bug fix: Qt wants data aligned to 32 bits

[ericfrederich]

Images in Qt show up incorrectly if each line is not aligned to 32 bits.

It is pretty common for an image's lines to be 32-bit aligned by chance.
Obviously any 32-bit image will not have any problem.
For the bug to manifest itself you'd need...

• a 1-bit image whose width is not a multiple of 32
• an 8-bit image who width is not a multiple of 4

Testing more images now and added a 7x13 png test image

[ericfrederich]

If this fix gets merged I can rebase #1429 on top of it since they both added the same tests.

[ericfrederich]

I just re-based this off of master. If you need me to do anything else let me know. I think it should be merged.
I wrote a little PyQt viewer that views an image after being converted through various Pillow modes.
Here you can see that it was clearly broken before my fix and that it works afterwards.
Further, this pull request includes tests for this image now.

before fix

after fix

[wiredfool]

Can you share the viewer?

[ericfrederich]

@wiredfool sure, here's the viewer...
Its more complex than is needed to exhibit the bug and the fix to it though.
Really just displaying a monochrome Tests/images/transparent.png through Qt in any way should show the problem.

Without this fix transparent.png will mess up with mode '1' but will be fine with mode 'L' and 'P' because its width is a multiple of 4. If you use an image like 7x13.png you'll see modes '1', 'L', and 'P' mess up.

Just something I wrote to test some stuff.
Didn't make any attempts to have it work on anything except the machine I was working on (Python2 and Qt4 with PyQt). You'll probably have to make minor changes to get it to work w/ Python3 or Qt5 or Pyside, etc.

#!/usr/bin/env python

import sys
from collections import OrderedDict

from PyQt4.QtCore import *
from PyQt4.QtGui import *

from PIL import Image

modes = OrderedDict()

modes['RGB'  ] = '(3x8-bit pixels, true color)'
modes['RGBA' ] = '(4x8-bit pixels, true color with transparency mask)'
modes['1'    ] = '(1-bit pixels, black and white, stored with one pixel per byte)'
modes['L'    ] = '(8-bit pixels, black and white)'
modes['P'    ] = '(8-bit pixels, mapped to any other mode using a color palette)'
# modes['CMYK' ] = '(4x8-bit pixels, color separation)'
# modes['YCbCr'] = '(3x8-bit pixels, color video format)'
# modes['LAB'  ] = '(3x8-bit pixels, the L*a*b color space)'
# modes['HSV'  ] = '(3x8-bit pixels, Hue, Saturation, Value color space)'
# modes['I'    ] = '(32-bit signed integer pixels)'
# modes['F'    ] = '(32-bit floating point pixels)'

class ClickableLabel(QLabel):
    clicked = pyqtSignal()
    def mousePressEvent(self, mouseEvent):
        self.clicked.emit()

class MyWidget(QWidget):
    def __init__(self, fname, parent=None):
        super(MyWidget, self).__init__(parent)

        layout = QVBoxLayout()
        self.img_label = ClickableLabel()
        self.desc_label = QLabel()
        self.combo = QComboBox()
        self.combo.addItems(modes.keys())
        layout.addWidget(self.combo)
        layout.addWidget(self.desc_label)
        layout.addWidget(self.img_label)
        self.setLayout(layout)
        self.combo.currentIndexChanged['QString'].connect(self.on_combo_change)
        self.img_label.clicked.connect(self.new_file)

        self.change_image(fname)

    def on_combo_change(self, s):
        s = str(s)

        # need to persist this one, so keep it on self
        self.qi = self.im.convert(s).toqimage()
        pm = QPixmap.fromImage(self.qi)
        self.img_label.setPixmap(pm.scaled(pm.width() * self.scale, pm.height() * self.scale, Qt.IgnoreAspectRatio, Qt.FastTransformation))
        self.desc_label.setText(modes[s])

    def new_file(self):
        fname = str(QFileDialog.getOpenFileName(
            self, "Open File",
            "Tests/images", "Images (*.png *.xpm *.jpg)"))
        self.change_image(str(fname))

    def change_image(self, fname):
        self.im = Image.open(fname)
        self.scale = 1
        while max(self.im.size) * (self.scale + 1) <= 800:
            self.scale += 1

        self.on_combo_change(self.combo.currentText())

def main():
    fname = sys.argv[1]
    app = QApplication(sys.argv)
    mw = MyWidget(fname)
    mw.show()
    sys.exit(app.exec_())

if __name__ == '__main__':
    main()

[wiredfool]

Thanks for the viewer. I'm not seeing the effects of this patch in the viewer, as it's working in master and this branch on 1 images. The tests, on the other hand, fail nicely in master and succeed here.