In this step of the Bitmap Painting tool development, we will add optional smooth falloff to the brush.

NATURAL LANGUAGE

We will add a new item to the brushes listbox.

We will extend the paintbrush function with a new case calculating the new color using the existing one and a falloff based on the pixel's distance to the center of the brush.

SCRIPT:

macroScript MicroPaint category: "HowTo"
(
global MicroPaint_CanvasRollout
try (destroyDialog MicroPaint_CanvasRollout) catch()
local isDrawing = false
local bitmapX = bitmapY = 512
local theCanvasBitmap = bitmap bitmapX bitmapY color:white
local currentPos = lastPos = [0,0]

rollout MicroPaint_CanvasRollout "MicroPaint"
(
  bitmap theCanvas pos:[0,0] width:bitmapX height:bitmapY bitmap:theCanvasBitmap
  colorpicker inkColor height:16 modal:false color:black across:4
  checkbutton airBrush "AirBrush"width:50
  spinner AirBrushSpeed "Speed" range:[0.1,50,10] fieldwidth:30
  spinner BrushSize "Size" range:[1,50,10] type:#integer fieldwidth:40
  listbox BrushShape items:#("Circle","Box","Circle Smooth") pos:[bitmapX+5,0] width:90

  fn paintBrush pos =
  (
    case BrushShape.selection of
    (
      1: (
        if distance pos currentPos <= BrushSize.value/2 do
          setPixels theCanvasBitmap pos #(inkColor.color)
      )
      2: setPixels theCanvasBitmap pos #(inkColor.color)
      3: (
        theFactor = (distance pos currentPos) / (BrushSize.value/2.0)
        if theFactor <= 1.0 do
        (
          theFactor = sin ( 90.0 * theFactor)
          thePixels = getPixels theCanvasBitmap pos 1
          if thePixels[1] != undefined do
          (
            thePixels[1] = (thePixels[1] * theFactor) + (inkColor.color * (1.0 - theFactor))
            setPixels theCanvasBitmap pos thePixels
          )
        )
      )--end case 3
    )--end case
  )--end fn

  fn drawStroke lastPos pos =
  (
    currentPos = lastPos
    deltaX = pos.x - lastPos.x
    deltaY = pos.y - lastPos.y
    maxSteps = amax #(abs(deltaX),abs(deltaY))
    deltaStepX = deltaX / maxSteps
    deltaStepY = deltaY / maxSteps
    for i = 0 to maxSteps do
    (
      if airBrush.checked then
      (
        for b = 1 to (BrushSize.value / AirBrushSpeed.value) do
        paintBrush (currentPos + (random [-BrushSize.value/2,-BrushSize.value/2] [BrushSize.value/2,BrushSize.value/2] ))
      )
      else
        for b = -BrushSize.value/2 to BrushSize.value/2 do
          for c = -BrushSize.value/2 to BrushSize.value/2 do
            paintBrush (currentPos + [c,b])
      currentPos += [deltaStepX, deltaStepY]
    )
    theCanvas.bitmap = theCanvasBitmap
  )

  on MicroPaint_CanvasRollout lbuttondown pos do
  (
    lastPos = pos
    isDrawing = true
    drawStroke lastPos pos
  )
  on MicroPaint_CanvasRollout lbuttonup pos do isDrawing = false
  on MicroPaint_CanvasRollout mousemove pos do
  (
    if isDrawing do drawStroke lastPos pos
    lastPos = pos
  )
)
createDialog MicroPaint_CanvasRollout (bitmapx+100) (bitmapy+30)
)

Step-By-Step

--Code in italic has no changes since the previous version. macroScript MicroPaint category:"HowTo"
(
global MicroPaint_CanvasRollout
try(destroyDialog CanvasRollout)catch()
local isDrawing = false
local bitmapX = bitmapY = 512
local theCanvasBitmap = bitmap bitmapX bitmapY color:white
local currentPos = lastPos = [0,0]

rollout MicroPaint_CanvasRollout "MicroPaint"
(
bitmap theCanvas pos:[0,0] width:bitmapX height:bitmapY bitmap:theCanvasBitmap
colorpicker inkColor height:16 modal:false color:black across:4
checkbutton airBrush "AirBrush" width:50
spinner AirBrushSpeed "Speed" range:[0.1,50,10] fieldwidth:40
spinner BrushSize "Size" range:[1,50,10] type:#integer fieldwidth:40
 listbox BrushShape items:#("Circle", "Box", "Circle Smooth") pos:[bitmapX+5,0] width:90 

We add a new brush to our "toolbox" - "Circle Smooth".

fn paintBrush pos =
(
case BrushShape.selection of
(
1: (
if distance pos currentPos <= BrushSize.value/2 do
setPixels theCanvasBitmap pos #(inkColor.color)
)
2: setPixels theCanvasBitmap pos #(inkColor.color)

3: ( 

When the "Circle Smooth" brush is selected,

theFactor =(distance pos currentPos)/ (BrushSize.value/2.0)

we calculate the distance from the pixel to be drawn to the center of the brush and divide by the radius of the brush.

if theFactor <= 1.0 do
(

If the result is less than 1.0, the pixel lies inside the circle.

theFactor = sin ( 90.0 * theFactor)

In that case, we multiply the value by 90 and use the Sine function to calculate a nice falloff curve. Note that remarking this line gives you a linear falloff curve instead of the sine-based falloff curve.

thePixels = getPixels theCanvasBitmap pos 1

Next, we read a single pixel from the position we are about to write to.

if thePixels[1] != undefined do
(

If the single pixel in the array is a valid color,

thePixels[1] = (thePixels[1] * theFactor) + (inkColor.color * (1.0 - theFactor))

we blend the color of the background with the brush color based on the distance factor. If the factor is 0.5, the two colors will be blended 50-50%. If the factor is 1 (at the edge of the circle, the background color will be used 100% and the ink will be 0%.

setPixels theCanvasBitmap pos thePixels

Finally, we write the color back into the bitmap.

) 
)
)  
)
)


fn drawStroke lastPos pos =
(
currentPos = lastPos
deltaX = pos.x - lastPos.x
deltaY = pos.y - lastPos.y
maxSteps = amax #(abs(deltaX),abs(deltaY))
deltaStepX = deltaX / maxSteps
deltaStepY = deltaY / maxSteps
for i = 0 to maxSteps do
(
if airBrush.checked then
(
for b = 1 to (BrushSize.value / AirBrushSpeed.value) do
paintBrush (currentPos + (random [-BrushSize.value/2,-BrushSize.value/2] [BrushSize.value/2,BrushSize.value/2] ))
)
else
for b = -BrushSize.value/2 to BrushSize.value/2 do
for c = -BrushSize.value/2 to BrushSize.value/2 do
paintBrush (currentPos + [c,b])
currentPos += [deltaStepX, deltaStepY]
) 
theCanvas.bitmap = theCanvasBitmap
)

on MicroPaint_CanvasRollout lbuttondown pos do
(
lastPos = pos
isDrawing = true
drawStroke lastPos pos
)
on MicroPaint_CanvasRollout lbuttonup pos do isDrawing = false
on MicroPaint_CanvasRollout mousemove pos do
(
if isDrawing do drawStroke lastPos pos
lastPos = pos
)
)
createDialog MicroPaint_CanvasRollout (bitmapx+100) (bitmapy+30)
) 

Result

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How To ... Develop a Bitmap Painting Tool - Airbrush and Shapes

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How To ... Develop a Bitmap Painting Tool - Load and Save