Overlay Overview

General information

Overlay bitmaps are separate 1-bit bitmaps that are drawn on top of you main bitmap. Every main bitmap can have up to 17 (MAX_OVERLAYS + 1) overlay bitmaps. The overlays are stored in an array containing MAX_OVERLAYS + 1 entries. Each array entry contains the overlay bitmap and its attributes. The array uses a 0-based index. (That is the index of the first entry is 0.)

For each overlay, the overlay pixels with a value of 0 are considered transparent. The overlay pixels with a value of 1 are considered non-transparent and will be painted using the color set by the use in OVERLAYATTRIBUTES.crColor.

The overlays can be painted automatically by the L_PaintDC function or you can paint them manually using L_PaintDCOverlay. L_PaintDC paints the overlays in ascending index order: The overlay with index 0 is painted first, overlay with index 1 is painted next, etc. If you need to change the order in which the overlays are painted, you must paint them manually with L_PaintDCOverlay. Another option is to rearrange the overlays and make sure that the overlay with index 0 is the overlay you want painted first, and then use L_PaintDC.

Overlay attributes

Each overlay has a number of attributes (or properties), which can be set by the user. These attributes are broken into two categories:

Display attributes

These properties specify how an overlay gets painted:

DICOM-related attributes

These properties are DICOM specific and are based on the "Overlay Plane Module Attributes" defined in PS 3.3 of the DICOM standard:

Overlay operations

The overlay array is automatically created the first time you set the attributes or the bitmap for a particular overlay index. The overlay array is automatically freed when the main bitmap is freed. To set the attributes for a specific overlay, use L_SetOverlayAttributes. One or more attributes may be set at a time. TO get the current attributes for a specific overlay, call L_GetOverlayAttributes. The L_GetOverlayCount gets the number of overlays that have been defined.

The overlay bitmaps are independent of the main bitmap. Separate memory is allocated for each overlay bitmap. You can set the overlay bitmap by:

To get the overlay bitmap for a specific overlay, call L_GetOverlayBitmap.

You can update the corresponding bitplane in the main bitmap with the bits from the overlay bitmap by calling L_UpdateBitmapOverlayBits and setting the SETOVERLAYBITS_FROMOVERLAY flag. This may be used to preserve a specific bitplane during an image processing function. To prevent a certain image processing function from affecting a bitplane,:

• get the bitplane into an overlay bitmap with L_UpdateBitmapOverlayBits(..., SETOVERLAYBITS_FROMBITMAP);

• apply the image processing function

• restore the desired main bitmap bitplane with L_UpdateBitmapOverlayBits(..., SETOVERLAYBITS_FROMOVERLAY);

To preserve the whole bitplane, make sure the overlay offset is 0,0 and that the overlay has the same size as the overlay bitmap. This is true by default, but if you change the overlay position and size, you must reset them.

The default overlay width is the display bitmap width - the overlay's left offset. The overlay's left offset is set in the OVERLAYATTRIBUTES.ptOrigin.x member. The display width depends on the bitmap's view perspective and it is the width of the bitmap painted on the screen with a zoom factor of 100%.

The default overlay height is the display bitmap height - the overlay's top offset. The overlay's top offset is set in the OVERLAYATTRIBUTES.ptOrigin.y member.

You can change the size of the overlay bitmap by calling L_SetOverlayBitmap. Note that you can call L_SetOverlayBitmap with an unallocated bitmap. In that case, only the width and height are used from the overlay bitmap.