How to plot positive and negative Y values in ImageItem

[alxcpa 01101]

I'm having trouble getting positive and negative Y values for a spectrogram.  I can use img.scale(dt,df) to scale properly but I would actually like to scale and shift down by the half of the max frequency, like img.scale(dt, df  (then - fmax/2)).  I can't seem to find the source for .scale() or .setPos(), in ImageView it references the functions to ImageItem, but it is not in the ImageItem source.  My only other idea is to create 2 images and scale one positive and one negative and combine them, but this doesn't seem necessary.  Any ideas?

[Patrick]

Hi,

ImageItem is a descendent of Qt QGraphicsItem, so the real work happens down there somewhere. Here's a an example of how I do what you describe:

# x, y are arrays of x and y axis labels, assumes regularly spaced grid
# imageItem has been added to the plotItem

x_scale = (self.x[-1] - self.x[0])/(self.x.shape[0]-1) if self.x.shape[0] > 1 else 1.0
y_scale = (self.y[-1] - self.y[0])/(self.y.shape[0]-1) if self.y.shape[0] > 1 else 1.0

self.plotItem.setLimits(xMin=self.x[0] - x_scale/2, xMax=self.x[-1] + x_scale/2,
                        yMin=self.y[0] - y_scale/2, yMax=self.y[-1] + y_scale/2)

self.imageItem.resetTransform()
self.imageItem.scale(x_scale, y_scale)
self.imageItem.setPos(self.x[0] - x_scale/2, self.y[0] - y_scale/2)

Patrick

[alxcpa 01101]

Thanks Patrick, this worked for me.