Symbology display issues in raster layers

Question

The code below aims to apply symbology based on NDVI values ​​to the selected raster layer. However, after running the script it does not display correct information (image 1). I need to access layer properties and click OK or Apply to display correctly (image 2 and 3). In other words, the code works for its purpose but there is an error in displaying or loading the layer. I'm using QGIS 3.30, Linux Mint 21.3.

Band statistics for this layer:

• STATISTICS_APPROXIMATE=YES
• STATISTICS_MAXIMUM=0.93173605203629
• STATISTICS_MEAN=0.67241566769275
• STATISTICS_MINIMUM=-0.69150775671005
• STATISTICS_STDDEV=0.28242418703578
• STATISTICS_VALID_PERCENT=100

ndvi_layer = iface.activeLayer()

stats = ndvi_layer.dataProvider().bandStatistics(1, QgsRasterBandStats.All, ndvi_layer.extent(), 0)

min_value = stats.minimumValue
max_value = stats.maximumValue

style = QgsStyle().defaultStyle()
color_ramp = style.colorRamp('RdYlGn')

shader = QgsColorRampShader()
shader.setColorRampType(QgsColorRampShader.Interpolated)

min_color = color_ramp.color(0.0)
mid_color = color_ramp.color(0.5)
max_color = color_ramp.color(1.0)

items = [
    QgsColorRampShader.ColorRampItem(min_value, min_color, f'{min_value}'),
    QgsColorRampShader.ColorRampItem((min_value + max_value) / 2, mid_color, f'{(min_value + max_value) / 2}'),
    QgsColorRampShader.ColorRampItem(max_value, max_color, f'{max_value}')
]
shader.setColorRampItemList(items)

raster_shader = QgsRasterShader()
raster_shader.setRasterShaderFunction(shader)

renderer = QgsSingleBandPseudoColorRenderer(ndvi_layer.dataProvider(), 1, raster_shader)
ndvi_layer.setRenderer(renderer)
ndvi_layer.triggerRepaint()
iface.mapCanvas().refreshAllLayers()

Answer 1

You can use the createShader() method of QgsSingleBandPseudoColorRenderer.

Example:

ndvi_layer = iface.activeLayer()
prov = ndvi_layer.dataProvider()
stats = prov.bandStatistics(1, QgsRasterBandStats.All, ndvi_layer.extent(), 0)
min_value = stats.minimumValue
max_value = stats.maximumValue
renderer = QgsSingleBandPseudoColorRenderer(prov, 1)
style = QgsStyle.defaultStyle()
color_ramp = style.colorRamp('RdYlGn')
renderer.setClassificationMin(min_value)
renderer.setClassificationMax(max_value)
renderer.createShader(color_ramp)
ndvi_layer.setRenderer(renderer)
ndvi_layer.triggerRepaint()

Result:

Side note: Since NDVI values are normalized with a fixed scale, I would consider setting the renderer classification min & max to -1 and 1 respectively (unless you just want to see local range/variation in NDVI values).