I use the fragment shader in the source code but green does not appear completely

[yulf]

I use this code but the green background turn transparent but not completely transparent.

I expect the green background disappear completely.

Is there anything I can improve for the code below?

varying highp vec2 vCoordinate;
uniform sampler2D uVideoframe;
uniform highp mat4 uMVPMatrix;
void main()
{
// lookup the color of the texel corresponding to the fragment being
// generated while rendering a triangle
lowp vec4 tempColor = texture2D(uVideoframe, vCoordinate);

// Calculate the average intensity of the texel's red and blue components
lowp float rbAverage = tempColor.r * 0.5 + tempColor.b * 0.5;

// Calculate the difference between the green element intensity and the
// average of red and blue intensities
lowp float gDelta = tempColor.g - rbAverage;

// If the green intensity is greater than the average of red and blue
// intensities, calculate a transparency value in the range 0.0 to 1.0
// based on how much more intense the green element is
tempColor.a = 1.0 - smoothstep(0.0, 0.25, gDelta);

// Use the cube of the of the transparency value. That way, a fragment that
// is partially translucent becomes even more translucent. This sharpens
// the final result by avoiding almost but not quite opaque fragments that
// tend to form halos at color boundaries.
tempColor.a = tempColor.a * tempColor.a * tempColor.a;

gl_FragColor = tempColor;

}