Pixels and Textures

1
Pixels and Textures

• Geb Thomas
• Adapted from the OpenGL Programming Guide

2
Learning Objectives

• Learn how to copy information from the
  framebuffer
• Learn what a texture is
• Learn how to use textures

3
Copying Framebuffers

• glReadPixels() - Reads a rectangular array of
  pixels from the framebuffer and stores the data
  in processor memory.
• glDrawPixels() - Writes a rectangular array of
  pixels from data kept in processor memory into
  the framebuffer at the current raster position
  specified by glRasterPos().

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ReadPixels

• void glReadPixels(GLint x, GLint y, GLsizei
  width, GLsizei height, GLenum format, GLenum
  type, GLvoid pixels)
• Reads pixel data from the framebuffer rectangle
  whose lower-left corner is at (x, y) and whose
  dimensions are width and height and stores it in
  the array pointed to by pixels. format indicates
  the kind of pixel data elements that are read,
  and type indicates the data type of each element.

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Format

• GL_COLOR_INDEX A single color index
• GL_RGB A red color component, followed by a
  green color component, followed by a blue color
  component
• GL_RGBA A red color component, followed by a
  green color component, followed by a blue color
  component, followed by an alpha color component

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Data Type

• GL_UNSIGNED_BYTE unsigned 8-bit integer
• GL_BYTE signed 8-bit integer
• GL_BITMAP single bits in unsigned 8-bit integers
  using the same format as glBitmap()

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Texture Mapping

• Create the texture
• Decide how to apply the texture
• Enable texture mapping
• Draw the scene, providing texture coordinates

8
Creating the Texture

• Normally you read in an image and create the
  texture pattern buffer
• OpenGL does not provide convenient ways to read
  in an image, you have to use an external library
  for this, such as this free library for reading
  and writing JPEG files.

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Decide how to apply the texture

• Decal?
• Modify the current colors?
• Blend with the current colors?
• Modify the current normals?
• Repeat at edges, or clip?

10
Learning Objectives

• Learn how to copy information from the
  framebuffer
• Learn what a texture is
• Learn how to use textures