How to solve performance issues with QPixmap (large drawingjobs)? - c++

Question

I am coding a small map editor (with rectangle tiles) and I need a way to draw a large amount of images OR one big image. The application is simple: You draw images on an empty screen with your mouse and when you are finished you can save it. A tile consists of a small image.
I tried out several solutions to display the tiles:
Each tile has its own QGraphicsItem (This works until you have a
1000x1000 map)
Each tile gets drawn on one big QPixmap (This means a very large image. Example: Map with 1000x100 and each tile has a size of 32x32 means that the QPixmap has a size of 32000x32000. This is a problem for QPainter.)
The current solution: Iterate through width & height of the TileLayer and draw each single tile with painter->drawPixmap(). The paint() method of my TileLayer looks like this:
void TileLayerGraphicsItem::paint(QPainter* painter, const QStyleOptionGraphicsItem* option,QWidget* /*widget*/)
{
painter->setClipRect(option->exposedRect);
int m_width=m_layer->getSize().width();
int m_height=m_layer->getSize().height();
for(int i=0;i<m_width;i++)
{
for(int j=0;j<(m_height);j++)
{
Tile* thetile=m_layer->getTile(i,j);
if(thetile==NULL)continue;
const QRectF target(thetile->getLayerPos().x()*thetile->getSize().width(),thetile->getLayerPos().y()*thetile->getSize().height(),thetile->getSize().width(),thetile->getSize().height());
const QRectF source(0, 0, thetile->getSize().width(), thetile->getSize().height());
painter->drawImage(target,*thetile->getImage(),source);
}
}}
This works for small maps with 100x100 or even 1000x100 tiles. But not for 1000x1000. The whole application begins to lag, this is of course because I have a for loop that is extremely expensive. To make my tool useful I need to be able to make at least 1000x1000 tilemaps without lags. Does anyone have an idea what I can do? How should I represent the tiles?
Update:
I changed the following: Only maps that exceed the window size of the minimap will be drawn with drawing single pixels for each tile. This is my render function now:
void RectangleRenderer::renderMinimapImage(QPainter* painter, TileMap* map,QSize windowSize)
{
for(int i=0;i<map->getLayers().size();i++)
{
TileLayer* currLayer=map->getLayers().at(i);
//if the layer is small draw it completly
if(windowSize.width()>currLayer->getSize().width()&&windowSize.height()>currLayer->getSize().height())
{
...
}
else // This is the part where the map is so big that only some pixels are drawn!
{
painter->fillRect(0,0,windowSize.width(),windowSize.height(),QBrush(QColor(map->MapColor)));
for(float i=0;i<windowSize.width();i++)
for(float j=0;j<windowSize.height();j++)
{
float tX=i/windowSize.width();
float tY=j/windowSize.height();
float pX=lerp(i,currLayer->getSize().width(),tX);
float pY=lerp(j,currLayer->getSize().height(),tY);
Tile* thetile=currLayer->getTile((int)pX,(int)pY);
if(thetile==NULL)continue;
QRgb pixelcolor=thetile->getImage()->toImage().pixel(thetile->getSize().width()/2,thetile->getSize().height()/2);
QPen pen;
pen.setColor(QColor::fromRgb(pixelcolor));
painter->setPen(pen);
painter->drawPoint(i,j);
}
}
}
}
This works not correct, however it is pretty fast. The problem is my lerp(linear interpolation) function to get the correct tiles to draw a pixel from.
Does anyone have a better solution to get the correct tiles while I iterate through the minimap pixels? At the moment I use linear interpolation between 0 and the maximum size of the tilemap and it does not work correctly.
UPDATE 2
//currLayer->getSize() returns how many tiles are in the map
// currLayer->getTileSize() returns how big each tile is (32 pixels width for example)
int raw_width = currLayer->getSize().width()*currLayer->getTileSize().width();
int raw_height = currLayer->getSize().height()*currLayer->getTileSize().height();
int desired_width = windowSize.width();
int desired_height = windowSize.height();
int calculated_width = 0;
int calculated_height = 0;
// if dealing with a one dimensional image buffer, this ensures
// the rows come out clean, and you don't lose a pixel occasionally
desired_width -= desired_width%2;
// http://qt-project.org/doc/qt-5/qt.html#AspectRatioMode-enum
// Qt::KeepAspectRatio, and the offset can be used for centering
qreal ratio_x = (qreal)desired_width / raw_width;
qreal ratio_y = (qreal)desired_height / raw_height;
qreal floating_factor = 1;
QPointF offset;
if(ratio_x < ratio_y)
{
floating_factor = ratio_x;
calculated_height = raw_height*ratio_x;
calculated_width = desired_width;
offset = QPointF(0, (qreal)(desired_height - calculated_height)/2);
}
else
{
floating_factor = ratio_y;
calculated_width = raw_width*ratio_y;
calculated_height = desired_height;
offset = QPointF((qreal)(desired_width - calculated_width)/2,0);
}
for (int r = 0; r < calculated_height; r++)
{
for (int c = 0; c < calculated_width; c++)
{
//trying to do the following: use your code to get the desired pixel. Then divide that number by the size of the tile to get the correct pixel
Tile* thetile=currLayer->getTile((int)((r * floating_factor)*raw_width)/currLayer->getTileSize().width(),(int)(((c * floating_factor)*raw_height)/currLayer->getTileSize().height()));
if(thetile==NULL)continue;
QRgb pixelcolor=thetile->getImage()->toImage().pixel(thetile->getSize().width()/2,thetile->getSize().height()/2);
QPen pen;
pen.setColor(QColor::fromRgb(pixelcolor));
painter->setPen(pen);
painter->drawPoint(r,c);
}
}
Trying to reverse engineer the example code, but it still does not work correctly.
Update 3
I tried (update 1) with linear interpolation again. And while I looked at the code I saw the error:
float pX=lerp(i,currLayer->getSize().width(),tX);
float pY=lerp(j,currLayer->getSize().height(),tY);
should be:
float pX=lerp(0,currLayer->getSize().width(),tX);
float pY=lerp(0,currLayer->getSize().height(),tY);
That's it. Now it works.

Answer 1

This shows how to do it properly. You use a level of detail (lod) variable to determine how to draw the elements that are currently visible on the screen, based on their zoom.
http://qt-project.org/doc/qt-5/qtwidgets-graphicsview-chip-example.html
Also don't iterate through all the elements that could be visible, but only go through the ones that have changed, and of those, only the ones that are currently visible.
Your next option to use is some other manual caching, so you don't have to repeatedly iterate through O(n^2) constantly.
If you can't optimize it for QGraphicsView/QGraphicsScene... then OpenGL is probably what you may want to look into. It can do a lot of the drawing and caching directly on the graphics card so you don't have to worry about it as much.
UPDATE:
Pushing changes to QImage on a worker thread can let you cache, and update a cache, while leaving the rest of your program responsive, and then you use a Queued connection to get back on the GUI thread to draw the QImage as a Pixmap.
QGraphicsView will let you know which tiles are visible if you ask nicely:
http://qt-project.org/doc/qt-5/qgraphicsview.html#items-5
UPDATE 2:
http://qt-project.org/doc/qt-5/qtwidgets-graphicsview-chip-chip-cpp.html
You may need to adjust the range of zooming out that is allowed on the project to test this feature...
Under where it has
const qreal lod = option->levelOfDetailFromTransform(painter->worldTransform());
if (lod < 0.2) {
if (lod < 0.125) {
painter->fillRect(QRectF(0, 0, 110, 70), fillColor);
return;
}
QBrush b = painter->brush();
painter->setBrush(fillColor);
painter->drawRect(13, 13, 97, 57);
painter->setBrush(b);
return;
}
Add in something like:
if(lod < 0.05)
{
// using some sort of row/col value to know which ones to not draw...
// This below would only draw 1/3 of the rows and 1/3 of the column
// speeding up the redraw by about 9x.
if(row%3 != 0 || col%3 != 0)
return;// don't do any painting, return
}
UPDATE 3:
Decimation Example:
// How to decimate an image to any size, properly
// aka fast scaling
int raw_width = 1000;
int raw_height = 1000;
int desired_width = 300;
int desired_height = 200;
int calculated_width = 0;
int calculated_height = 0;
// if dealing with a one dimensional image buffer, this ensures
// the rows come out clean, and you don't lose a pixel occasionally
desired_width -= desired_width%2;
// http://qt-project.org/doc/qt-5/qt.html#AspectRatioMode-enum
// Qt::KeepAspectRatio, and the offset can be used for centering
qreal ratio_x = (qreal)desired_width / raw_width();
qreal ratio_y = (qreal)desired_height / raw_height();
qreal floating_factor = 1;
QPointF offset;
if(ratio_x < ratio_y)
{
floating_factor = ratio_x;
calculated_height = raw_height*ratio_x;
calculated_width = desired_width;
offset = QPointF(0, (qreal)(desired_height - calculated_height)/2);
}
else
{
floating_factor = ratio_y;
calculated_width = raw_width*ratio_y;
calculated_height = desired_height;
offset = QPointF((qreal)(desired_width - calculated_width)/2);
}
for (int r = 0; r < calculated_height; r++)
{
for (int c = 0; c < calculated_width; c++)
{
pixel[r][c] = raw_pixel[(int)(r * floating_factor)*raw_width][(int)(c * floating_factor)];
}
}
Hope that helps.