Beyond QPainter::setBrush() - Alternative Filling Techniques in Qt

Purpose

• The setBrush() function of QPainter sets the brush that will be used to fill the interior of shapes subsequently drawn with the painter.
• In Qt GUI programming, QPainter is a fundamental class used for drawing various graphical elements.

Brush Types

• A brush in Qt defines the color or pattern used for filling shapes. Qt provides various brush styles:
  • Solid color brushes (using QColor)
  • Gradient brushes (linear, radial, conical) for creating smooth color transitions
  • Texture brushes for applying an image pattern
  • Pattern brushes for repeating a smaller image tile

Setting the Brush

#include <QtGui/QPainter>
#include <QtGui/QBrush>

// ...

QPainter painter(&widget);  // Create a painter for the widget

// Set a solid color brush
QColor red(255, 0, 0);  // Create a red color
QBrush brush(red);
painter.setBrush(brush);

// Draw a rectangle
painter.drawRect(10, 10, 100, 50);

// ... (more drawing commands)

1. Include the necessary headers (QPainter and QBrush).
2. Create a QPainter object, typically associated with a widget you want to draw on.
3. Define the brush you want to use:
   • Create a QColor object to specify the solid color.
   • Construct a QBrush object using the QColor.
4. Call painter.setBrush(brush) to apply the brush to the painter.
5. Use drawing functions like drawRect to fill shapes with the set brush.

Key Points

• To revert to no filling (transparent interior), use a brush with style Qt::NoBrush.
• You can change the brush multiple times within a painting process to create diverse fills.
• setBrush() only affects subsequent drawing commands with the same QPainter object.

• Experiment with different brush styles and properties to achieve the desired visual effects in your Qt applications.
• For gradient, texture, or pattern brushes, explore the constructors and properties of the corresponding QGradient, QTextureBrush, and QPatternBrush classes, respectively.

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Linear Gradient Brush

#include <QtGui/QPainter>
#include <QtGui/QBrush>
#include <QtGui/QGradient>

// ...

QPainter painter(&widget);

// Create a linear gradient brush
QLinearGradient gradient(0, 0, 100, 100);
gradient.setColorAt(0.0, Qt::blue);
gradient.setColorAt(1.0, Qt::yellow);
QBrush brush(gradient);
painter.setBrush(brush);

// Draw a rectangle with a blue to yellow gradient
painter.drawRect(10, 10, 100, 50);

• Constructs a QBrush using the gradient and sets it on the painter.
• Sets colors at specific positions within the gradient (0.0 for start, 1.0 for end).
• Creates a QLinearGradient object specifying the start and end points for the gradient.

Radial Gradient Brush

#include <QtGui/QPainter>
#include <QtGui/QBrush>
#include <QtGui/QGradient>

// ...

QPainter painter(&widget);

// Create a radial gradient brush
QRadialGradient gradient(50, 50, 25);
gradient.setColorAt(0.0, Qt::white);
gradient.setColorAt(0.7, Qt::lightGray);
gradient.setColorAt(1.0, Qt::darkGray);
QBrush brush(gradient);
painter.setBrush(brush);

// Draw a circle with a white to dark gray radial gradient
painter.drawEllipse(25, 25, 50, 50);

• Constructs a QBrush and sets it on the painter.
• Sets colors at different positions within the gradient to create a radial effect.
• Creates a QRadialGradient object specifying the center point and radius.

Texture Brush

#include <QtGui/QPainter>
#include <QtGui/QBrush>
#include <QtGui/QImage>

// ...

QPainter painter(&widget);

// Load an image for the texture
QImage texture("texture.png");

// Create a texture brush
QBrush brush(texture);
painter.setBrush(brush);

// Draw a rectangle with the texture pattern
painter.drawRect(10, 100, 100, 50);

• Sets the texture brush on the painter.
• Creates a QTextureBrush using the loaded image.
• Loads an image using QImage.

#include <QtGui/QPainter>
#include <QtGui/QBrush>
#include <QtGui/QPixmap>

// ...

QPainter painter(&widget);

// Load a small image for the pattern
QPixmap pattern("pattern.png");

// Create a pattern brush
QBrush brush(pattern);
painter.setBrush(brush);

// Draw a rectangle with the repeating pattern
painter.drawRect(10, 150, 200, 50);

• Sets the pattern brush on the painter.
• Creates a QPatternBrush using the loaded image, which will be repeated as a tile.
• Loads a small image using QPixmap.

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1. Using QPen with Solid Color
   

   • If you only need a solid color fill and don't require advanced features like gradients or textures, you can use QPen with a set width of 0. This effectively fills the shape with the pen's color.

   QPainter painter(&widget);
   
   // Set a solid color pen with width 0 for filling
   QColor red(255, 0, 0);
   QPen pen(red);
   pen.setWidth(0);
   painter.setPen(pen);
   
   // Draw a rectangle filled with red
   painter.drawRect(10, 10, 100, 50);
   

   Note
   This approach has limitations:

   • It only works with solid colors.
   • The fill respects the outline drawn by the pen (e.g., rounded corners if the pen has a cap style).

2. Drawing Filled Shapes Directly
   

   • Certain QPainter drawing functions have a filled variant that takes a brush as an argument. These directly draw the shape with the specified brush.

   QPainter painter(&widget);
   
   // Set a solid color brush
   QColor red(255, 0, 0);
   QBrush brush(red);
   
   // Draw a filled rectangle with the red brush
   painter.fillRect(10, 100, 100, 50, brush);
   

   Note
   This approach limits flexibility as it's tied to specific shapes.

Choosing the Right Approach

• Use filled shape drawing functions like fillRect for specific cases where you don't require advanced brush features.
• Consider a solid color QPen with width 0 if you only need a simple solid fill and don't need outline effects.
• Use QPainter::setBrush() for the most versatile control over fill behavior, including gradients, textures, and patterns.