Add the quad to
viz.ORTHO instead of
viz.SCREEN. Objects added to
viz.ORTHO will maintain the same pixel size and aspect ratio, regardless of the window size. The scale and position of the object will need to be specified in pixels though.
Code:
clusterTex = viz.addTexture('resources\ClusterGraphics.png')
clusterQuad = viz.addTexQuad(parent=viz.ORTHO)
clusterQuad.texture(clusterTex)
# Set size of quad to size of texture
clusterQuad.setScale(clusterTex.getSize())