[GPS]: The code below draws some nice gradients on [canvas] widgets.  It's reasonably quick, and seems to be bug free. 
----
======
  proc + {n1 n2} {
    expr {$n1 + $n2}
  }
  proc - {n1 n2} {
    expr {$n1 - $n2}
  }
  proc * {n1 n2} {
    expr {$n1 * $n2}
  }
  proc / {n1 n2} {
    expr {$n1 / $n2}
  }
  proc toInt {n} {
    expr int($n)
  }
  
  proc drawGradient {win type col1Str col2Str} {
    $win delete gradient
    
    set width [winfo width $win]
    set height [winfo height $win]
    
    foreach {r1 g1 b1} [winfo rgb $win $col1Str] break
    foreach {r2 g2 b2} [winfo rgb $win $col2Str] break
    set rRange [- $r2.0 $r1]
    set gRange [- $g2.0 $g1]
    set bRange [- $b2.0 $b1]
  
    if {$type == "x"} {
      set rRatio [/ $rRange $width]
      set gRatio [/ $gRange $width]
      set bRatio [/ $bRange $width]
    
      for {set x 0} {$x < $width} {incr x} {
        set nR [toInt [+ $r1 [* $rRatio $x]]]
        set nG [toInt [+ $g1 [* $gRatio $x]]]
        set nB [toInt [+ $b1 [* $bRatio $x]]]
  
        set col [format {%4.4x} $nR]
        append col [format {%4.4x} $nG]
        append col [format {%4.4x} $nB]
        $win create line $x 0 $x $height -tags gradient -fill #${col}
      }
    } else {
      set rRatio [/ $rRange $height]
      set gRatio [/ $gRange $height]
      set bRatio [/ $bRange $height]
  
      for {set y 0} {$y < $height} {incr y} {
        set nR [toInt [+ $r1 [* $rRatio $y]]]
        set nG [toInt [+ $g1 [* $gRatio $y]]]
        set nB [toInt [+ $b1 [* $bRatio $y]]]
  
        set col [format {%4.4x} $nR]
        append col [format {%4.4x} $nG]
        append col [format {%4.4x} $nB]
        $win create line 0 $y $width $y -tags gradient -fill #${col}
      }
    }
    return $win
  }
  
  canvas .grad1
  bind .grad1 <Configure> [list drawGradient .grad1 x red royalblue]
  
  canvas .grad2
  bind .grad2 <Configure> [list drawGradient .grad2 y yellow red]
  
  pack .grad1 .grad2 -fill both -expand 1
======

___

[uniquename] 2014jan27

For those who do not have the facilities or time to implement the GPS (G. P. Staplin) code above,
here is an image of the 2 Tk canvases that the code creates.

[colorGradientOnCanvas_staplin_wiki6100_381x557.jpg]

----

Here's a slightly more portable version of that code with some error-checking thrown in for good measure. -- [Damon Courtney]
----
======
 proc DrawGradient {win axis col1Str col2Str} {
    if {[winfo class $win] != "Canvas"} {
        return -code error "$win must be a canvas widget"
    }

    $win delete gradient

    set width  [winfo width $win]
    set height [winfo height $win]
    switch -- $axis {
        "x" { set max $width; set x 1 }
        "y" { set max $height; set x 0 }
        default {
            return -code error "Invalid axis $axis: must be x or y"
        }
    }

    if {[catch {winfo rgb $win $col1Str} color1]} {
        return -code error "Invalid color $col1Str"
    }

    if {[catch {winfo rgb $win $col2Str} color2]} {
        return -code error "Invalid color $col2Str"
    }

    foreach {r1 g1 b1} $color1 break
    foreach {r2 g2 b2} $color2 break
    set rRange [expr $r2.0 - $r1]
    set gRange [expr $g2.0 - $g1]
    set bRange [expr $b2.0 - $b1]

    set rRatio [expr $rRange / $max]
    set gRatio [expr $gRange / $max]
    set bRatio [expr $bRange / $max]

    for {set i 0} {$i < $max} {incr i} {
        set nR [expr int( $r1 + ($rRatio * $i) )]
        set nG [expr int( $g1 + ($gRatio * $i) )]
        set nB [expr int( $b1 + ($bRatio * $i) )]

        set col [format {%4.4x} $nR]
        append col [format {%4.4x} $nG]
        append col [format {%4.4x} $nB]
        if {$x} {
            $win create line $i 0 $i $height -tags gradient -fill #${col}
        } else {
            $win create line 0 $i $width $i -tags gradient -fill #${col}
        }
    }
    bind $win <Configure> [list DrawGradient $win $axis $col1Str $col2Str]
    return $win
 }

 canvas .grad1
 DrawGradient .grad1 y darkblue royalblue

 canvas .grad2
 DrawGradient .grad2 x yellow red

 pack .grad1 .grad2 -fill both -expand 1
======

___

[uniquename] 2014jan27

For those who do not have the facilities or time to implement the Courtney code above,
here is an image of the 2 Tk canvases that the code creates.

[colorGradientOnCanvas_courtney_wiki6100_384x555.jpg]


----

[DKF]: Both versions are cool!
----

[pw]: Here is a library for drawing simple gradients. It's currently capable of five styles: vertical, horizontal, circular, central, and arc.
----
======
# Create color gradients of various shapes and sizes on a canvas.
#   Usage:
#       gradient draw    <canvas> <tags> <x> <y> <width> <height> <vertical|horizontal|circular|central|arc> <color1> <color2>  ?-animate? ?option? ...
#       gradient redraw  <canvas> <tags> <x> <y> <width> <height> <vertical|horizontal|circular|central|arc> <color1> <color2>  ?-animate? ?option? ...
#       gradient resize  <canvas> <tags> <x> <y> <width> <height>
#       gradient recolor <canvas> <tags> <color1> <color2>

namespace eval gradient {
    namespace ensemble create -subcommands {
        draw
        redraw
        recolor
        resize
        demo
    }

    # Create a gradient on a canvas. The gradient will be placed at the top of the display list.
    proc draw {canvas tags x y width height orient color1 color2 args} {
        # Ensure valid colors are provided while also converting to lists of 16-bit RGB values.
        if { [catch {winfo rgb . $color1} rgb1] } {
            error "invalid color: $color1"
        }
        if { [catch {winfo rgb . $color2} rgb2] } {
            error "invalid color: $color2"
        }
        
        # Delete the gradient if it already exists.
        $canvas delete [join $tags &&]
        
        # Create a hidden canvas item to store meta data about this gradient.
        set meta_data [list x $x  y $y  width $width  height $height  orient $orient  color1 $color1  color2 $color2  args $args]
        $canvas create line $x $y $x $y  -state hidden  -tags [list {*}$tags meta [list gradient: {*}$meta_data]]
        
        # Intercept the -animate option.
        if { "-animate" in $args} {
            set index [lsearch $args "-animate"]
            set args  [lreplace $args $index $index]
            set animate 1
        } else {
            set animate 0
        }
        
        # Draw the gradient.
        if { $orient eq "central" } {
            # Create the canvas rectangle items.
            foreach {x1 y1 x2 y2 color} [get_rects $x $y $width $height $rgb1 $rgb2] {
                $canvas create rectangle $x1 $y1 $x2 $y2  -outline $color  -width 2  -tags $tags  {*}$args
                if {$animate} {update}
            }
        } elseif { $orient eq "circular" } {
            # Create the canvas oval items.
            foreach {x1 y1 x2 y2 color} [get_rects $x $y $width $height $rgb1 $rgb2] {
                $canvas create oval $x1 $y1 $x2 $y2  -outline $color  -width 2  -tags $tags  {*}$args
                if {$animate} {update}
            }
        } elseif { $orient eq "arc" } {
            # Create the canvas arc items.
            foreach {x1 y1 x2 y2 color} [get_rects $x $y $width $height $rgb1 $rgb2] {
                $canvas create arc $x1 $y1 $x2 $y2  -outline $color  -width 2  -tags $tags  -style arc  {*}$args
                if {$animate} {update}
            }
        } elseif { $orient in [list "vertical" "horizontal"] } {        
            # Create the canvas line items.
            foreach {x1 y1 x2 y2 color} [get_lines $x $y $width $height $orient $rgb1 $rgb2] {
                $canvas create line $x1 $y1 $x2 $y2  -fill $color  -tags $tags  -capstyle projecting  {*}$args
                if {$animate} {update}
            }
        } else {
            # Delete the meta data and throw an error.
            $canvas delete [join $tags &&]
            error "invalid orientation: $orient; must be: vertical, horizontal, circular, central, or arc"
        }
        
        return
    }
    
    # Same as draw, but if the gradient already exists then it will retain its position in the canvas's display list after being redrawn.
    proc redraw {canvas tags x y width height orient color1 color2 args} {
        # If the gradient already exists, remember its location in the display list.
        set item_above [$canvas find above [join $tags &&]]
        
        # Draw the gradient.
        draw $canvas $tags $x $y $width $height $orient $color1 $color2 {*}$args
        
        # Move the gradient to its previous location in the display list, if any.
        if { $item_above ne "" } {
            $canvas lower [join $tags &&] $item_above
        }
        
        return
    }
    
    # Shortcut procedure to change the colors of the gradient. Same as redraw but introspectively determines some meta data. 
    # This will use the gradient's x/y location when it was last drawn.
    proc recolor {canvas tags color1 color2} {
        # Retrieve the gradient's meta data.
        set meta [get_meta_data $canvas $tags]
        set meta [dict remove $meta color1 color2]
        dict with meta {}
        
        # Redraw the gradient with the new colors.
        redraw $canvas $tags $x $y $width $height $orient $color1 $color2 {*}$args
        return     
    }
    
    # Shortcut procedure to resize and reposition a gradient. Same as redraw but introspectively determines some meta data. 
    proc resize {canvas tags x y width height} {
        # Retrieve the gradient's meta data.
        set meta [get_meta_data $canvas $tags]
        set meta [dict remove $meta x y width height]
        dict with meta {}
        
        # Recreate the canvas lines with the new dimensions.
        redraw $canvas $tags $x $y $width $height $orient $color1 $color2 {*}$args
        return
    }
    
    # Retrieve meta information on a gradient.
    proc get_meta_data {canvas tags} {
        if { [string is integer -strict $tags] } {
            # This is supposedly a canvas item ID instead of a list of tags describing a gradient. Avoid introducing subtle logic errors.
            error "cannot provide canvas item ID in lieu of gradient tags: $tags"
        }
        # This is a list of tags for a gradient. Find the hidden meta item for this gradient.
        set all_tags [$canvas gettags "meta&&[join $tags &&]"]
        set meta_data [lrange [lsearch -inline -index 0 $all_tags "gradient:"] 1 end]
        if { [llength $meta_data] == 0 } {
            error "cannot find meta data for gradient with tags: $tags"
        }
        return $meta_data
    }
    
    # For a canvas item ID, return all tags that are not gradient meta tags.
    proc get_non_meta_tags {canvas id} {
        set tags [$canvas gettags $id]
        set index [lsearch $tags "meta"]
        set tags [lreplace $tags $index $index]
        set index [lsearch -index 0 $tags "gradient:"]
        set tags [lreplace $tags $index $index]
        return $tags
    }

    # Calculate a list of center-to-outward colored rectangles suitable for [canvas create rectangle] or [canvas create oval].
    proc get_rects {x y width height rgb1 rgb2} {
        set rects [list]
        set x1 [expr {int($x)+1}]
        set y1 [expr {int($y)+1}]
        set x2 [expr {$x1+$width-1}]
        set y2 [expr {$y1+$height-1}]
        
        # Calculate each rectangle.
        if { $width > $height } {
            # Decrement X by one pixel in both directions. Decrement Y by a fraction in both directions.
            # The Y axis will be more heavily concentrated.
            set ratio  [expr { 1.0*$height/$width }]
            set length [expr { $width/2 }]
            while { $x2 > $x1 } {
                lappend rects $x1 $y1 $x2 $y2 [get_color $rgb1 $rgb2 $length [expr { ($x2-$x1)/2 }]]
                set x1 [expr { $x1+1 }]
                set x2 [expr { $x2-1 }]
                set y1 [expr { $y1+$ratio }]
                set y2 [expr { $y2-$ratio }]
            }
        } else {
            # Decrement Y by one pixel in both directions. Decrement X by a fraction in both directions.
            # The X axis will be more heavily concentrated (unless width==height).
            set ratio  [expr { 1.0*$width/$height }]
            set length [expr { $height/2 }]
            while { $y2 > $y1 } {
                lappend rects $x1 $y1 $x2 $y2 [get_color $rgb1 $rgb2 $length [expr { ($y2-$y1)/2 }]]
                set x1 [expr { $x1+$ratio }]
                set x2 [expr { $x2-$ratio }]
                set y1 [expr { $y1+1 }]
                set y2 [expr { $y2-1 }]
            }
        }
        return $rects
    }

    # Calculate a list of left-to-right or top-to-bottom colored lines suitable for [canvas create line].
    proc get_lines {x1 y1 width height orient rgb1 rgb2} {
        set lines [list]
        set x1 [expr {int($x1)}]
        set y1 [expr {int($y1)}]
        set x2 [expr {$x1+$width}]
        set y2 [expr {$y1+$height}]
        if { $orient eq "vertical" } {
            # Calculate the color for each horizontal line.
            for {set y $y1} {$y < $y2} {incr y} {
                lappend lines $x1 $y $x2 $y [get_color $rgb1 $rgb2 $height [expr {$y-$y1}]]
            }
        } elseif { $orient eq "horizontal" } {
            # Calculate the color for each vertical column.
            for {set x $x1} {$x < $x2} {incr x} {
                lappend lines $x $y1 $x $y2 [get_color $rgb1 $rgb2 $width [expr {$x-$x1}]]
            }
        } else {
            error "invalid orientation: $orient; must be: vertical or horizontal"
        }
        
        return $lines
    }

    # Calculates the color at the specified index between rgb1 and rgb2 where rgb1 and rgb2 are the specified length apart.
    proc get_color {rgb1 rgb2 length index} {
        # Throw an error if the index is out of bounds.
        if { $index < 0  ||  $index >= $length } {
            error "index $index is out of bounds for length $length"
        }
        
        lassign $rgb1 r1 g1 b1
        lassign $rgb2 r2 g2 b2
        
        # Determine the ratio between each starting component color and ending component color.
        set r_ratio [expr { 1.00*($r2-$r1+1)/$length }]
        set g_ratio [expr { 1.00*($g2-$g1+1)/$length }]
        set b_ratio [expr { 1.00*($b2-$b1+1)/$length }]

        # Calculate the new component colors at the given index. 
        set r [expr { int($r_ratio*$index+$r1) }]
        set g [expr { int($g_ratio*$index+$g1) }]
        set b [expr { int($b_ratio*$index+$b1) }]

        # A hacky workaround to make up for a lack of precision (or faulty math?).
        # The final pixel of the gradient should exactly match the color of rgb2
        if { $index == [expr {$length-1}] } {
            lassign $rgb2 r g b
        }

        # Convert the integer RGB values to a hex color.
        return [rgb_to_hex [list $r $g $b]]
    }
    
    # Convert a list of 16-bit RGB values to an 8-bit hex color. Using 8-bit hex colors instead of 16-bit
    # speeds up drawing of images two-fold (I haven't benchmarked this for canvases).
    proc rgb_to_hex {rgb} {
        lassign $rgb r g b
        set r [format %02x [expr {$r/256}]]
        set g [format %02x [expr {$g/256}]]
        set b [format %02x [expr {$b/256}]]
        return #$r$g$b
    }
    
    # Converts 8-bit RGB values to 16-bit RGB values.
    proc rgb_8_to_16_bit {rgb} {
        lassign $rgb r g b
        return [list [expr {$r*256}] [expr {$g*256}] [expr {$b*256}]]
    }
    
    # Returns a random 8-bit hex color.
    proc random_color {} {
        return [rgb_to_hex "[expr {int(rand()*65536)}] [expr {int(rand()*65536)}] [expr {int(rand()*65536)}]"]
    }

    # Redraw all gradients with random colors
    proc randomize_all_gradient_colors {canvas} {
        foreach id [$canvas find withtag meta] {
            set tags [get_non_meta_tags $canvas $id]
            recolor $canvas $tags [random_color] [random_color]
        }
      return
    }
    
    # Randomize colors for all canvas items.
    proc randomize_all_canvas_item_colors {canvas} {
        foreach id [$canvas find withtag all] {
            if { [$canvas type $id] in {window} } {
                continue
            }
            $canvas itemconfigure $id -fill [random_color]
            if { [$canvas type $id] in {rectangle arc oval} } {
                 $canvas itemconfigure $id -outline [random_color]
            }
        }
        return
    }
    
    # Run a demonstration.
    proc demo {} {
        # Create a toplevel window with a canvas filling the contents.
        set win .gradients
        if { [winfo exists $win] } {
            destroy $win
        }
        toplevel $win
        set cvs [canvas $win.cvs -highlightthickness 0]
        pack $cvs -fill both -expand yes
        wm geometry $win 800x600
        raise $win

        # Fill the bottom of the screen with a striped green fade.
        gradient draw $cvs footer 0 0 10 10 vertical white darkgreen -dash {3 1}
        bind $cvs <Configure> {
            gradient resize %W footer 0 [expr {%h-300}] %w 300
        }
        
        # Draw a strip of rainbow.
        set width  100
        set colors {white red orange yellow green blue purple violet white}
        for {set i 1} {$i < [llength $colors]} {incr i} {
            set c1 [lindex $colors $i-1]
            set c2 [lindex $colors $i]
            gradient draw $cvs "wave $i" [expr {$width*($i-1)}] 0 $width 50 horizontal $c1 $c2
        }
        
        # Draw some balls with diameters of 100.
        set colors {green red yellow blue orange white}
        for {set i 0} {$i < 6} {incr i} {
            gradient draw $cvs "beachball $i" 0 55 100 100 arc [lindex $colors $i] #333 -start [expr {$i*60}] -extent 60
        }
        set colors {cyan pink orange firebrick}
        for {set i 0} {$i < [llength $colors]} {incr i} {
            gradient draw $cvs "quadball $i" 110 55 100 100 arc [lindex $colors $i] #eee -start [expr {$i*90}] -extent 90
        }
        gradient draw $cvs ball1    220 55 100 100 circular #bbbbbb white
        gradient draw $cvs ball2    330 55 100 100 circular #888 #eee
        gradient draw $cvs ball3    440 55 100 100 circular black grey
        gradient draw $cvs ball4    550 55 100 100 circular black #555
        gradient draw $cvs ball5    660 55 100 100 circular cyan black
        
        # Draw some boxes.
        gradient draw $cvs box1   0 160 150 100 horizontal white firebrick
        gradient draw $cvs box2 150 160 200 100 central    black firebrick
        gradient draw $cvs box3 350 160 100 100 horizontal firebrick black
        gradient draw $cvs box4 450 160 200 100 central    firebrick black
        gradient draw $cvs box5 650 160 150 100 horizontal black white
        
        # Display some control buttons.
        frame $cvs.f
        button $cvs.f.b1 -text "Redraw with random gradient colors" -command [list [namespace current]::randomize_all_gradient_colors $cvs]
        button $cvs.f.b2 -text "Relaunch demo with default colors" -command [list [namespace current]::demo]
        button $cvs.f.b3 -text "Chaos" -command [list [namespace current]::randomize_all_canvas_item_colors $cvs]
        pack $cvs.f.b1 $cvs.f.b2 $cvs.f.b3 -fill x
        $cvs create window 400 400 -window $cvs.f

        return
    }
}

gradient demo
======

[MaxJarek]: We have also tklib http://core.tcl.tk/tklib/doc/trunk/embedded/www/tklib/files/modules/canvas/canvas_gradient.html%|%canvas::gradient%|% module.


----
'''[arjen] - 2013-10-04 07:37:28'''

I tried the example in the documentation for the canvas::gradient package. Unfortunately a small extension of that example showed a glitch:

======
# grad.tcl --
#     Show a gradient on the canvas
#
package require canvas::gradient
canvas .c
canvas::gradient .c -direction x -color1 yellow -color2 blue
pack .c -fill both -expand 1
set poly [.c create polygon 200 100 300 200 200 300 200 100 -fill blue]
======

The polygon does not show up. My guess is that the gradient items are drawn later, so that a ".c lower canvas::gradient" is required.

(I should add this to the Tklib tracker too - will do later)

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