Get certain pixels with their coordonates from an Image with Qt/QML - c++

Question

We are creating a game where there are maps. On those maps, players can walk, but to know if they can walk somewhere, we have another image, where the path is paint.
The player can move by clicking on the map, if the click match with the collider image, the character should go to the clicked point with a pathfinder. If not, the character don't move.
For example, here is a map and its collision path image :
How can I know if I've clicked on the collider (this is a png with one color and transparency) in Qt ?
I'm using QML and Felgo for rendering so if there is already a way to do it with QML, it's even better, but I can implement it in C++ too.
My second question is how can I do a pathfinder ? I know the algorithms for that but should I move by using pixels ?
I've seen the QPainterPath class which could be what i'm looking for, how can I read all pixels with a certain color of my image and know their coordonates ?
Thanks

Answer 1

QML interface doesn't provide efficient way to resolve your task. It should be done at C++ side.
To get image data you can use:
QImage to load image
Call N times QImage::constScanLine, each time read K pixels. N equals to image height in pixels, K equals to width.
How to deal with returned uchar* of QImage::constScanLine?
You should call QImage::format() to determine pixel format hidden by uchar*. Or you can call QImage::convertToFormat(QImage::Format_RGB32) and always cast pixel data from uchar* to your custom struct like PixelData:
#pragma pack(push, 1)
struct PixelData {
uint8_t padding;
uint8_t r;
uint8_t g;
uint8_t b;
};
#pragma pack(pop)
according to this documentation: https://doc.qt.io/qt-5/qimage.html#Format-enum
Here is compilable solution for loading image into RAM for further effective working with it's data:
#include <QImage>
#pragma pack(push, 1)
struct PixelData {
uint8_t padding;
uint8_t r;
uint8_t g;
uint8_t b;
};
#pragma pack(pop)
void loadImage(const char* path, int& w, int& h, PixelData** data) {
Q_ASSERT(data);
QImage initialImage;
initialImage.load(path);
auto image = initialImage.convertToFormat(QImage::Format_RGB32);
w = image.width();
h = image.height();
*data = new PixelData[w * h];
PixelData* outData = *data;
for (int y = 0; y < h; y++) {
auto scanLine = image.constScanLine(y);
memcpy(outData, scanLine, sizeof(PixelData) * w);
outData += w;
}
}
void pathfinder(const PixelData* data, int w, int h) {
// Your algorithm here
}
void cleanupData(PixelData* data) {
delete[] data;
}
int main(int argc, char *argv[])
{
int width, height;
PixelData* data;
loadImage("D:\\image.png", width, height, &data);
pathfinder(data, width, height);
cleanupData(data);
return 0;
}
You can access each pixel by calling this function
inline const PixelData& getPixel(int x, int y, const PixelData* data, int w) {
return *(data + (w * y) + x);
}
... or use this formula somewhere in your pathfinding algorithm, where it could be more efficient.