Bug report #1533
WMS raster transparency problem
|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 almost 12 years ago
I realize now that the link can be shortened to:
#2 Updated by ersts - almost 12 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
#4 Updated by ersts - almost 12 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.