Reset image with color

[SebastianStehle]

Hi,

how can I reset an image with a color?

I tried this:

                    new Drawables()
                        .FillColor(ParseColor(options))
                        .Rectangle(0, 0,
                            image.Width,
                            image.Height)
                        .Draw(image):

But it does not work, because it will always blend transparent colors over.

[dlemstra]

Can you add an input and output image that demonstrates your issue?

[SebastianStehle]

I have a class that supports thumbnail generation and is build with imagesharp, which works great. To support more file formats I also want to support Magick.

To support the resize modes I have migrated a class called ResizeHelper from Imagesharp. It also supports to set a background color. It works basically like this:

var clone = image.Clone();

new Drawables()
	.FillColor(ParseColor(options)) // Background color
	.Rectangle(0, 0,
		image.Width,
		image.Height)
	.Draw(image);

var (size, rect) = ResizeHelper.CalculateTargetLocationAndBounds(resizeMode, new Size(image.Width, image.Height), targetWidth, targetHeight);

var targetRectangle = new MagickGeometry(size.Width, size.Height)
{
	IgnoreAspectRatio = true
};

image.Resize(targetRectangle);

var sourceRectangle = new MagickGeometry(rect.Width, rect.Height)
{
	IgnoreAspectRatio = true
};

clone.Resize(sourceRectangle);

image.Composite(clone, rect.X, rect.Y, CompositeOperator.Over);

The source image is this:

The expected image is this:

But it is actual this (the original image is still there):

With a background color it works as expected:

[SebastianStehle]

I have found this solution:

public static void Clear(this IMagickImage<byte> image, MagickColor color)
        {
            const int BufferSize = 50;

            using (var pixels = image.GetPixels())
            {
                var colorChannels = pixels.Channels;
                var colorArray = color.ToByteArray();

                var buffer = new byte[BufferSize * BufferSize * colorChannels];

                // Create a buffer to reduce native calls and keep memory footprint low.
                for (var x = 0; x < BufferSize; x++)
                {
                    for (var y = 0; y < BufferSize; y++)
                    {
                        var destination = colorChannels * (x + (y * BufferSize));

                        Array.Copy(colorArray, 0, buffer, destination, colorChannels);
                    }
                }

                for (var x = 0; x < image.Width; x += BufferSize)
                {
                    for (var y = 0; y < image.Height; y += BufferSize)
                    {
                        var w = Math.Min(BufferSize, image.Width - x);
                        var h = Math.Min(BufferSize, image.Height - y);

                        var bufferLength = w * h * colorChannels;

                        var actualBuffer = buffer.AsSpan()[..bufferLength];

                        pixels.SetArea(x, y, w, h, actualBuffer);
                    }
                }
            }
        }

[dlemstra]

It looks like you want to do this:

using (var image = new MagickImage("input.png"))
{
    var originalWidth = image.Width;
    var originalHeight = image.Height;

    var targetWidth = 400;
    var targetHeight = 400;
    image.Resize(targetWidth, targetHeight);

    image.Extent(originalWidth, originalHeight, Gravity.Center, MagickColors.Transparent);
    image.Write("output.png");
}

[SebastianStehle]

No, not really, because I guess the color is only applied to the extended area. But the problem is that the two images overlap and cause problems for transparent areas.