[SDPhantom]

Here's the modified function. The default values for the arguments noted below will apply if an argument is missing or is nil. Values marked required are as such.

Syntax:
SetBarValue(bar,val1,val2,flipH,flipV,cropx1,cropx2,cropy1,cropy2)

bar = Bar texture (required)
val1, val2 = Upper and lower values for the bar to be drawn (default 0)
flipH, flipV = Boolean true/false, flips texture horizontally/vertically if true (default false)
cropx1, cropx2, cropy1, cropy2 = Crop values, swapping high and low values within the xy pairs will not affect flipping (default 0, 1, 0, 1)

local function SetBarValue(bar,val1,val2,flipH,flipV,cropx1,cropx2,cropy1,cropy2)-- Accepts 9 values, bar texture, start value, end value, flip flags, and crop offsets (any value exceptbar can be nil)
--  Keep the values within a 0-1 range
    val1=math.min(math.max(val1 or 0,0),1);
    val2=math.min(math.max(val2 or 0,0),1);
 
--  Sometimes textures glitch when trying to set a width or height of 0, so handle it here
    if val1==val2 then
        bar:Hide();
        return;
    else
        bar:Show();
    end
 
--  Texture crop/padding
    cropx1=math.min(math.max(cropx1 or 0,0),1);
    cropx2=math.min(math.max(cropx2 or 1,0),1);
    cropy1=math.min(math.max(cropy1 or 0,0),1);
    cropy2=math.min(math.max(cropy2 or 1,0),1);
 
--  Set our low and high values
    local low,high=math.min(val1,val2),math.max(val1,val2);
 
--  Find our parent width and height
    local parent=bar:GetParent();
    local pw,ph=parent:GetWidth(),parent:GetHeight();
 
--  Get our point set, we'll use this to get our bar orientation
    local point=bar:GetPoint(1);
    if not point then return; end-- Handle if point isn't set, perhaps cleared?
 
--  These are our temp origin variables (note one of these pairs can have both vars nil, just use default texcoords for those)
    local origL=point:find("LEFT$");
    local origR=point:find("RIGHT$");
    local origT=point:find("^TOP");
    local origB=point:find("^BOTTOM");
 
--  Reset point
    bar:SetPoint(point
        ,(origL or origR) and (origL and low*pw or -low*pw) or 0--  X offset
        ,(origT or origB) and (origB and low*ph or -low*ph) or 0--  Y offset
    );
 
--  Texcoord calc
    local l,r,t,b=0,1,0,1;--    Texcoords are in the range of 0-1 with their origin at top left
    if origL then
        l,r=low,high;
    elseif origR then
        l,r=1-high,1-low;
    end
    if origT then
        t,b=low,high;
    elseif origB then
        t,b=1-high,1-low;
    end
 
--  Process flip (affects which side the bar piece is drawn from too)
    if flipH then l,r=1-l,1-r; end
    if flipV then t,b=1-t,1-b; end
 
--  Constrain to crop size
    local cropL,cropR,cropT,cropB=math.min(cropx1,cropx2),math.max(cropx1,cropx2),math.min(cropy1,cropy2),math.max(cropy1,cropy2);
    local cropW,cropH=cropR-cropL,cropB-cropT;
    l,r,t,b=l*cropW+cropL,r*cropW+cropL,t*cropH+cropT,b*cropH+cropT;
 
--  Apply texcord/size
    bar:SetTexCoord(l,r,t,b);
    bar:SetWidth(pw*((origL or origR) and math.abs(high-low) or 1));
    bar:SetHeight(ph*((origT or origB) and math.abs(high-low) or 1));
end