Fix various issues with application of raster styles from QML-files #38082
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When applying a style file with a colorrampshader node but without a colorramp node, the actual color values of the items of the ramp are overwritten by the default ramp (currently 'Spectral'). Files with not colorramp node happened to be created by earlier QGIS versions - at least with 2.10. This is because the default ramp is applied after populating the color ramp item list. Applying a color ramp emits a colorRampChanged() signal. This signal causes new colors from the ramp applied to the items, overwriting the already set ones. This is not desired. Fix by populating the item list after applying the color ramp.
…derer When populating QgsSingleBandPseudoColorRenderer from an existing one the min/max values where set last. However if that renderers shader was a QgsColorRampShader without an actual color ramp, a default one was used. Now, if the color ramp shader contained a color map, we try to interpolate the colors with these min/max values, which are not set yet. So we set these values before loading any raster shader.
if mMin or mMax are not set yet for some reason, we should try our best to apply a reasonable color ramp.
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Description
This PR fixes various issues with the application of styles from QML-files to raster layers. The general behavior is that a style is loaded from a QML with a colormap. Then this style seems to be show up OK in the canvas, but the actual color map assignment is sometimes wrong, and when you hit apply a wrong style is actually applied or the color scheme reverts back to a default one.
An example of this behavior (using ESACCI-LC data from http://maps.elie.ucl.ac.be/CCI/viewer/download.php)
This behavior is described in issues
Two major reasons for this behavior are probably the culprits:
applying a color ramp overwrites an already set colormap
When the QgsColorRampShaderwidget is populated by data from a QgsColorRampShader, the colormapTreeWidget is filled first (containing specific value->color assignments). After that it is checked if the shader has a colorramp and if it has none the default colorramp is applied. The problem is that this in the end emits colorRampChanged() which in turn repopulates the colorrampTreeWidget with new colors from the set colorramp (in this case the default one) overwriting the ones from the applied shader.
Fix: populate the colormap after the color ramp is applied. (7368e7f)
QgsColorRampShaderWidget::applyColorRamp() relies on valid mMin and mMax
A second problem is that the function QgsColorRampShaderWidget::applyColorRamp() assumes that the member variables mMin and mMax are already set when calculating new colors for the colormap. This is not necessarily the case as when a QgsSingleBandPseudoColorRendererWidget is populated by a QgsSingleBandPseudoColorRenderer the min/max form fields are set after the color ramp shader is applied. Then the colors for all the entries in the colormap are picked with a nan-value.
Fix:
This PR fixes #28229, #28926 and #29427 and may also fix #26701.