Convert RAW 8 bit pixels into a QImage

[danielperaza]

Hi, I would like to know how can I convert an array of unsigned chars which represents a grayscale image in raw format into a QImage object, since I have been trying with Qt's QImage::loadFromData() unsuccessfully. Also, if there is any source of information on Qt's native image processing API, please let me know because Trolltech's official C++ Qt GUI Programming with Qt has become not enough for me.

[jpn]

QImage::loadFromData() expects the data to contain a header. Just construct a QImage object with suitable size and format and access its bits().

[pherthyl]

Hi, I would like to know how can I convert an array of unsigned chars which represents a grayscale image in raw format into a QImage object, since I have been trying with Qt's QImage::loadFromData() unsuccessfully. Also, if there is any source of information on Qt's native image processing API, please let me know because Trolltech's official C++ Qt GUI Programming with Qt has become not enough for me.

This is how I do it:

uchar** currImageData = &(frameData.value(frameName)->imageData);
 
	// check if the frame dimensions have changed
	if(!(*currImageData) || frame->width != frameData.value(frameName)->width || frame->height != frameData.value(frameName)->height) { 
		if(*currImageData) {
			qDebug() << "deleting data";
			delete[] *currImageData;
		}
		qDebug() << "allocating" << frame->nChannels << " channel image of size" << frame->width << frame->height << "and depth" << frame->depth;
		imageHeight = frame->height;
		*currImageData = new unsigned char[4*imageWidth*imageHeight];
		for(int i = 0; i < imageWidth*imageHeight; i++) {
			(*currImageData)[i*4+3] = 0xFF;
		}
	}
 
	int pixels =frame->width * frame->height;
 
		uchar* src = (uchar*)(frame->imageData);
		uchar* srcEnd = src + (frame->nChannels*pixels);
		uchar* dest = *currImageData;
		if(frame->nChannels == 3) {
			do {
				memcpy(dest, src, 3);
				dest += 4;
				src += 3;
			} while(src < srcEnd);
		}
		else if(frame->nChannels == 1) {
			//qDebug() << "single channel";
			int index = 0;
			do {
				// RGB pixels are written with the same value
				memset(dest, *src, 3);
				// increment past alpha
				dest+=4;
				// increment source to next pixel
				src++;
				index++;
			} while(src < srcEnd);
		}
	}
}

uchar** currImageData = &(frameData.value(frameName)->imageData);

	// check if the frame dimensions have changed
	if(!(*currImageData) || frame->width != frameData.value(frameName)->width || frame->height != frameData.value(frameName)->height) { 
		if(*currImageData) {
			qDebug() << "deleting data";
			delete[] *currImageData;
		}
		qDebug() << "allocating" << frame->nChannels << " channel image of size" << frame->width << frame->height << "and depth" << frame->depth;
		imageHeight = frame->height;
		*currImageData = new unsigned char[4*imageWidth*imageHeight];
		for(int i = 0; i < imageWidth*imageHeight; i++) {
			(*currImageData)[i*4+3] = 0xFF;
		}
	}

	int pixels =frame->width * frame->height;
	
		uchar* src = (uchar*)(frame->imageData);
		uchar* srcEnd = src + (frame->nChannels*pixels);
		uchar* dest = *currImageData;
		if(frame->nChannels == 3) {
			do {
				memcpy(dest, src, 3);
				dest += 4;
				src += 3;
			} while(src < srcEnd);
		}
		else if(frame->nChannels == 1) {
			//qDebug() << "single channel";
			int index = 0;
			do {
				// RGB pixels are written with the same value
				memset(dest, *src, 3);
				// increment past alpha
				dest+=4;
				// increment source to next pixel
				src++;
				index++;
			} while(src < srcEnd);
		}
	}
}

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The first part is just memory allocation for the data, then it copies it (depending on the number of channels) and when you want a QImage from that data you can use the QImage constructor that takes uchar* data and give it the correct dimensions.

[spud]

I would follow JPN's suggestion, but using QImage::scanLine() like this:

QImage image(width, height, QImage::Format_Indexed8);
for (int y = 0;y<height; y++)
	memcpy(image.scanLine(y), rawData + y*width, width);

QImage image(width, height, QImage::Format_Indexed8);
for (int y = 0;y<height; y++)
	memcpy(image.scanLine(y), rawData + y*width, width);

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The problem with QImage::bits() is that Qt might add padding to each row to align the rows to 32 bits.

Then you need to set the Color table like this:

QVector<QRgb> colorTable(256);
for(int i=0;i<256;i++)
	colorTable[i] = qRgb(i,i,i);
image.setColorTable(colorTable);

QVector<QRgb> colorTable(256);
for(int i=0;i<256;i++)
	colorTable[i] = qRgb(i,i,i);
image.setColorTable(colorTable);

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