Bitmap to ASCII Art for Android

A simple Bitmap image converter to ASCII art on Kotlin. The function accepts a Bitmap

Features

• Conversion of any Bitmap into ASCII text art.
• Control of the output width (in characters).
• Customizable character set from dark to light.
• Suitable for Android projects (uses android.graphics.Bitmap).

Example

                                  =:              .:--::.                               
                                . .#+:.............:-=+=:=:                             
                                   .+#+---------==---+-=-=+*=..                         
                               .   ...-=+=========++*###*+*###=.                        
                                  ...:::=#+:.:.:-----==**%#%#+-:.                       
                                  ...:....=*-.    .....-+##*##*=::..                    
                                 ....:..   .++..... .....-++++**=:::.                   
                                ....:..      .*:...::.::::-++*=+***+++=                 
                               .:.....       .:            ..:::::....:                 
                           =-..::....        .             ..:--:..  .                  
                         ..%*+=-::...       ..            ..:-+=:..                     
                         .%%#**==+-..       .             .:-+#+:....                   
                         .+%*%%***+=:..    .             ..:+##=:...                    
                          .-**+%#%##*=:.  .             ..:+#%+-:..                     
                           ..=%##*###*+-.-.            ..:=#%#=:..                      
                             ..#+-:*#*=--#.     .......::-*%%+=::.                      
                               .==::-++=#-....:--=+*+*#%%#++-.                      
                                 .-:::+%%-::::---===-=*%%%*#%%%%:.                      
                                   .::+%:.... .......:+%%%#*+#*:.                       
                                     :%:..   ..:---=++###+--=...                                         

How it works

1. Image scaling
   Bitmap.createScaledBitmap(src, dstWidth, dstHeight, filter) is used to create a scaled-down copy of the source with the desired width and calculated height to maintain proportions.

2. Traversing all pixels
   Two nested loops run along the x and y coordinates from 0 to the width/height of the reduced `Bitmap'.

3. Getting the pixel color
   For each pixel, GetPixel(x, y) is called, then the components r, g, b (red, green, blue) are extracted from the integer ARGB value using bit operations.

4. Brightness calculation (grayscale)
   The brightness is calculated using the formula
   gray = 0.299 * r + 0.587 * g + 0.114 * b, where r, g, b are values from 0 to 255.

5. Converting brightness to symbol
   The brightness is normalized to the range from 0 to 1 and multiplied by the index of the last character in the string chars'. A symbol is selected based on the resulting index: the darkest pixels turn into "dense" symbols. (@, %, #`), and the lightest ones are in dots or spaces.

6. Formation of lines
   For each row of pixels, a sequence of characters is collected and a newline is added. In the end, you get one big String with ASCII art.

Function code

fun bitmapToAscii(
        bitmap: Bitmap,
        width: Int = 80,
        chars: String = "@%#*+=-:. "
    ): String {
        val scaled = Bitmap.createScaledBitmap(
            bitmap,
            width,
            (bitmap.height * width / bitmap.width.toFloat() / 2).toInt(),
            true
        )
        val sb = StringBuilder()
        for (y in 0 until scaled.height) {
            for (x in 0 until scaled.width) {
                val pixel = scaled.getPixel(x, y)
                val r = (pixel shr 16 and 0xFF)
                val g = (pixel shr 8 and 0xFF)
                val b = (pixel and 0xFF)
                val gray = (0.299 * r + 0.587 * g + 0.114 * b).toInt()
                val idx = ((gray / 255.0) * (chars.lastIndex)).toInt()
                sb.append(chars[idx])
            }
            sb.append('\n')
        }
        return sb.toString()
    }