Setting pixel color of 8-bit grayscale image using pointer - c++

Question

I have this code:
QImage grayImage = image.convertToFormat(QImage::Format_Grayscale8);
int size = grayImage.width() * grayImage.height();
QRgb *data = new QRgb[size];
memmove(data, grayImage.constBits(), size * sizeof(QRgb));
QRgb *ptr = data;
QRgb *end = ptr + size;
for (; ptr < end; ++ptr) {
int gray = qGray(*ptr);
}
delete[] data;
It is based on this: https://stackoverflow.com/a/40740985/8257882
How can I set the color of a pixel using that pointer?
In addition, using qGray() and loading a "bigger" image seem to crash this.
This works:
int width = image.width();
int height = image.height();
for (int y = 0; y < height; ++y) {
for (int x = 0; x < width; ++x) {
image.setPixel(x, y, qRgba(0, 0, 0, 255));
}
}
But it is slow when compared to explicitly manipulating the image data.
Edit
Ok, I have this code now:
for (int y = 0; y < height; ++y) {
uchar *line = grayImage.scanLine(y);
for (int x = 0; x < width; ++x) {
int gray = qGray(line[x]);
*(line + x) = uchar(gray);
qInfo() << gray;
}
}
And it seems to work. However, when I use an image that has only black and white colors and print the gray value, black color gives me 0 and white gives 39. How can I get the gray value in a range of 0-255?

Answer 1

First of all you are copying too much data in this line:
memmove(data, grayImage.constBits(), size * sizeof(QRgb));
The size ob Qrgb is 4 bytes, but according to the documentation, the size of a Format_Grayscale8 pixel is only 8 bits or 1 byte. If you remove sizeof(QRgb) you should be copying the correct amount of bytes, assuming all the lines in the bitmap are consecutive (which, according to the documentation, they are not -- they are aligned to at minimum 32-bits, so you would have to account for that in size). The array data should not be of type Qrgb[size] but ucahr[size]. You can then modify data as you like. Finally, you will probably have to create a new QImage with one of the constructors that accept image bits as uchar and assign the new image to the old image:
auto newImage = QImage( data, image.width(), image.height(), QImage::Format_Grayscale8, ...);
grayImage = std::move( newImage );
But instead of copying image data, you could probably just modify grayImage directly by accessing its data through bits(), or even better, through scanLine(), maybe something like this:
int line, column;
auto pLine = grayImage.scanLine(line);
*(pLine + column) = uchar(grayValue);
EDIT:
According to scanLine documentation, the image is at least 32-bit aligned. So if your 8-bit grayScale image is 3 pixels wide, a new scan line will start every 4 bytes. If you have a 3x3 image, the total size of the memory required to hold the image pixels will be 12. The following code shows the required memory size:
int main() {
auto image = QImage(3, 3, QImage::Format_Grayscale8);
std::cout << image.bytesPerLine() * image.height() << "\n";
return 0;
}
The fill method (setting all gray values to 0xC0) could be implemented like this:
auto image = QImage(3, 3, QImage::Format_Grayscale8);
uchar gray = 0xc0;
for ( int i = 0; i < image.height(); ++i ) {
auto pLine = image.scanLine( i );
for ( int j = 0; j < image.width(); ++j )
*pLine++ = gray;
}