CODEWITHSUNDEEP
renderingalphablendingskia
sanjay
sanjay

Reputation: 765

Skia library not able to use alpha masking

I want to achieve alpha masking with two complex shapes with skia.

Shape 1 : Complex draw operating , with multiple canvas save and restore. In the example text("Skia").

Shape 2 : Mask layer, complex drawing operation with multiple canvas save and restore. In example it's a rectangle.

Deseried Result: Only the intersection is drawn but only with Shape 1 pixels.

I am not able to achieve this result without involving bitmap operations. Is there any effective solution to achieve it without bitmap drawing (taking snapshot from canvas surface) ?

void draw(SkCanvas *canvas)
{
    SkFont font1;
    font1.setSize(100);
    font1.setEdging(SkFont::Edging::kAntiAlias);

    // clear canvas, transparent
    canvas->clear(SK_ColorTRANSPARENT);
    sk_sp<SkTextBlob> blob1 = SkTextBlob::MakeFromString("Skia", font1);

    SkPaint paint1;

    paint1.setAntiAlias(true);
    paint1.setColor(SkColorSetARGB(0xFF, 0x42, 0x85, 0xF4));

    // Text draw
    canvas->drawTextBlob(blob1.get(), 0, 100, paint1);

    SkPaint p;
    p.setColor(SK_ColorRED);
    p.setAntiAlias(true);
    canvas->saveLayer(nullptr, nullptr);
    // canvas bounds, 400x400
    canvas->drawRect(SkRect::MakeXYWH(0, 0, 400, 400), p);
    p.setBlendMode(SkBlendMode::kClear);

    // Alpha drawing is complex operation with multiple save and restore
    // There can be multiple draw commands
    canvas->drawRect(SkRect::MakeXYWH(0, 0, 400, 50), p);
    canvas->restore();
}

Example: 400x400 canvas enter image description here

Upvotes: 0

Views: 301

Answers (1)

sanjay
sanjay

Reputation: 765

I was able to achieve it after some changes

void draw(SkCanvas *canvas)
{
    SkFont font1;
    font1.setSize(100);
    font1.setEdging(SkFont::Edging::kAntiAlias);

    // clear canvas, transparent
    canvas->clear(SK_ColorTRANSPARENT);
    sk_sp<SkTextBlob> blob1 = SkTextBlob::MakeFromString("Skia", font1);

    SkPaint paint1;

    paint1.setAntiAlias(true);
    paint1.setColor(SkColorSetARGB(0xFF, 0x42, 0x85, 0xF4));

    // Text draw
    canvas->drawTextBlob(blob1.get(), 0, 100, paint1);

    SkPaint p;
    p.setColor(SK_ColorRED);
    p.setAntiAlias(true);
    p.setBlendMode(SkBlendMode::kDstOut);
    canvas->saveLayer(nullptr, &p);
    canvas->clear(SK_ColorWHITE);
    p.setBlendMode(SkBlendMode::kClear);
    canvas->drawRect(SkRect::MakeXYWH(0, 0, 400, 70), p);
    canvas->restore();
}

Upvotes: 1

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