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Realtime image pixelmap from Numpy array data in Qt
June 3rd, 2013

Realtime image pixelmap from Numpy array data in Qt

Consider realtime spectrograph software like QRSS VD. It's primary function is to scroll a potentially huge data-rich image across the screen. In Python, this is often easier said than done.__ If you're not careful, you can tackle this problem inefficiently and get terrible frame rates (<5FPS) or eat a huge amount of system resources (I get complaints often that QRSS VD takes up a lot of processor resources, and 99% of it is drawing the images). In the past, I've done it at least 4 different ways (one, two, three, four, five). Note that "four" seems to be the absolute fastest option so far. I've been keeping an eye out for a while now contemplating the best way to rapidly draw color-mapped 8-bit data in a python program. Now that I'm doing a majority of my graphical development with PyQt and QtDesigner (packaged with PythonXY), I ended-up with a solution that looks like this (plotting random data with a colormap):

1.) in QtDesigner, create a form with a scrollAreaWidget

2.) in QtDesigner, add a label inside the scrollAreaWidget

3.) in code, resize label and also scrollAreaWidgetContents to fit data (disable "widgetResizable")

4.) in code, create a QImage from a 2D numpy array (dtype=uint8)

5.) in code, set label pixmap to QtGui.QPixmap.fromImage(QImage)

That's pretty much it! Here are some highlights of my program. Note that the code for the GUI is in a separate file, and must be downloaded from the ZIP provided at the bottom. Hope it helps someone else out there who might want to do something similar!

import ui_main
import sys
from PyQt4 import QtCore, QtGui

import sys
from PyQt4 import Qt
import PyQt4.Qwt5 as Qwt
from PIL import Image
import numpy
import time

spectroWidth=1000
spectroHeight=1000

a=numpy.random.random(spectroHeight*spectroWidth)*255
a=numpy.reshape(a,(spectroHeight,spectroWidth))
a=numpy.require(a, numpy.uint8, 'C')

COLORTABLE=[]
for i in range(256): COLORTABLE.append(QtGui.qRgb(i/4,i,i/2))

def updateData():
    global a
    a=numpy.roll(a,-5)
    QI=QtGui.QImage(a.data, spectroWidth, spectroHeight, QtGui.QImage.Format_Indexed8)
    QI.setColorTable(COLORTABLE)
    uimain.label.setPixmap(QtGui.QPixmap.fromImage(QI))

if __name__ == "__main__":
    app = QtGui.QApplication(sys.argv)
    win_main = ui_main.QtGui.QWidget()
    uimain = ui_main.Ui_win_main()
    uimain.setupUi(win_main)

    # SET UP IMAGE
    uimain.IM = QtGui.QImage(spectroWidth, spectroHeight, QtGui.QImage.Format_Indexed8)
    uimain.label.setGeometry(QtCore.QRect(0,0,spectroWidth,spectroHeight))
    uimain.scrollAreaWidgetContents.setGeometry(QtCore.QRect(0,0,spectroWidth,spectroHeight))

    # SET UP RECURRING EVENTS
    uimain.timer = QtCore.QTimer()
    uimain.timer.start(.1)
    win_main.connect(uimain.timer, QtCore.SIGNAL('timeout()'), updateData)

    ### DISPLAY WINDOWS
    win_main.show()
    sys.exit(app.exec_())
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