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Fix CairoMakie heatmap cell sizes #3625

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Fix CairoMakie heatmap cell sizes #3625

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b0798f8
fix CairoMakie heatmap cell sizes
Feb 12, 2024
def0bbb
calculate margin per pixel individually
Feb 12, 2024
d855982
compute dx only when needed
Feb 13, 2024
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22 changes: 15 additions & 7 deletions CairoMakie/src/primitives.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -771,27 +771,35 @@ function draw_atomic(scene::Scene, screen::Screen, @nospecialize(primitive::Unio
end
end

function _draw_rect_heatmap(ctx, xys, ni, nj, colors)
function _draw_rect_heatmap(ctx, xys, ni, nj, colors, margin_factor=0.05f0)
@inbounds for i in 1:ni, j in 1:nj
p1 = xys[i, j]
p2 = xys[i+1, j]
p3 = xys[i+1, j+1]
p4 = xys[i, j+1]

# Rectangles and polygons that are directly adjacent usually show
# white lines between them due to anti aliasing. To avoid this we
# increase their size slightly.

if alpha(colors[i, j]) == 1 && margin_factor > 0
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What happens if using a transparent colormap here?

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The problem mostly goes away, but when you set alpha=0.99 for example, there are still some cases where artifacts occur. For example thin lines that shouldn't be there.

# dx and dy are the margin widths that are added to the
# heatmap cell sizes further down
# adding 5% seems like a good compromise between avoiding artifacts
# and not changing pixel sizes too much
# how much is added can be controlled via margin_factor
dx = abs(p2[1] - p1[1]) * 0.5f0 * margin_factor
dy = abs(p4[2] - p1[2]) * 0.5f0 * margin_factor

if alpha(colors[i, j]) == 1
# sign.(p - center) gives the direction in which we need to
# extend the polygon. (Which may change due to rotations in the
# model matrix.) (i!=1) etc is used to avoid increasing the
# outer extent of the heatmap.
center = 0.25f0 * (p1 + p2 + p3 + p4)
p1 += sign.(p1 - center) .* Point2f(0.5f0 * (i!=1), 0.5f0 * (j!=1))
p2 += sign.(p2 - center) .* Point2f(0.5f0 * (i!=ni), 0.5f0 * (j!=1))
p3 += sign.(p3 - center) .* Point2f(0.5f0 * (i!=ni), 0.5f0 * (j!=nj))
p4 += sign.(p4 - center) .* Point2f(0.5f0 * (i!=1), 0.5f0 * (j!=nj))
p1 += sign.(p1 - center) .* Point2f(dx * (i!=1), dy * (j!=1))
p2 += sign.(p2 - center) .* Point2f(dx * (i!=ni), dy * (j!=1))
p3 += sign.(p3 - center) .* Point2f(dx * (i!=ni), dy * (j!=nj))
p4 += sign.(p4 - center) .* Point2f(dx * (i!=1), dy * (j!=nj))
end

Cairo.set_line_width(ctx, 0)
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