Drawing Tab Properties Reference

This section provides background for several of the concepts in the Form Properties Drawing topic, including:

• Inheritable Properties

• Z order Imaging

• Logical Bit-map Drawing Operations

• Palette Mode

For a full treatment of drawing text and graphics on a form, see Form Text and Graphics.

Inheritable Properties

Form controls inherit properties from the form. For example, if the form background color is set to light gray, form controls inherit that background color and will have a light gray background also. You can think of form properties as being inheritable properties used by all elements within the form.

These properties can be overridden as shown in the following example. The appearance of drawing commands (lines, arcs, circles, text, and so on) are controlled by a combination of:

• inheritable form property settings.

• specific procedure coding (overriding the inherited settings).

Z Order Imaging

The image you see on your computer display looks like a flat, paper-like image. In reality, the image is composed of many image layers. "Z order" is a relative ordering which determines how multiple layers overlay one another. Three major graphical layers are associated with forms. The back layer is the drawing space where the results of the graphics methods are displayed. Next is the middle layer where graphical objects and ALabel controls are displayed. The front layer is where all nongraphical controls like AButton, ACheckBox, or AListBox are displayed. Anything contained in a layer closer to the front covers anything contained in the layer (s) behind it. "Z order" arranges objects only within the layer where the object is displayed. Objects can be sent to the front or the back of the Z order within their original layer.

For example:

Logical bit-map drawing operations

At first sight, the modes under which drawing operations take place may look a little forbidding. The DrawMode property (found on the Drawing tab in Form Properties) has 16 possible settings, 0-15, with such scintillating and easy-to-distinguish names as MaskNotPen and MaskPenNot. How do you make sense of them? The secret is that of these 15 settings, only six are truly important; the others are for the most part inversions or combinations of these six. Each mode determines the nature of the graphic "pen" with which objects are drawn.

The essential six DrawMode properties are as follows:

With these six DrawMode properties, you can accomplish most of the drawing tasks necessary for the creation of a form. With the addition of mode 3, NotCopyPen, and using mode 6, XorPen, you can also devise credible animations. (NotCopyPen draws the inverse of the line over the background; XorPen has the unique characteristic that if you draw a line twice with it, the original background re-appears.)

If you have time, experiment with the remaining DrawMode properties; the results are less predictable, but for some applications, they are just right. They are:

PaletteMode

Although many computers today have displays capable of displaying thousands of colors ("high color") or millions of colors ("true color"), there are still plenty of machines out there that can only display 25

You can choose how you want the system to select the colors for display by setting the PaletteMode property of the form. All controls on the form, especially including the AImage control, will be displayed based on the PaletteMode.

The settings for PaletteMode include: