.setPageStyleSymbol( transparent_fill )

x, y = 0, 0
w, h = c.paperWidth(), c.paperHeight()
composerMap = QgsComposerMap(c, x ,y, w, h)
composerMap.setBackgroundEnabled(False)
c.addItem(composerMap)

dpmm = dpi / 25.4
width = int(dpmm * c.paperWidth())
height = int(dpmm * c.paperHeight())

# create output image and initialize it
image = QImage(QSize(width, height), QImage.Format_ARGB32)
image.setDotsPerMeterX(dpmm * 1000)
image.setDotsPerMeterY(dpmm * 1000)
image.fill(Qt.transparent)

imagePainter = QPainter(image)

c.setPlotStyle(QgsComposition.Print)
c.renderPage( imagePainter, 0 )
imagePainter.end()

imageFilename =  mainPath + filename + '.' + imageType
image.save(imageFilename, imageType)
print 'image saved'
print 'done'

Best Answer

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