ImBuf: Optimize GPU memory by using 1 component format for grayscale images

This is done by checking the number of bitplanes from the image buffer.
We assume that for float buffer to use the same bitplanes as it was a
byte buffer.

Then, the data of the image buffer is packed at the start of the `rect` or
`float_rect` before upload.

**Statistics - einar.v004.blend **

Note that not all grayscale textures have been stored as BW images so the
amount of memory that can be reduced would be more.

Without patch
```
104 Textures - 3294.99 MB (3294.47 MB over 32x32), 37 RTs - 192.52 MB.
Avg. tex dimension: 2201.88x1253.51 (2283.53x2202.13 over 32x32)
464 Buffers - 25.01 MB total 1.24 MB IBs 23.50 MB VBs.
3512.52 MB - Grand total GPU buffer + texture load
```

Patch applied
```
104 Textures - 2917.66 MB (2917.14 MB over 32x32), 39 RTs - 215.45 MB.
Avg. tex dimension: 2221.38x1252.75 (2323.28x2253.47 over 32x32)
467 Buffers - 25.01 MB total 1.24 MB IBs 23.51 MB VBs.
3158.13 MB - Grand total GPU buffer + texture load.
```

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D15484

1 files changed, 10 insertions, 0 deletions

diff --git a/source/blender/editors/sculpt_paint/paint_image.cc b/source/blender/editors/sculpt_paint/paint_image.cc
index 24290fed323..5a6ac9463e2 100644
--- a/source/blender/editors/sculpt_paint/paint_image.cc
+++ b/source/blender/editors/sculpt_paint/paint_image.cc
@@ -158,6 +158,16 @@ void imapaint_image_update(
                                             imapaintpartial.dirty_region.xmax,
                                             imapaintpartial.dirty_region.ymax);
 
+  /* When buffer is partial updated the planes should be set to a larger value than 8. This will
+   * make sure that partial updating is working but uses more GPU memory as the gpu texture will
+   * have 4 channels. When so the whole texture needs to be reuploaded to the GPU using the new
+   * texture format.*/
+  if (ibuf != nullptr && ibuf->planes == 8) {
+    ibuf->planes = 32;
+    BKE_image_partial_update_mark_full_update(image);
+    return;
+  }
+
   /* TODO: should set_tpage create ->rect? */
   if (texpaint || (sima && sima->lock)) {
     const int w = BLI_rcti_size_x(&imapaintpartial.dirty_region);