[BUG] image tick positions misplaced when specifying image axes

[sefffal]

Details

See #4087 and #4088, and tagging @t-bltg.

With GR, the tick positions of images are not placed correctly when the axes of the image are passed to plot:

using Images, Plots
i = rand(RGB, 10, 10)
plot(i) # Fixed by #4088
plot(1:10, 1:10, i) # Still broken

Image plot without specifying axes:

Image plot with axes specified:

It's subtle, but to match the normal behaviour of images and other backends, the +/- 5s should be at the centres of their pixels. The +/-2.5s are also slightly off.
Thanks!

Backends

This bug occurs on ( insert x below )

Backend	yes	no	untested
gr (default)	x
pyplot		x
plotlyjs		x
pgfplotsx			x
unicodeplots			x
inspectdr			x
gaston			x

Versions

Plots.jl version:
[91a5bcdd] Plots v1.27.0
Backend version (]st -m <backend(s)>):
[28b8d3ca] GR v0.64.0
Output of versioninfo():

Julia Version 1.7.2
Commit bf53498635 (2022-02-06 15:21 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, skylake)
Environment:
  JULIA_NUM_THREADS = 4
  JULIA_EDITOR = code

[t-bltg]

I'll have a look in the next few days.

In the mean time does plot(..., xticks=1:10, yticks=1:10) as specified in #4088 (comment) work ?

[sefffal]

Thanks @t-bltg!

i = rand(RGB, 10, 10)
plot(i, xticks=1:10, yticks=1:10)

Works!

But

plot(1:10, 1:10, i, xticks=1:10, yticks=1:10)

does not.

[t-bltg]

I've had a look. The function involved in the example is

Plots.jl/src/recipes.jl

Line 1469 in 0f4a0c7

@recipe function f(x::AVec, y::AVec, mat::AMat{T}) where {T<:Colorant}

Since the explicit x and y vectors are given, I'm afraid we can't tweak them to fit, because other use cases will be broken.
The following patch could work but it is only increasing complexity, and it does not cover all possible cases:

 
--- a/Plots.jl/src/recipes.jl	2022-03-29 12:37:35.927406939 +0200
+++ b/Plots.jl/src/recipes.jl	2022-03-29 12:34:27.166126958 +0200
@@ -1450,18 +1450,31 @@
 # 3 arguments
 # --------------------------------------------------------------------
 
+function check_image_axes(x, y, z)
+    # fix for github.com/JuliaPlots/Plots.jl/issues/4158
+    if x isa AbstractRange && length(x) == size(z, 2)
+        half_step = step(x) / 2
+        x = range(first(x) - half_step, stop = last(x) + half_step)
+    end
+    if y isa AbstractRange && length(y) == size(z, 1)
+        half_step = step(y) / 2
+        y = range(first(y) - half_step, stop = last(y) + half_step)
+    end
+    x, y, z
+end
+
 # images - grays
 @recipe function f(x::AVec, y::AVec, mat::AMat{T}) where {T<:Gray}
     if is_seriestype_supported(:image)
         seriestype := :image
         yflip --> true
-        SliceIt, x, y, Surface(mat)
+        (SliceIt, check_image_axes(x, y, Surface(mat))...)
     else
         seriestype := :heatmap
         yflip --> true
         colorbar --> false
         fillcolor --> cgrad([:black, :white])
-        SliceIt, x, y, Surface(convert(Matrix{Float64}, mat))
+        (SliceIt, check_image_axes(x, y, Surface(convert(Matrix{Float64}, mat)))...)
     end
 end
 
@@ -1470,13 +1483,13 @@
     if is_seriestype_supported(:image)
         seriestype := :image
         yflip --> true
-        SliceIt, x, y, Surface(mat)
+        (SliceIt, check_image_axes(x, y, Surface(mat))...)
     else
         seriestype := :heatmap
         yflip --> true
         colorbar --> false
         z, plotattributes[:fillcolor] = replace_image_with_heatmap(mat)
-        SliceIt, x, y, Surface(z)
+        (SliceIt, check_image_axes(x, y, Surface(z))...)
     end
 end

I'd suggest to be explicit about what you want to be drawn by using either:

i = rand(RGB, 12, 10)
plot(i, xticks=axes(i, 2), yticks=axes(i, 1))  # should be preferred
plot(1:13, 1:11, i)  # nodes (un-shifted, but confuses the ticking algorithm) [1]
plot(.5:12.5, .5:10.5, i)  # cell centers (un-shifted, but confuses the ticking algorithm) [2]

Note that the plot(x, y, z) syntax plots at x, y coordinates which is different from setting the appearing ticks using plot(..., xticks=...). If you use the former, then you should follow the internal convention of [1,2] which uses the pixel borders (hence nodes) and not the pixel center (cells).

It's very easy to use a helper in the form of:

plot_img(img) = plot(img, xticks=axes(img, 2), yticks=axes(img, 1))

[sefffal]

Hi @t-bltg, here's a motivating example for why I believe this is a bug. The root issue is not the tick positions but the location the image is rendered on the plot.
First, as background I'm working on the AstroImages package, so I'm trying to find a general solution rather than fixing a single plot: JuliaAstro/AstroImages.jl#30

using Images
using Plots
i = rand(RGB, 4,4)

p = plot()
# Plot image
plot!(p, 4:2:10, 4:2:10, i)
# Plot other series
plot!(p, 3:10, 3:10)

In GR, this produces:

This behaviour is necessary to be able to offset the position of the image in addition to modifying the tick marks.

As you can see, the tick location still drifts from the left of the pixels to the right of the pixels.

The behaviour is correct in plotly:

And also correct in PyPlot:

[dorn-gerhard]

the wrong behaviour still exists for RGB / RGBA input:

heatmap(1:4, 1:4, [RGBA(rand(),rand(),rand(),rand()) for i = 1:4, j = 1:4])

[sefffal]

1 year bump on this issue

[JeffFessler]

Also facing this issue and hoping to help resolve it or find a work-around.
I want to emphasize that it works fine for numeric data, but not for RGB data.

using Plots
using ColorTypes: RGB
p1 = heatmap(1:6, 1:4, rand(4,6); aspect_ratio=:equal, yticks=1:4, color=:grays, widen=true)
p2 = heatmap(1:6, 1:4, rand(RGB{Float32}, 4,6); aspect_ratio=:equal, yticks=1:4, widen=true)

p1 with numeric data is perfect:

but p2 with RGB data has wrong tick locations:

(Using Julia 1.9.1 with Plots v1.38.16 on Intel Mac.)