Drawing with high precision alpha blending

Asked Apr 26 '13 at 16:14

Active Apr 26 '13 at 16:14

Viewed 138 times

I need to blend together about 1 million semi-transparent rectangles, while being able to manage transparency accuracy by increment of 1e-6.

Typically, if my 1 millions rectangle would be drawn on top of each other, I want to have a resulting alpha value for these pixels of exactly 1.0 (0.5 for 500 000 rectangles, and so on).

Using the cairo library, it would ideally look like:

const int NB_RECT = 1000000;
//[...]
cairo_set_operator(cr, CAIRO_OPERATOR_ADD);
cairo_set_source_rgba(cr, 1.0, 0, 0, 1.0/NB_RECT);
for(int i = 0 ; i < NB_RECT ; i++) {
    //[...]
    cairo_rectangle(cr, x, y, w, h);
    cairo_fill(cr);
}
// [...]

This does not work because below alpha~=0.01, the drawing commands seem to be simply discarded (probably due to the internal representation of colors inside cairo).

Could you suggest a drawing library that handle high precision transparency, or possible workaround?

asked Apr 26 '13 at 16:14

skadge

Can you pre-calculate the colour/transparency, or do the rectangles overlap only partially? – Kninnug Apr 26 '13 at 16:39
Standard alpha blending doesn't add up linearly, it approaches 1.0 asymptotically. You're going to need a completely custom approach. – Mark Ransom Apr 26 '13 at 17:02
@Mark Ransom: with operator CAIRO_OPERATOR_ADD, alpha blending is indeed linear. Cf http://cairographics.org/operators/ – skadge Apr 28 '13 at 15:26
@Kninnug: the rectangle overlap only partially – skadge Apr 28 '13 at 15:26

Drawing with high precision alpha blending

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