#fill_keep_premul

1fn fill_keep_premul(inout self, source: vec4) -> vec4

The fill_keep_premul function blends a pre-multiplied source color with the current result color of the SDF2D drawing context, preserving the existing content while applying the new fill.

#Parameters

• self (inout): A reference to the Sdf2d instance where the blending operation is performed. The function modifies the result field of self in place.
• source (vec4): A pre-multiplied RGBA color to be blended with the existing content.

#Returns

• vec4: The updated result color after the blending operation, reflecting the combination of the source color and the existing content.

#Example

1fn pixel(self) -> vec4 {
2    // Create an Sdf2d drawing context for the current viewport.
3    let sdf = Sdf2d::viewport(self.pos * self.rect_size);
4
5    // Draw a rectangle with rounded corners.
6    sdf.box(10.0, 10.0, 80.0, 80.0, 5.0);
7
8    // Apply a semi-transparent red fill while keeping existing content.
9    sdf.fill_keep_premul(vec4(1.0, 0.0, 0.0, 0.5));
10
11    // Rotate the drawing by 45 degrees around the center of the rectangle.
12    sdf.rotate(PI * 0.25, 50.0, 50.0);
13
14    // Return the final color result.
15    return sdf.result;
16}

In this example:

• Create Drawing Context: We initialize an Sdf2d drawing context using the current position (self.pos) and size (self.rect_size) of the viewport.
• Draw Shape: We draw a box starting at position (10.0, 10.0) with a width and height of 80.0 units and rounded corners with a radius of 5.0.
• Apply Fill: The fill_keep_premul function is used to blend a semi-transparent red color (vec4(1.0, 0.0, 0.0, 0.5)) with the existing content. This preserves any previous fills or drawings while adding the new color.
• Rotate Drawing: We rotate the entire drawing context by 45 degrees (PI * 0.25 radians) around the point (50.0, 50.0), which is the center of the rectangle.
• Return Result: The final color result is returned, which includes all the drawing operations performed.

This function is particularly useful when you want to add a new fill color to your drawing without overwriting the existing content, allowing for complex blending effects in your shaders.