Bug report #1533

WMS raster transparency problem

Added by John Tull over 15 years ago. Updated almost 15 years ago.

Assignee:ersts -
Category:Map Canvas
Affected QGIS version: Regression?:No
Operating System:OS X Easy fix?:No
Pull Request or Patch supplied: Resolution:fixed
Crashes QGIS or corrupts data: Copied to github as #:11593


It appears that a WMS raster layer with transparency set in the layer properties will increase transparency with every screen refresh until it becomes invisible. To replicate, try the NED shaded relief layer from:



#1 Updated by John Tull over 15 years ago

#2 Updated by ersts - over 15 years ago

  • Status changed from Open to In Progress

This is a known issue. If you keep repeatedly hitting apply, the transparency it is additive. Likewise, if you decrease the transparency, it is still additive. However, if you pan the map canvas a little and cause a new fetch request, your layer will be at the correct/expected transparency level.

It seems to be a bug in Qt with setting the alpha value for an image more than once. Each apply causes a refresh and thus a reapplication of the alpha value. Too speed things up, the WMS provider does not request a new version of the image if the extent has not changed, it uses a cached version.

I am going to downgrade this a little, while it needs to get fixed but it does not cause a crash or a damage of data

#3 Updated by John Tull over 15 years ago

Peter: Thanks for the detailed explanation. I did not see a bug filed, so added it this weekend. It sounds like I need to be filing bugs against Qt instead... Also, it's good to know that forcing a wms refresh works around the problem.

#4 Updated by ersts - over 15 years ago

I think it is more just that setAlphaChannel() does not work quite a expected. When you use that method it changes the image format to QImage::Format_ARGB32_Premultiplied

The image is stored using a premultiplied 32-bit 

ARGB format (0xAARRGGBB), i.e. the red, green,

and blue channels are multiplied by the alpha component 

divided by 255. (If RR, GG, or BB has a

higher value than the alpha channel, the results 

are undefined.) Certain operations (such as 

image composition using alpha blending) are faster 

using premultiplied ARGB32 than with plain ARGB32.

So I changed it around a little have had the WMS Provider store a QImage::Format_ARGB32 then just adjusted the pixel values on each refresh. WMS images should be small enough to not see any performance hit.

The mods were made to the trunk r:10146. If you can take it for a spin and make sure it works more as expected. If it does I will close the ticket and add it to the Version-1_0 branch for inclusion in the next release.

#5 Updated by John Tull over 15 years ago

  • Status changed from In Progress to Closed
  • Resolution set to fixed

It does appear to be working now. Great! I closed the ticket.

#6 Updated by Anonymous almost 15 years ago

Milestone Version 1.0.1 deleted

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