Need floating point precision, GUI gui uses int - c++

Question

I have a flow layout. Inside it I have about 900 tables. Each table is stacked one on top of the other. I have a slider which resizes them and thus causes the flow layout to resize too.
The problem is, the tables should be linearly resizing. Their base size is 200x200. So when scale = 1.0, the w and h of the tables is 200.
I resize by a fixed amount each time making them 4% bigger each time. This means I would expect them to grow by 8 pixels each time. What happens is, every few resizes, the tables grow by 9 pixels. I use doubles everywhere. I have tried rounding, floor and ceil but the problem persists. What could I do so that they always grow by the correct amount?
void LobbyTableManager::changeTableScale( double scale )
{
setTableScale(scale);
}
void LobbyTableManager::setTableScale( double scale )
{
scale += 0.3;
scale *= 2.0;
std::cout << scale << std::endl;
agui::Gui* gotGui = getGui();
float scrollRel = m_vScroll->getRelativeValue();
setScale(scale);
rescaleTables();
resizeFlow();
...
double LobbyTableManager::getTableScale() const
{
return (getInnerWidth() / 700.0) * getScale();
}
void LobbyFilterManager::valueChanged( agui::Slider* source,int val )
{
if(source == m_magnifySlider)
{
DISPATCH_LOBBY_EVENT
{
(*it)->changeTableScale((double)val / source->getRange());
}
}
}
void LobbyTableManager::renderBG( GraphicsContext* g, agui::Rectangle& absRect, agui::Rectangle& childRect )
{
int cx, cy, cw, ch;
g->getClippingRect(cx,cy,cw,ch);
g->setClippingRect(absRect.getX(),absRect.getY(),absRect.getWidth(),absRect.getHeight());
float scale = 0.35f;
int w = m_bgSprite->getWidth() * getTableScale() * scale;
int h = m_bgSprite->getHeight() * getTableScale() * scale;
int numX = ceil(absRect.getWidth() / (float)w) + 2;
int numY = ceil(absRect.getHeight() / (float)h) + 2;
float offsetX = m_activeTables[0]->getLocation().getX() - w;
float offsetY = m_activeTables[0]->getLocation().getY() - h;
int startY = childRect.getY() + 1;
if(moo)
{
std::cout << "TS: " << getTableScale() << " Scr: " << m_vScroll->getValue() << " LOC: " << childRect.getY() << " H: " << h << std::endl;
}
if(moo)
{
std::cout << "S=" << startY << ",";
}
int numAttempts = 0;
while(startY + h < absRect.getY() && numAttempts < 1000)
{
startY += h;
if(moo)
{
std::cout << startY << ",";
}
numAttempts++;
}
if(moo)
{
std::cout << "\n";
moo = false;
}
g->holdDrawing();
for(int i = 0; i < numX; ++i)
{
for(int j = 0; j < numY; ++j)
{
g->drawScaledSprite(m_bgSprite,0,0,m_bgSprite->getWidth(),m_bgSprite->getHeight(),
absRect.getX() + (i * w) + (offsetX),absRect.getY() + (j * h) + startY,w,h,0);
}
}
g->unholdDrawing();
g->setClippingRect(cx,cy,cw,ch);
}
void LobbyTable::rescale( double scale )
{
setScale(scale);
float os = getObjectScale();
double x = m_baseHeight * os;
if((int)(x + 0.5) > (int)x)
{
x++;
}
int oldH = getHeight();
setSize(m_baseWidth * os, floor(x));
...
I added the related code. The slider sends a value changed which is multiplied to get a 4 percent increase (or 8 percent if slider moves 2 values etc...) then the tables are rescaled with this.
The first 3 are when the table size increased by 9, the 4th time it increased by 8px. But the scale factor increases by 0.04 each time.
Why is the 4th time inconsistant?
the pattern seems like 8,8,8,9,9,9,8,8,8,9,9,9...
It increases by 1 pixel more for a few and then decreases by 1 ten increases by 1 etc, thats my issue...

Answer 1

I still don't see the "add 4%" code there (in a form I can understand, anyway), but from your description I think I see the problem: adding 4% twice is not adding 8%. It is adding 8.16% (1.04 * 1.04 == 1.0816). Do that a few more times and you'll start getting 9 pixel jumps. Do it a lot more times and your jumps will get much bigger (they will be 16 pixel jumps when the size gets up to 400x400). Which, IMHO is how I like my scaling to happen.