[Interest] QImage::Format_Grayscale8 work working

[Jason H]

It returns black. All black. Hence the issue :-)
The original file is an 8MP 4:3 image as png it saves to ~10mb. 
- as a Grayscale8 it saves as 8 KB-kilo. (should be 8,121,344)
- as Indexed8 it saves as 5.5mb 

I'd expect Greyscale8 to be about the same size on disk as Indexed8 +/- color table. 

> Sent: Friday, March 16, 2018 at 8:38 AM
> From: "Nikolai Tasev" <nikolai at nand.bg>
> To: "Jason H" <jhihn at gmx.com>
> Cc: "Interestqt-project.org" <Interest at qt-project.org>
> Subject: Re: [Interest] QImage::Format_Grayscale8 work working
>
> What is not working for Grayscale? Looking at the code it will do 
> different things when you are passing Qt::red, Qt::blue, Qt::green.
> It will not return a color image but a grayscale image of the 
> corresponding channel.
> 
> On 3/16/2018 2:08 PM, Jason H wrote:
> >> Sent: Friday, March 16, 2018 at 9:12 AM
> >> From: "Nikolai Tasev" <nikolai at nand.bg>
> >> To: "Jason H" <jhihn at gmx.com>, "Interestqt-project.org" <Interest at qt-project.org>
> >> Subject: Re: [Interest] QImage::Format_Grayscale8 work working
> >>
> >>
> >> Format_Indexed8 and Format_Grayscale8 both have 8bit per pixel. The
> >> difference is that to convert from the 8bit pixel to RGB values
> >> (QImage::pixel() method)
> >> for Indexed8u you need to set the Palette for conversion and for
> >> Grayscale8 you don't (it just assumes that R=G=B=gray value)
> > I am aware of that.
> >
> >> Are you trying to extract a channel from a multichannel image and put it
> >> into a grayscale image? The code seems unnecessary complex
> >> and inefficient for such a task. You just need to get the data pointer
> >> and skip the uneeded channels and keep watch for the end of the row.
> > That assumes I be want to do the math for every Format. ARGB is easy, RGB888 is not, etc. Pixel() alerts need to not care. Yes, I could use scan lines, but then each format requires code. Which I haven't got time to write now.
> >
> > Anyway, I be still believe my code is correct, and it's still not working.
> >
> >> On 3/15/2018 10:21 PM, Jason H wrote:
> >>> Given the following functions, I should be able to create a non-all black image (assuming input is good)? The only success I have is using Indexed8
> >>>
> >>> QHash<QRgb, qint32 (*)(QRgb)> colorFuncs {
> >>> 	{ Qt::red,   qRed},
> >>> 	{ Qt::green, qGreen},
> >>> 	{ Qt::blue,  qBlue},
> >>> 	{ Qt::gray,  qGray},
> >>> };
> >>>
> >>> QImage color8(const QImage &image, int channel) { // Channel is one of Qt::red Qt::green Qt::blue or Qt::gray
> >>> 	QImage out(image.width(), image.height(), QImage::Format_Indexed8);  // Change to Format_Grayscale8, and get nothing
> >>> // for indexed8
> >>> 	QVector<QRgb> values;
> >>> 	values.reserve(256);
> >>> 	if (channel==Qt::gray) { for (int c=0; c<256; c++) values.append(qRgb(c,c,c)); }
> >>> 	if (channel==Qt::red)  { for (int c=0; c<256; c++) values.append(qRgb(c,0,0)); }
> >>> 	if (channel==Qt::green){ for (int c=0; c<256; c++) values.append(qRgb(0,c,0)); }
> >>> 	if (channel==Qt::blue) { for (int c=0; c<256; c++) values.append(qRgb(0,0,c)); }
> >>> 	out.setColorTable(values);
> >>> // end for indexed8
> >>>
> >>> 	int (*colorFunc)(QRgb rgb) = colorFuncs[channel];
> >>> 	for (int y=0; y < image.height(); y++) {
> >>> 		for (int x=0; x < image.width(); x++) {
> >>> 			out.setPixel(x,y, colorFunc(image.pixel(x,y)));
> >>> 		}
> >>> 	}
> >>>
> >>> 	return out;
> >>> }
> >>>
> >>> I'm not familar with Qt and Grayscale8... Anyone know what is going wrong?
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> >>>
> >>
> 
>