Determine if Alpha Channel is Used in an Image

Question

As I\'m bringing in images into my program, I want to determine if:

1. they have an alpha-channel
2. if that alpha-channel is used

#1

Answer 1

You don't have to loop through every pixel (well you might, but it depends on the image). Set up to loop over all the pixels, but just break out of the loop when you find an alpha value other than 255 use the following pseudo code:

bool hasAlpha = false;
foreach (var pixel in image)
{
    hasAlpha = pixel.Alpha != 255;
    if (hasAlpha)
    {
        break;
    }
}

You'll only have to check all the pixels for images that don't have any alpha. For images that do have alpha this will break out quite quickly.

Answer 2

You won't find a solution better than this, it took me hours to optimize:

public bool IsAlphaBitmap(ref System.Drawing.Imaging.BitmapData BmpData)
{
    byte[] Bytes = new byte[BmpData.Height * BmpData.Stride];
    Marshal.Copy(BmpData.Scan0, Bytes, 0, Bytes.Length);
    for (p = 3; p < Bytes.Length; p += 4) {
        if (Bytes[p] != 255) return true;
    }
    return false;
}

Answer 3

The simplest method using ImageMagick (command line) is to test the alpha channel if the mean is less than 1 (on a scale of 0 to 1). 1 is fully opaque. So

convert image -channel a -separate -format "%[fx:(mean<1)?1:0]" info:

If the return value is 1, then at least one pixel is transparent; otherwise (if 0), the image is fully opaque.

Answer 4

Since posting my first answer here , I found out that the LockBits command can actually convert image data to a desired pixel format. This means that, no matter the input, you can simply check the bytes 'as' 32 bit per pixel ARGB data. Since that format has 4-byte pixels, and the stride in the .Net framework is always a multiple of 4 bytes, the normally very important issue of correctly adjusting data reading to scanline lengths becomes irrelevant. This all vastly simplifies the code.

Of course, the first two checks from my other answer still apply; checking the HasAlpha flag on the bitmap flags and the alpha on the palette entries of indexed formats is a very quick initial way to determine if an image can have transparency, before switching to the full data sweep.

I have also since found out that indexed png with alpha-capable palettes is actually a thing (though poorly supported in .Net), so only checking on a single alpha-capable colour on indexed formats is too naive.

With all that in mind, and a linq operation that turns the palette check into a one-liner, the final adjusted code becomes this:

public static Boolean HasTransparency(Bitmap bitmap)
{
    // Not an alpha-capable color format. Note that GDI+ indexed images are alpha-capable on the palette.
    if (((ImageFlags)bitmap.Flags & ImageFlags.HasAlpha) == 0)
        return false;
    // Indexed format, and no alpha colours in the image's palette: immediate pass.
    if ((bitmap.PixelFormat & PixelFormat.Indexed) != 0 && bitmap.Palette.Entries.All(c => c.A == 255))
        return false;
    // Get the byte data 'as 32-bit ARGB'. This offers a converted version of the image data without modifying the original image.
    BitmapData data = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
    Int32 len = bitmap.Height * data.Stride;
    Byte[] bytes = new Byte[len];
    Marshal.Copy(data.Scan0, bytes, 0, len);
    bitmap.UnlockBits(data);
    // Check the alpha bytes in the data. Since the data is little-endian, the actual byte order is [BB GG RR AA]
    for (Int32 i = 3; i < len; i += 4)
        if (bytes[i] != 255)
            return true;
    return false;
}

This works for any input pixel format, be it paletted or not.

Answer 5

While I know the OP is not about MagickNet, it might help s/o.

Magick.Net provides a wrapper around Imagick lib and includes a feature to easily access channel statistics.

Example

    public bool HasTransparentBackground(MagickImage image)
    {
        if (!image.HasAlpha) return false;

        var statistics = image.Statistics();
        var alphaStats = statistics.GetChannel(PixelChannel.Alpha);

        var alphaMax = Math.Pow(2, alphaStats.Depth);
        return alphaStats.Minimum < alphaMax * .2;
    }

We first check if the image supports transparency an return if not. Then we get statistics for alpha channel and can simply check the Min property. There's also a Mean property that allows you to check "how much transparent" your image is.

See also

• Detect Alpha Channel with ImageMagick
• https://github.com/dlemstra/Magick.NET

Answer 6

I get a more advanced solution, based on ChrisF answer:

public bool IsImageTransparent(Bitmap image,string optionalBgColorGhost)
    {
        for (int i = 0; i < image.Width; i++)
        {
            for (int j = 0; j < image.Height; j++)
            {
                var pixel = image.GetPixel(i, j);
                if (pixel.A != 255)
                    return true;
            }
        }

        //Check 4 corners to check if all of them are with the same color!
        if (!string.IsNullOrEmpty(optionalBgColorGhost))
        {
            if (image.GetPixel(0, 0).ToArgb() == GetColorFromString(optionalBgColorGhost).ToArgb())
            {
                if (image.GetPixel(image.Width - 1, 0).ToArgb() == GetColorFromString(optionalBgColorGhost).ToArgb())
                {
                    if (image.GetPixel(0, image.Height - 1).ToArgb() ==
                        GetColorFromString(optionalBgColorGhost).ToArgb())
                    {
                        if (image.GetPixel(image.Width - 1, image.Height - 1).ToArgb() ==
                            GetColorFromString(optionalBgColorGhost).ToArgb())
                        {
                            return true;
                        }
                    }
                }
            }
        }

        return false;
    }

    public static Color GetColorFromString(string colorHex)
    {
        return ColorTranslator.FromHtml(colorHex);
    }

It has a optional bg color string to non transparent images:

Example of usage:

IsImageTransparent(new Bitmap(myImg),"#FFFFFF");