Color tables of grayscale images

Question

I have a grayscale image in PyQt and want to get the color of a specific pixel. Grayscale images use a color table with up to 256 entries.

from PyQt4 import QtGui

def test():
    image = QtGui.QImage(100, 100, QtGui.QImage.Format_Indexed8)
    image.load("d:/1.bmp")
    print image.pixel(1, 1)
    print image.pixelIndex(1, 1)

    image.setColorTable(list([i] for i in range(256)))
    print image.colorTable()

import sys
app = QtGui.QApplication(sys.argv)
window = test()
sys.exit(app.exec_())

This is d:/1.bmp:

I have the following issues:

• image.colorTable() returns a list of 256 times the number 4294967295L (which is 2^32-1) altough I have just set the color table to 0 to 255.

• image.pixelIndex(1, 1) gives the message "QImage::pixelIndex: Not applicable for 32-bpp images (no palette)" although the format is set to Indexed8 (and isGrayscale() returns true).

• image.pixel(1, 1) returns 4278190080 (also when I set the format to Format_RGB32). What is this color? (It should be black.)

New code according to the answer by ekhumoro:

from PyQt4 import QtGui

def test():
    image = QtGui.QImage(100, 100, QtGui.QImage.Format_Indexed8)
    image.load("d:/1.bmp")
    image2 = image.convertToFormat(QtGui.QImage.Format_Indexed8)
    print "format:", image2.format()
    print "pixel color:", QtGui.qGray(image2.pixel(1, 1))

    image2.setColorTable(list([QtGui.qRgb(i, i, i)] for i in range(256)))
    print "color table:", image2.colorTable()

import sys
app = QtGui.QApplication(sys.argv)
window = test()
sys.exit(app.exec_())

Answer 1

The docs for QImage have all the answers to your questions:

• From the entry for setColorTable():

  Sets the color table used to translate color indexes to QRgb values, to the specified colors. When the image is used, the color table must be large enough to have entries for all the pixel/index values present in the image, otherwise the results are undefined.

• From the entry for load():

  The loader attempts to read the image using the specified format, e.g., PNG or JPG. If format is not specified (which is the default), the loader probes the file for a header to guess the file format.

  So the format you passed to the QImage constructor is irrelevant, and I would predict that print image.format() will output a value > 3. Also, looking at the entry for pixelIndex() I see this:

  If position is not valid, or if the image is not a paletted image (depth() > 8), the results are undefined.

• From the entry for pixel():

  QRgb QImage::pixel(int x, int y) const.

  So this function returns a value of type QRgb, which the docs describe thus:

  An ARGB quadruplet on the format #AARRGGBB, equivalent to an unsigned int.

  Handily, Qt provides some functions for extracting the various components of QRgb values. One of these is qGray, which is described thus:

  Returns a gray value (0 to 255) from the given ARGB quadruplet rgb.

  (NB: these functions are in the global namespace, so in PyQt, you'll find them in the QtGui module).