Image scaling (KeepAspectRatioByExpanding) through OpenGL

by

Simply do the same math you did with KeepAspectRatio, but this time keep the height instead of the width. You’ll get a width bigger than 1, so you’ll have to use the inverse of it for your texture coordinate.

Normalized coordinates yes/no won’t matter, you just have to add these factors for the texture coordinates. For this to work I assume the image fills the whole texture (from (0,0) to (1,1); this makes it easy to multiply the values with your width/height). Texture wrapping has to be off.

Default texture coordinates:

(0,0)-(1,0)
|     |
(0,1)-(1,1)

Aspect ratios:

tex_ar = tex_width / tex_height; // aspect ratio for the texture being used
scr_ar = scr_width / scr_height; // aspect ratio for the quad being drawn

KeepAspectRatio (touch from the inside):

(0, 0)-----------------------(max(1, scr_ar / tex_ar), 0)
|                            |
(0, max(1, tex_ar / scr_ar))-(max(1, scr_ / tex_ar), max(1, tex_ar / scr_ar))

KeepAspectRatioByExpanding (touch from outside; just replace max() with min()):

(0, 0)-----------------------(min(1, scr_ar / tex_ar), 0)
|                            |
(0, min(1, tex_ar / scr_ar))-(min(1, scr_ / tex_ar), min(1, tex_ar / scr_ar))

For your case you’d just have to multiply the resulting texture coordinates with your width/height.

More Related Contents:

  1. Qt3d. Draw transparent QSphereMesh over triangles
  2. Paint a rect on qglwidget at specifit times
  3. Reflection and refraction impossible without recursive ray tracing?
  4. Detect if stdin is a terminal or pipe?
  5. Deploying Qt 5 App on Windows
  6. Transform the modelMatrix
  7. Serialization with Qt
  8. How to compile for Windows on Linux with gcc/g++?
  9. Qt 5.1.1: Application failed to start because platform plugin “windows” is missing
  10. How to compile Qt 5 under Windows or Linux, 32 or 64 bit, static or dynamic on Visual Studio or g++
  11. How to efficiently display OpenCV video in Qt?
  12. How to make an expandable/collapsable section widget in Qt
  13. How do I create a pause/wait function using Qt?
  14. When should Q_OBJECT be used?
  15. QML and C++ image interoperability
  16. Get index of QPushButton on 2D array QPushButton
  17. Segmentation fault at glGenVertexArrays( 1, &vao );
  18. How do I draw text with GLUT / OpenGL in C++?
  19. Console output in a Qt GUI app?
  20. “Symbol(s) not found for architecture x86_64” on QtCreator project
  21. Open QFile for appending
  22. How to load a bmp on GLUT to use it as a texture?
  23. How do I construct an ISO 8601 datetime in C++?
  24. What are consequences of forcing QObject as a parent of QWidget?
  25. C++ – std::thread crashes upon execution
  26. QTableView printing
  27. Cmake link library target link error
  28. Where can I get windows.h for Mac?
  29. Two colours text in QPushButton
  30. true isometric projection with opengl

Leave a Comment