PPTX resolution: set transparent color

In PPT, we can set the specified color to be transparent through the picture format options - > Color - > set transparent color, so as to realize the need to remove the solid color background.

Effect principle

The logic of setting transparent color in PPT is very simple and clear:

·Replace color A with color B

When color B is a transparent color, the requirement of setting a color as a transparent color can be met.

Resolve color substitution node (OpenXml)

First, let's look at the xml node where PPT stores transparent colors:

<p:pic>
	......
	<p:blipFill>
		<a:blip ......>
			<a:clrChange>
				<a:clrFrom>
					<a:srgbClr val="FFAAFF" >
						<a:alpha val="100000" />
					</a:srgbClr>
				</a:clrFrom>
				<a:clrTo>
					<a:srgbClr val="FFAAFF">
						<a:alpha val="0" />
					</a:srgbClr>
				</a:clrTo>
			</a:clrChange>
		</a:blip>
		......
	</p:blipFill>
	......
</p:pic>

Node attribute resolution:

Alpha channel resolution of color

There is an attribute value val in the node * * < A: srgbclr / > * * which represents the hexadecimal representation of the RGB channel of the color.

There is an attribute value val in the child node * * < A: Alpha / > * * which represents the thousandfold percentage of the Alpha channel of the color (for example, 100000 actual value represents 100%).

Note: when the val value in node * * < A: Alpha / > is 100000, this node can be omitted. That is, when there is no child node < A: Alpha / > * * in the node < A: srgbclr / >, the percentage of Alpha channels representing the color is 100% (255).

In this case, we resolved to replace the color #FFAAFF in the picture with the color #00FFAAFF.

Effect realization (C#)

You can refer to the following code to replace the color of the specified Bitmap:

/// <summary>
///Replace the color < paramref name = "colora" / > specified on the picture < paramref name = "bitmap" / > with the color < paramref name = "colorb" / >
/// </summary>
///< param name = "bitmap" > picture < / param >
///< param name = "colora" > color to be replaced < / param >
///< param name = "colorb" > replace < paramref name = "colora" / > with the color < / param >
public void ReplaceColor(Bitmap bitmap, System.Drawing.Color colorA, System.Drawing.Color colorB)
{
    //Here is to traverse every pixel in the picture
    bitmap.PerPixelProcess(color =>
    {
        //Replace if the current color is similar to colorA
        var isSimilar = IsSimilarColors(color, colorA);
        return isSimilar ? colorB : color;
    });
}

/// <summary>
///Is it an approximate color
/// </summary>
/// <param name="x"></param>
/// <param name="y"></param>
///< param name = "accuracy" > allowable error of RGB channel < / param >
/// <returns></returns>
private bool IsSimilarColors(System.Drawing.Color x, System.Drawing.Color y, int accuracy = 36)
{
    var offsetA = x.A - y.A;
    var offsetR = x.R - y.R;
    var offsetG = x.G - y.G;
    var offsetB = x.B - y.B;

    if (Math.Abs(offsetA) > 1)
    {
        return false;
    }

    if (offsetR == offsetG && offsetR == offsetB)
    {
        if (Math.Abs(offsetR) > 1)
        {
            return ColorDifference(x, y) <= accuracy / 3d;
        }
    }

    var difference = ColorDifference(x, y);
    return difference <= accuracy;
}

/// <summary>
///The weighted Euclidean distance is calculated by formula in RGB space
/// </summary>
/// <param name="x"></param>
/// <param name="y"></param>
/// <returns></returns>
private double ColorDifference(System.Drawing.Color x, System.Drawing.Color y)
{
    var m = (x.R + y.R) / 2.0;
    var r = Math.Pow(x.R - y.R, 2);
    var g = Math.Pow(x.G - y.G, 2);
    var b = Math.Pow(x.B - y.B, 2);

    return Math.Sqrt((2 + m / 256) * r + 4 * g + (2 + (255 - m) / 256) * b);
}

GitHub project warehouse:

If you want to refer to the complete case, please refer to the following items:
Set transparent color case

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