GLART week 8: Text

Assigment: Add text to any of the previous homework assignments. Reading: Redbook pages 255-277 "Chapter 7: Display Lists" Examples: - flat text drawn over a scene - text drawn in a 3D scene Requires images/font_tahoma.png Also see demo/ and the jocode/ to generate text using Java's Font capabilities. Resources: Bitmap Font Builder -- creates texture images of character sets NOTES: ================================================================= OpenGL does not have any font or text capabilities built-in, but there are several ways to create text, including building geometry from font shapes, or drawing images of letters. This second method is simple and works well in most situations so we'll explore it here. The idea is to draw an image of a letter textured onto a quad. That's not so complex by itself, but the problem becomes more substantial when you consider that there are 26 letters in the alphabet, plus upper and lower case, and then there are numbers, punctuation, etc. Fortunately with some clever use of texture coordinates and display lists, we can create a character set and draw any text string we want with minimal effort. JOApp contains a print(String) function that will render a text string by texturing images of characters onto quads. There are several steps that happen for this function to work. First we'll need a texture image (font_tahoma.png) that contains a full character set arranged in order by ascii value. The JOApp.buildFont() function creates a set of quads, each textured with one character from the character set image. The textured quads are rendered into a set of display lists, so that we can later draw any character just by calling its display list. The JOApp.print() function draws text by matching each character in a text string with the correct display list, then running callList() to draw that character. print() calls setOrthoOn() to set the projection mode to ortho, and disables the depth test so text always draws on top of other geometry. We don't need any new functions to use text in OpenGL, but there are some settings that affect how text will appear: texture - be sure to glEnable(GL_TEXTURE_2D) light - disable lighting (glDisable(GL_LIGHTING)), to insure that the text color is even across the screen. With lighting on, some or all of the text may appear shadowed. blend - enable blending so the background of the text will be transparent. glEnable(GL_BLEND) glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA) ortho - set the projection matrix to an orthoganal projection to make it easy to place the text flat on the screen (glOrtho()) depth - disable the depth test (glDisable(GL_DEPTH_TEST)) before drawing text to insure that the text is drawn on top of the rendered scene. pushAttrib - The glPushAttrib() function is a convenient way to save the current OpenGL settings. Subsequent code can change settings (such as enabling/disabling, setting lights, textures, materials, etc.) then can call glPopAttrib() to return to the previous settings. Call gl.glPushAttrib(GL.GL_ALL_ATTRIB_BITS) to save all OpenGL settings, or you can preserve specific settings, for example just those affecting light and texture: glPushAttrib(GL.GL_TEXTURE_BIT | GL.GL_LIGHTING_BIT) The JOApp.print() function calls these settings for you, but if you want to work with text in some other customized situation you can use the print() function as a starting point.