Functionality:

This program implements two painting brushes. One is a simple overpainting rectangular brush and the other is weighted mask-driven tinting brush. Brushes are applied by depressing the mouse left button and moving the mouse pointer over the image while keeping the mouse left button depressed.

Both brushes allow to change their size and aspect ratio. The color of each brush can be controlled by three RGB or three HSV sliders. The RGB and HSV sliders are interdependent. This means if the color is changed in one color space the sliders that represent color in the other color space are adjusted correspondingly.

For the tinting brush the user can select or unselect each of HSV components. If a component is not selected, it is not affected on the image canvas when the brush is applied to the image. The user can increase or decrease the transparency (or thickness) of the tinting brush using the 'Thickness' slider. These controls have no effect on the overpainting brush.

Visualization of the brush is implemented on a separate canvas. The visualization allows the user to see the magnified image of brush given a momentary setting of its mode, shape, size, color and transparency, the latter for the tinting brush only. The brush displayed in its original and scaled sizes. For scaled size the user can control the magnification of the brush image by using a slider. After the demo was already done, I added Sampling mode, which allows the user to click on the image canvas and change the color of the visualization canvas background. In the sampling mode the image of the brush is adjusted automatically to the changing color of the background. This allows the user to see the brush visualization in the same color as it would have on the image canvas at the location of the user's click.

Finally, a software cursor is implemented. The program disables the hardware mouse pointer when it enters the image canvas and draws a custom cursor. The cursor admits the contour shape of a tinting brush. It has two color states, bright and dark. In bright state the cursor color is bright yellow and it seen well on darker areas of the image. When the cursor moved to the lighter areas of the image its color dynamically changed to black and v.v. When the brush size or shape changed the cursor modified correspondingly. Except, for the overpainting brush it does not become rectangular cursor.

Implementation details:

The fall-off function of the tinging brush is

alpha = MAX(0, (sin(pi * x/W) + sin(pi * y/H) - 1)).

In this expression, x is the x coordinate of the array of pixels that represent the rectangle of the brush, y is the y coordinate, W is the width and H is the height of the brush.

The transparency change is accomplished by multiplying the alpha by a number from the [0.1, 0.8] interval.

The software cursor is implemented by saving the original pixels of the canvas, drawing the cursor pixels and restoring the original pixels of the canvas when cursor moved.

The implementation is very straightforward and described in some detail in the source file paint.c. The implementation is in C and does not rely on any special packages, other than xsupport.

The application was tested primarily by running it and using different controls of the GUI. Some debug prints are added to display inconsistent states of the application, if they ever occur.