How do I override the default icon selection algorithm?

Raymond Chen

Raymond

The usual way of loading an icon from a resource is to use the
Load­Icon
or
Load­Image function
and letting the window manager’s icon selection algorithm
decide which image to use
based on the images available in the icon,
the desired icon size, and the current color depth.
A customer wanted to override that algorithm,
because the window manager uses the current display
color depth to select an image,
but they were obtaining the icon for printing purposes,
so they wanted to get the highest-color-quality icon
rather than the one that matched the screen’s color depth.
How do you override the default algorithm?

You basically do the same thing the window manager does.
As we saw earlier,

icon resources are actually stored in multiple pieces
.
The thing you use to talk about icons is actually the
icon directory,
which in turn points to a set of images.
The first step, then, is to obtain the icon directory.

HRSRC hrsrcIcon = FindResource(hResources,
                     MAKEINTRESOURCE(IDI_MY_ICON), RT_GROUP_ICON);
HGLOBAL hIcon = LoadResource(hResources, hrsrcIcon);
auto lpIcon = static_cast<GRPICONDIR *>(LockResource(hIcon));

You then take the images listed in the GRPICONDIR and
apply your custom algorithm to decide which one you like best.
(If you want to use the default algorithm, you can call
Lookup­Icon­Id­From­Directory
or
Lookup­Icon­Id­From­Directory­Ex.
But if you want to use the default algorithm, then just use
Load­Image already!)

When you’ve found the image you like, take the nId,
and that’s the resource ID for the RT_ICON.

HRSRC hrsrcImage = FindResource(hResources,
                     MAKEINTRESOURCE(nId), RT_ICON);
HGLOBAL hImage = LoadResource(hResources, hrsrcImage);
auto lpImage = static_cast<PBYTE>(LockResource(hImage));

You can then convert the icon image data into an icon by
using the
Create­Icon­From­Resource or
Create­Icon­From­Resource­Ex function.

Raymond Chen
Raymond Chen

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